NRCA Slate cheat sheet

Slate is a metamorphic rock derived from shale; commercial roofing slate is valued for fissility (splitting into thin sheets), density, and low absorption, not for mineral purity alone.

Standard slate thickness grades: 3/16 in (quarter-inch or "¼") is lightest common; 1/4 in ("standard") is typical; 3/8 in and 1/2 in are heavier grades used where greater durability or wind uplift resistance is required.

Slate is sold by "square" = 100 sq ft of finished roof area; because of headlap and side lap, 1 square of roof requires more than 100 sq ft of slate (order factor commonly 1.10–1.30 depending on exposure).

Exposure = the exposed length of slate = (slate length − headlap). For 3/16 in to 3/8 in slate, headlap is typically 3 in; for 1/2 in and heavier, 3 in minimum but often 4 in.

Standard slate sizes range roughly from 6x8 in to 14x24 in; larger slate requires thicker material and greater headlap, and the same roof may mix sizes (graduated or random-width) for appearance.

Color comes from mineral content: carbonaceous matter gives black/gray; hematite gives red/purple; chlorite gives green; weathering pyrite can produce rust-colored stains. Color is not a reliable durability indicator.

ASTM C406 classifies roofing slate by grade (S1 for uniform, unfading, free of harmful minerals; S2 for durable but with some variation; S3 for limited use), and by type based on weathering/absorption characteristics.

Slate life expectancy is commonly cited as 75–200 years depending on grade, thickness, and climate; hard, low-absorption slate (often called "hard" or "ribbon" slate) outlasts soft, high-absorption slate.

Fasteners: copper or stainless steel slate hooks/nails are preferred; galvanized steel and electroplated copper are avoided because corrosion products stain slate and shorten service life.

Headlap rule of thumb: minimum headlap = 3 in for slate up to 3/8 in thick; 4 in for 1/2 in and thicker slate. Exposure must never exceed slate length minus required headlap.

Nail placement: two nails per slate, located 1 to 1.5 in from each side edge and about 1 in above the bottom edge of the slate above (i.e., just above the headlap line), so the nail is covered by the next course.

Slate must be able to move: nails are driven snug, not tight, and slates are hung on the nail (not butted tight against it), because thermal expansion/contraction will crack tight-hung slate.

Side lap: minimum 3 in side lap for standard slate; joints in adjacent courses must be offset at least 3 in so no four corners meet (no "cross joints").

Course layout: use a story pole or gauge line to establish exposure; the first course is doubled (a "starter" or "double" course) at the eave, and the top course is often doubled at the ridge.

Slate is laid from eave to ridge and from the rake toward the center or from one end, working so that each slate overlaps the one below; installers stand on slate hooks or roof jacks, not on the slate field.

Hips and ridges use special hip/ridge slate or saddle slate; valleys are typically open metal valleys (16–20 in wide) or closed valleys depending on design.

Fastener pattern for wide slate may require four nails; slate wider than about 14 in or in high-wind zones should be nailed with two nails per slate minimum plus a copper or stainless clip where specified.

Slate roofs are heavy: 3/16 in slate weighs roughly 700–800 lb per square; 1/4 in about 900–1,000 lb; 3/8 in about 1,300–1,500 lb; 1/2 in about 1,800–2,000 lb per square - structural design must account for this dead load.

Minimum deck for slate is typically 3/4 in nominal solid wood sheathing (boards not over 8 in wide, or plywood/OSB rated for the load); skip sheathing with spaced boards is acceptable only where slate length spans the boards and underlayment is designed for it.

Underlayment for slate must be a high-temperature-rated material: common choices are 30 lb asphalt-saturated felt (two layers), ASTM D6757 or D8257 high-temperature underlayment, or a self-adhering membrane at eaves/valleys.

Underlayment is lapped shingle-style: side laps at least 6 in, end laps at least 4 in (more in low-slope areas), with laps oriented to shed water down-slope.

Eave protection (ice dam membrane) is required in cold climates; extend it from the eave edge to at least 24 in inside the interior wall line, following the local code (IRC R905.2.7 for asphalt; slate follows manufacturer/code provisions).

Chimney flashing consists of: base flashing (apron, step flashing on sides, backer/cricket flashing on the uphill side), counterflashing set into a reglet, and a cricket or saddle for chimneys wider than 30 in.

Step flashing is installed in individual pieces interwoven with each slate course along sidewalls; each piece extends at least 2 in under the slate and 4 in up the wall, with a 2 in lap to the next piece.

Valley flashing: open valleys use metal (copper, lead-coated copper, or stainless) at least 16–20 in wide with a W or crimped center; closed valleys are woven with slate and require careful layout.

Flashing materials should be compatible with slate: copper, lead-coated copper, lead, and stainless are standard; bare galvanized steel and aluminum are avoided near slate because runoff can stain and because of galvanic interaction.

Drip edge and rake flashing should be copper or compatible metal; gutters and hangers must be sized for the high runoff from steep slate roofs and anchored to resist snow/ice loads.

First step in restoration is a condition survey: map broken/slipped slate by area, check for missing or corroded flashing, and assess deck and underlayment condition from the attic side.

Slate can be repaired individually using a slate ripper (a long flat bar with a hook) to cut or pull the nails of the broken slate, then a new slate is set and secured with a copper or stainless slate hook or a nail driven through the head of the slate above.

Slate hooks are used where the slate cannot be nailed (repair situations); the hook is slipped under the slate above and holds the replacement slate's head.

Matching replacement slate: match thickness, size, color, and grade as closely as possible; salvaged slate from the same roof or a known source is preferred, and new slate should be sorted to blend.

Never walk on slate without protection: use roof jacks, ladders with standoffs, or plywood walkways padded with foam; broken slate from foot traffic is the most common avoidable damage.

Flashing repairs often outlast slate: replace failed step flashing, counterflashing, and valley metal with copper or lead-coated copper; re-point mortar in reglets and repaint/repair chimney masonry.

Snow guards and snow fences protect slate from snow slides and ice; their absence in snow country is a common cause of slate and gutter damage and should be part of restoration scope.

Slate roofs can be cleaned of moss/lichen with a soft brush and approved treatment; avoid pressure washing and harsh chemicals that abrade or stain slate and void warranties.

Restoration vs. replacement decision: if more than about 20–25% of slate is broken or the underlayment has failed over large areas, full replacement or a tear-off to deck is often more economical than spot repair.

OSHA fall protection: for residential steep roofs, fall protection (guardrail, safety net, or personal fall arrest system) is required when working 6 ft or more above a lower level in construction (29 CFR 1926.501); roof anchors must be engineered/rated.

Ladder safety: 4:1 rule (1 ft out for every 4 ft up), extend 3 ft above the roof edge, and secure the ladder; use ladder stabilizers/standoffs to avoid gutter and slate damage.

Roof jacks and toe boards: OSHA requires employees on steep roofs to use roof jacks or a crawling board; roofing brackets must be properly seated on the deck and not bear on slate.

Personal protective equipment for slate work: fall harness and lanyard, hard hat, safety glasses, cut-resistant gloves (slate edges are sharp), and dust protection when cutting.

Relevant codes/standards: IRC Chapter 9 (Roof Assemblies) covers roof covering requirements; ASTM C406 covers roofing slate; ASTM C1311 and D6757/D8257 cover underlayments; SMACNA and NRCA manuals give installation detail.

Ice barrier and underlayment requirements come from the locally adopted code (IRC R905); slate roofs must meet the same eave protection and flashing rules as other steep-slope coverings, plus manufacturer instructions.

Structural code: the roof dead load of slate must be included in design per ASCE 7 and the adopted building code; adding slate over an existing roof without checking capacity is a common code violation.

Fire rating: slate is a Class A noncombustible covering; the assembly rating depends on the deck and underlayment, so verify the rated assembly when required by code.

Inspection and permits: re-roofing and structural alterations typically require a permit and inspection; historic districts may require review before altering a slate roof's appearance or materials.

Slate "ribbon" or "graduated" patterns refer to color banding and size graduation, not to material grade; they are design choices that affect ordering and layout, not durability.

Fissility: slate splits along cleavage planes formed during metamorphism; splitting direction determines the "grain" and the natural surface texture (smooth, honed, or sanded/rubbed).

Absorption and weathering: slate with high water absorption and abundant pyrite tends to fail faster (rust staining, delamination); ASTM C406 type testing addresses this.

Slate terminology: "head" is the top edge, "butt" the bottom exposed edge, "bed" the underside, "face" the exposed surface; "headlap" is the overlap at the head.

Ordering: add 10–20% waste for breakage and cutting, and more for hips/ridges/valleys and complex roofs; keep a stock of matching slate for future repairs.

Fastener corrosion: copper and stainless steel are the standard; galvanized nails rust and stain slate, and electroplated copper is not a substitute for solid copper or stainless.

Weight per square formula: weight (lb/square) ≈ slate thickness (in) × 100 × density factor; practical figures are ~800 lb for 3/16 in, ~1,000 lb for 1/4 in, ~1,500 lb for 3/8 in, and ~2,000 lb for 1/2 in per square.

Slate sizes and exposure: exposure = length − headlap; a 12 in slate with 3 in headlap gives 9 in exposure, so one square needs about 100/(9/12 × width factor) slate depending on width.

Compatibility: avoid contact between slate and acidic or alkaline cleaners, and avoid copper runoff onto galvanized steel below; copper staining of slate and masonry is a known aesthetic issue.

Nail length: nails should penetrate the deck at least 3/4 in (through sheathing) and be long enough to pass through the slate and into solid wood; do not overdrive.

Starter course: a double layer of slate at the eave, with the lower layer set in a bed of mortar or on a cant strip, sheds water off the fascia and protects the underlayment edge.

Rake and eave treatment: rake slate should overhang the rake edge 1–1.5 in; eave slate should project over the gutter 1–2 in so water clears the fascia and enters the gutter.

Valley layout: open valleys require the slate to be cut back from the valley centerline (typically 3–4 in on each side), and the metal valley must be wide enough (16–20 in) to handle runoff.

Hip and ridge: hip slate is laid from the bottom up, alternating sides, with each piece overlapping the one below; ridge slate is laid similarly from one end or from the center.

Ventilation: slate roofs need balanced intake and exhaust ventilation (e.g., 1 sq ft net free vent area per 150 sq ft of attic floor with vapor barrier, per IRC); slate itself is not breathable enough to substitute.

Wind uplift: in high-wind zones, use thicker slate, larger headlap, and clips or hooks; follow local code and manufacturer guidance for edge zones.

Cutting and fitting: slate is cut with a slate cutter or scored and snapped; use a template for repetitive cuts and keep cut edges away from exposure where possible.

Quality control: check exposure with a gauge every few courses, verify nail placement and that no slate is tight-hung, and inspect from the attic for daylight and nail penetration.

Deck attachment: sheathing must be fastened per code (e.g., 8d nails at 6 in on center at edges and 12 in in the field for plywood), and gaps of 1/8 in between panels are required for expansion.

Deck flatness: slate requires a relatively flat deck; uneven or cupped boards telegraph through and cause slate breakage; replace or plane defective boards before installation.

Underlayment fastening: use corrosion-resistant fasteners (galvanized or stainless) with large heads or cap nails; staples are generally not acceptable for high-temperature underlayment.

Ice dam protection: self-adhering membrane at eaves, in valleys, and around penetrations in cold climates; laps must be at least 6 in and the membrane must be applied directly to the deck.

Chimney cricket: required for chimneys wider than 30 in (some codes 24 in); cricket slope should be at least 1/4 in per foot and flashed into the chimney with base and counterflashing.

Skylight and vent flashing: use manufactured flashing kits rated for the roof pitch; curb-mounted units require step flashing and counterflashing, and the slate must be cut to fit without gaps.

Wall flashing: step flashing for masonry and siding walls, with a reglet cut at least 1 in deep; sealant alone is not a substitute for proper metal flashing.

Valley metal: copper or lead-coated copper, minimum 16 in wide for open valleys, with a W-shaped center or a standing seam; the metal must extend at least 6 in under the slate on each side.

Drip edge: use a metal drip edge at eaves and rakes where required by code; it should be installed over the underlayment at the eave and under the underlayment at the rake (or per manufacturer).

Slate ripper technique: slide the ripper under the broken slate to hook and cut the nails, then pull the slate out; avoid prying against good slate, which cracks it.

Replacement sequence: insert the new slate from below the slate above, slide it up into place, then secure with a nail or a slate hook; ensure the new slate's exposure matches the surrounding courses.

Slate hook selection: copper or stainless hooks sized to slate thickness; hooks are driven into the deck above the slate being repaired and bent over the slate head.

Flashing repair priority: failed flashing causes more water damage than broken slate; inspect step flashing, valleys, and chimney counterflashing first during any restoration.

Moss and lichen: remove with a soft brush and a diluted approved cleaner; avoid pressure washing, which erodes slate surfaces and forces water under the slate.

Snow retention: add snow guards or snow fences where snow slides are a hazard; they must be anchored into the deck, not just the slate.

Salvage: when replacing a roof, salvage sound slate for future repairs and for matching on partial restorations; label and store by size and color.

Repair documentation: photograph and map repairs by roof area, and keep a maintenance log; this supports warranty claims and future restoration planning.

When to replace: widespread broken slate, failed underlayment (visible from attic), sagging deck, or more than ~20–25% damage usually justifies full replacement rather than continued spot repair.

OSHA 1926.501(b)(13): residential steep roofs require fall protection when working 6 ft or more above a lower level; 1926.502 covers guardrail, safety net, and personal fall arrest requirements.

Ladder and scaffold rules: ladders must be inspected, rated for the load, and secured; scaffolds must be designed and erected by qualified persons per OSHA 1926 Subpart L.

Roof anchors: permanent anchors for fall arrest must be rated (commonly 5,000 lb) and installed per manufacturer; temporary anchors must be engineered for the load.

Hazard communication: slate dust from cutting contains crystalline silica; use wet cutting or dust collection and respiratory protection per OSHA 1926.1153.

Code adoption: the IRC is the model code; local amendments govern slate installation details, ice barriers, and permit requirements - always check the locally adopted edition.

ASTM standards relevant to slate: C406 (roofing slate), C1311 (solvent-based primer for flashing), D6757/D8257 (underlayment), and C1177/C1178 for gypsum sheathing where used.

Fire and wind: slate is Class A noncombustible, but the assembly rating depends on deck and underlayment; wind design must follow ASCE 7 and local code for edge zones.

Historic preservation: the Secretary of the Interior's Standards apply to historic buildings; in-kind slate replacement and retention of original material are preferred over substitution.

Inspection: final inspection should verify exposure, fastener type and placement, flashing details, and underlayment installation before the roof is closed in.

Slate grading: ASTM C406 Grade S1 is the highest (uniform color, no harmful minerals); S2 allows some variation; S3 is for limited use where appearance and durability requirements are lower.

Slate types by weathering: Type I (weathering) and Type II (non-weathering) classifications relate to how the slate changes with exposure; check the current ASTM C406 designation for exact type definitions.

Density: good roofing slate has a high specific gravity (roughly 2.6–2.9) and low absorption (often under 0.5–1%); high absorption correlates with faster deterioration.

Pyrite and carbonates: pyrite inclusions oxidize and cause rust stains and pop-outs; carbonate inclusions (calcite) can cause spalling - avoid slate with visible harmful minerals on the exposed face.

Slate surface finishes: natural cleft (split), sanded/rubbed (smooth), and honed; surface finish affects appearance and slightly affects water shedding, not structural performance.

Slate patterns: standard (uniform), random-width, graduated (increasing size toward the eave), and diamond/hexagonal; pattern affects ordering, layout, and waste.

Slate accessories: ridge and hip slate (saddle slate), slate-and-copper snow guards, copper valleys, and copper or stainless fasteners should be specified with the slate to ensure compatibility.

Storage: store slate on edge in a protected area, not flat, and keep it dry; stacked flat slate can warp or break and absorb moisture before installation.

Substitutes: synthetic slate, composite, and fiber-cement products mimic slate but have different weights, fastener requirements, and warranties; do not mix specifications across materials.

Exposure gauge: a story pole marked with the exposure is used to set each course; check exposure at the rake and at the ridge to keep courses level and parallel.

Nail position: nails should be placed so they are covered by the slate above (about 1 in above the butt line of the slate above), and never in the exposed area of the slate.

Two-nail vs four-nail: slate up to about 12 in wide uses two nails; wider slate or high-wind areas may require four nails, with the outer nails set about 1.5 in from the edges.

Clipping: copper or stainless clips are used at the top corners of slate in some specifications to prevent lifting; clips must not restrict slate movement.

Overhang at rakes and eaves: maintain consistent overhang (typically 1–1.5 in at rakes, 1–2 in at eaves) so water sheds clear of fascia and into gutters.

Valley weave (closed valley): alternate courses are woven across the valley; this requires careful cutting and is used where the design calls for a closed valley rather than open metal.

Ridge and hip: use saddle slate or metal ridge; fasten with copper or stainless nails and ensure each piece overlaps the one below by at least the headlap.

Penetrations: pipes, vents, and antennas require flashed penetrations with a lead or copper boot and a collar set into the slate; avoid cutting slate unnecessarily.

Final inspection: verify no broken slate, correct exposure, proper nail coverage, and that all flashing is lapped to shed water before signing off.

Sheathing types: nominal 1 in boards (up to 8 in wide) or 3/4 in plywood/OSB are common; board sheathing must be sound and nailed to rafters, and gaps should be minimized for underlayment support.

Underlayment layers: two layers of 30 lb felt are common under slate, with the second layer lapped over the first; high-temperature underlayment is used where heat buildup is a concern.

Self-adhering membrane: applied at eaves, valleys, and penetrations; it must be installed over a clean, dry deck and rolled to ensure adhesion.

Step flashing size: pieces typically 4x4 in or 5x5 in, bent to fit the wall/slate junction, with at least 2 in under the slate and 4 in up the wall, lapped 2 in to the next piece.

Counterflashing: set into a reglet at least 1 in deep, with a 1/4 in gap at the bottom to allow for movement; sealant alone is not a substitute for a reglet.

Chimney flashing components: apron (front), step flashing (sides), backer flashing and cricket (rear), counterflashing, and a cap; all must be compatible metals (copper, lead-coated copper, stainless).

Valley flashing width: open valleys typically 16–20 in wide, with the metal extending at least 6 in under the slate on each side and the center crimped or W-shaped to stiffen and direct water.

Flashing laps: all flashing must lap shingle-style (upper piece over lower piece) and be at least 2 in lapped; vertical laps should be at least 4 in.

Gutter sizing: steep slate roofs shed water fast; size gutters and downspouts for the roof area and pitch, and use hangers rated for snow and ice loads.

Inspection from the attic: look for daylight, water stains, corroded nails, and deteriorated underlayment; attic-side evidence often reveals problems not visible from the roof surface.

Slate ripper alternatives: a flat pry bar or a slate hook can be used in tight spots; always protect adjacent slate with a shim or pad.

Replacement slate sizing: match the length and width of surrounding slate as closely as possible; a slightly larger slate can be trimmed, but a smaller one will leave gaps.

Nail vs hook for repairs: use a nail through the head of the slate above where accessible; use a slate hook where the head is not accessible or the slate above is fragile.

Flashing replacement: when step flashing fails, remove the slate in the affected area, replace the flashing, and reinstall or replace the slate; do not attempt to seal from the surface.

Moss removal: use a soft brush and approved cleaner; avoid pressure washing and harsh chemicals; moss indicates moisture retention, so check for underlying flashing or ventilation issues.

Snow guard installation: anchor into the deck through the slate with proper flashing; snow guards should be spaced per manufacturer to distribute loads.

Historic roofs: retain original slate where possible, match color and size, and document repairs; replacement with synthetic materials may violate preservation standards.

Maintenance schedule: inspect annually and after major storms; keep gutters clear and repair broken slate promptly to prevent water infiltration.

Fall protection trigger: 6 ft for construction (OSHA 1926.501); for general industry, 4 ft (29 CFR 1910.28) - know which standard applies to the work.

Ladder angle and extension: 4:1 slope, 3 ft above the landing, and secured at the top; use a ladder standoff to protect gutters and slate.

Roof jack use: roofing brackets must be seated on the deck and not bear on slate; they must be spaced per manufacturer and rated for the load.

Silica dust: cutting slate produces respirable crystalline silica; use wet cutting or dust collection and respiratory protection per OSHA 1926.1153.

Code references: IRC Chapter 9 (Roof Assemblies), R905 (roof covering requirements), and local amendments; ASTM C406 for slate; ASCE 7 for loads.

Ice barrier: required in cold climates per IRC R905.2.7 (asphalt) and extended to slate per manufacturer and local code; typically from eave to 24 in inside the wall line.

Fire rating: slate is Class A noncombustible, but the assembly rating depends on deck and underlayment; verify the rated assembly when required.

Historic standards: the Secretary of the Interior's Standards for Rehabilitation apply to historic buildings; in-kind replacement and retention of original material are preferred.

Permits and inspections: re-roofing and structural changes require permits; final inspection should verify exposure, fasteners, flashing, and underlayment.

Slate weight per square: 3/16 in ≈ 800 lb; 1/4 in ≈ 1,000 lb; 3/8 in ≈ 1,500 lb; 1/2 in ≈ 2,000 lb - structural design must account for this dead load plus snow load.

Exposure calculation: exposure = slate length − headlap; a 12 in slate with 3 in headlap has 9 in exposure, and one square covers 100 sq ft of roof area, not 100 sq ft of slate.

Ordering factor: because of headlap and side lap, order 1.10–1.30 times the roof area in slate, plus waste for cuts and breakage.

Headlap minimums: 3 in for slate up to 3/8 in thick; 4 in for 1/2 in and thicker; never reduce headlap to increase exposure.

Side lap: minimum 3 in; joints in adjacent courses must be offset at least 3 in so no four corners meet.

Fasteners: copper or stainless steel nails or hooks; galvanized and electroplated copper are avoided because corrosion products stain slate.

Slate grades (ASTM C406): S1 (uniform, no harmful minerals), S2 (some variation), S3 (limited use); types classify weathering behavior.

Color sources: carbon (black/gray), hematite (red/purple), chlorite (green); pyrite weathering causes rust stains and is a durability concern.

Storage and handling: store slate on edge, keep dry, and handle carefully to avoid chipping; chipped slate should not be installed in exposed areas.

Course layout: establish exposure with a story pole, snap chalk lines, and lay from eave to ridge; keep courses level and parallel to the ridge.

Starter course: double layer at the eave; the lower layer is set in mortar or on a cant strip to shed water off the fascia.

Nail placement: two nails per slate, 1–1.5 in from each side edge and about 1 in above the butt line of the slate above, so nails are covered.

Snug, not tight: drive nails so the slate can hang and move; tight-hung slate cracks with thermal movement.

Side lap and offset: minimum 3 in side lap; offset joints at least 3 in between courses to avoid cross joints.

Wide slate: slate wider than about 12–14 in or in high-wind zones may require four nails or clips per manufacturer.

Hip and ridge: lay hip slate from the bottom up, alternating sides, each overlapping the one below; use saddle slate or metal ridge.

Valleys: open metal valleys (16–20 in wide) with slate cut back 3–4 in from the centerline; closed valleys are woven and require careful layout.

Ventilation: balance intake and exhaust (e.g., 1 sq ft net free vent area per 150 sq ft of attic floor with vapor barrier) to prevent condensation and ice dams.

Deck requirements: 3/4 in nominal solid sheathing (boards up to 8 in wide or plywood/OSB); deck must be flat and sound, with gaps for expansion.

Underlayment: 30 lb felt (two layers) or high-temperature underlayment; lap side 6 in and end 4 in, shingle-style, and fasten with corrosion-resistant cap nails.

Ice dam membrane: self-adhering membrane at eaves, valleys, and penetrations in cold climates; extend from the eave to at least 24 in inside the interior wall line per code.

Step flashing: individual pieces interwoven with each slate course along sidewalls; at least 2 in under the slate and 4 in up the wall, lapped 2 in.

Counterflashing: set into a reglet at least 1 in deep with a 1/4 in gap at the bottom for movement; sealant alone is not a substitute.

Chimney cricket: required for chimneys wider than 30 in (some codes 24 in); slope at least 1/4 in per foot and flash into the chimney with base and counterflashing.

Valley metal: copper or lead-coated copper, 16–20 in wide, with a W-shaped center; extend at least 6 in under the slate on each side.

Drip edge: metal drip edge at eaves and rakes; install over the underlayment at the eave and under it at the rake (or per manufacturer).

Compatibility: use copper, lead-coated copper, lead, or stainless flashing; avoid bare galvanized steel and aluminum near slate because of staining and galvanic interaction.

Condition survey: map broken/slipped slate by area, check flashing, and assess deck and underlayment from the attic before planning repairs.

Slate ripper: a long flat bar with a hook used to cut or pull the nails of a broken slate so it can be removed without damaging adjacent slate.

Replacement: slide the new slate up into place from below the slate above, then secure with a nail or slate hook; match exposure and size to surrounding courses.

Slate hooks: copper or stainless hooks sized to slate thickness, driven into the deck above the repair and bent over the slate head where nailing is not possible.

Matching slate: match thickness, size, color, and grade; salvaged slate from the same roof is preferred, and new slate should be sorted to blend.

Flashing first: failed flashing causes more water damage than broken slate; inspect step flashing, valleys, and chimney counterflashing first.

Moss and lichen: remove with a soft brush and approved cleaner; avoid pressure washing and harsh chemicals that erode slate and force water under it.

Snow retention: add snow guards or snow fences where snow slides are a hazard; anchor into the deck, not just the slate.

Replacement threshold: widespread broken slate, failed underlayment, sagging deck, or more than ~20–25% damage usually justifies full replacement rather than spot repair.

OSHA fall protection: 6 ft trigger for construction (1926.501); guardrail, safety net, or personal fall arrest system required; anchors must be rated (commonly 5,000 lb).

Ladder safety: 4:1 angle, 3 ft above the landing, secured; use standoffs to protect gutters and slate.

Roof jacks: required for steep-roof work; must be seated on the deck, not on slate, and spaced per manufacturer.

PPE: fall harness and lanyard, hard hat, safety glasses, cut-resistant gloves, and dust protection when cutting slate.

Silica: cutting slate produces respirable crystalline silica; use wet cutting or dust collection and respiratory protection per OSHA 1926.1153.

Code references: IRC Chapter 9 and R905, ASTM C406, ASCE 7, and local amendments; always check the locally adopted edition.

Ice barrier: required in cold climates; extend from the eave to at least 24 in inside the interior wall line per code and manufacturer instructions.

Fire rating: slate is Class A noncombustible, but the assembly rating depends on deck and underlayment; verify the rated assembly.

Historic preservation: the Secretary of the Interior's Standards apply; in-kind replacement and retention of original material are preferred over substitution.

Slate is metamorphic rock from shale; fissility (splitting into thin sheets) is the key property for roofing slate.

Thickness grades: 3/16 in (lightest common), 1/4 in (standard), 3/8 in, and 1/2 in (heaviest); thicker slate requires more headlap and adds dead load.

Standard sizes: roughly 6x8 in to 14x24 in; larger slate requires thicker material and greater headlap.

ASTM C406 grades: S1 (uniform, no harmful minerals), S2 (some variation), S3 (limited use); types classify weathering behavior.

Durability: hard, low-absorption slate can last 75–200 years; soft, high-absorption slate fails faster and may stain.

Color: carbon (black/gray), hematite (red/purple), chlorite (green); pyrite weathering causes rust stains.

Weight per square: ~800 lb (3/16 in), ~1,000 lb (1/4 in), ~1,500 lb (3/8 in), ~2,000 lb (1/2 in).

Ordering: 1.10–1.30 times roof area plus 10–20% waste for cuts and breakage; keep matching stock for repairs.

Fasteners: copper or stainless steel nails/hooks; galvanized and electroplated copper stain slate and shorten service life.

Exposure = slate length − headlap; 3 in headlap for slate up to 3/8 in, 4 in for 1/2 in and thicker.

Nail placement: two nails per slate, 1–1.5 in from side edges and about 1 in above the butt line of the slate above.

Snug, not tight: slate must hang on the nail and move with thermal expansion; tight-hung slate cracks.

Side lap: minimum 3 in; offset joints at least 3 in between courses to avoid cross joints.

Starter course: double layer at the eave, set in mortar or on a cant strip to shed water off the fascia.

Rake and eave overhang: 1–1.5 in at rakes, 1–2 in at eaves, so water clears fascia and enters gutters.

Hip and ridge: lay from the bottom up, alternating sides, each piece overlapping the one below; use saddle slate or metal ridge.

Valleys: open metal valleys 16–20 in wide with slate cut back 3–4 in from the centerline; closed valleys are woven.

Ventilation: balance intake and exhaust (e.g., 1 sq ft net free vent area per 150 sq ft of attic floor with vapor barrier).

Deck: 3/4 in nominal solid sheathing (boards up to 8 in wide or plywood/OSB); must be flat, sound, and gapped for expansion.

Underlayment: 30 lb felt (two layers) or high-temperature underlayment; side lap 6 in, end lap 4 in, shingle-style.

Fastening underlayment: corrosion-resistant cap nails; staples generally not acceptable for high-temperature underlayment.

Ice dam membrane: self-adhering membrane at eaves, valleys, and penetrations; extend to at least 24 in inside the interior wall line per code.

Step flashing: 4x4 or 5x5 in pieces, at least 2 in under the slate and 4 in up the wall, lapped 2 in to the next piece.

Counterflashing: reglet at least 1 in deep with a 1/4 in gap at the bottom for movement; sealant alone is not a substitute.

Chimney cricket: required for chimneys wider than 30 in (some codes 24 in); slope at least 1/4 in per foot.

Valley metal: copper or lead-coated copper, 16–20 in wide, W-shaped center, extending at least 6 in under the slate on each side.

Compatibility: copper, lead-coated copper, lead, and stainless are standard; avoid bare galvanized and aluminum near slate.

Condition survey: map broken/slipped slate, check flashing, and assess deck and underlayment from the attic before planning repairs.

Slate ripper: a long flat bar with a hook used to cut or pull the nails of a broken slate without damaging adjacent slate.

Replacement: slide the new slate up into place from below the slate above, then secure with a nail or slate hook; match exposure and size.

Slate hooks: copper or stainless hooks sized to slate thickness, driven into the deck above the repair and bent over the slate head.

Matching slate: match thickness, size, color, and grade; salvaged slate from the same roof is preferred and should be sorted to blend.

Flashing first: failed flashing causes more water damage than broken slate; inspect step flashing, valleys, and chimney counterflashing first.

Moss and lichen: remove with a soft brush and approved cleaner; avoid pressure washing and harsh chemicals.

Snow retention: add snow guards or snow fences where snow slides are a hazard; anchor into the deck, not just the slate.

Replacement threshold: widespread broken slate, failed underlayment, sagging deck, or more than ~20–25% damage usually justifies full replacement.

OSHA fall protection: 6 ft trigger for construction (1926.501); guardrail, safety net, or personal fall arrest system required; anchors rated (commonly 5,000 lb).

Ladder safety: 4:1 angle, 3 ft above the landing, secured; use standoffs to protect gutters and slate.

Roof jacks: required for steep-roof work; seated on the deck, not on slate, and spaced per manufacturer.

PPE: fall harness and lanyard, hard hat, safety glasses, cut-resistant gloves, and dust protection when cutting slate.

Silica: cutting slate produces respirable crystalline silica; use wet cutting or dust collection and respiratory protection per OSHA 1926.1153.

Code references: IRC Chapter 9 and R905, ASTM C406, ASCE 7, and local amendments; check the locally adopted edition.

Ice barrier: required in cold climates; extend from the eave to at least 24 in inside the interior wall line per code and manufacturer instructions.

Fire rating: slate is Class A noncombustible, but the assembly rating depends on deck and underlayment; verify the rated assembly.

Historic preservation: the Secretary of the Interior's Standards apply; in-kind replacement and retention of original material are preferred.

Slate is metamorphic rock from shale; fissility (splitting into thin sheets) is the key property for roofing slate.

Thickness grades: 3/16 in, 1/4 in, 3/8 in, and 1/2 in; thicker slate requires more headlap and adds dead load.

Standard sizes: roughly 6x8 in to 14x24 in; larger slate requires thicker material and greater headlap.

ASTM C406 grades: S1 (uniform, no harmful minerals), S2 (some variation), S3 (limited use); types classify weathering behavior.

Durability: hard, low-absorption slate can last 75–200 years; soft, high-absorption slate fails faster and may stain.

Color: carbon (black/gray), hematite (red/purple), chlorite (green); pyrite weathering causes rust stains.

Weight per square: ~800 lb (3/16 in), ~1,000 lb (1/4 in), ~1,500 lb (3/8 in), ~2,000 lb (1/2 in).

Ordering: 1.10–1.30 times roof area plus 10–20% waste; keep matching stock for repairs.

Fasteners: copper or stainless steel nails/hooks; galvanized and electroplated copper stain slate and shorten service life.

Exposure = slate length − headlap; 3 in headlap for slate up to 3/8 in, 4 in for 1/2 in and thicker.

Nail placement: two nails per slate, 1–1.5 in from side edges and about 1 in above the butt line of the slate above.

Snug, not tight: slate must hang on the nail and move with thermal expansion; tight-hung slate cracks.

Side lap: minimum 3 in; offset joints at least 3 in between courses to avoid cross joints.

Starter course: double layer at the eave, set in mortar or on a cant strip to shed water off the fascia.

Rake and eave overhang: 1–1.5 in at rakes, 1–2 in at eaves, so water clears fascia and enters gutters.

Hip and ridge: lay from the bottom up, alternating sides, each piece overlapping the one below; use saddle slate or metal ridge.

Valleys: open metal valleys 16–20 in wide with slate cut back 3–4 in from the centerline; closed valleys are woven.

Ventilation: balance intake and exhaust (e.g., 1 sq ft net free vent area per 150 sq ft of attic floor with vapor barrier).

Deck: 3/4 in nominal solid sheathing (boards up to 8 in wide or plywood/OSB); must be flat, sound, and gapped for expansion.

Underlayment: 30 lb felt (two layers) or high-temperature underlayment; side lap 6 in, end lap 4 in, shingle-style.

Fastening underlayment: corrosion-resistant cap nails; staples generally not acceptable for high-temperature underlayment.

Ice dam membrane: self-adhering membrane at eaves, valleys, and penetrations; extend to at least 24 in inside the interior wall line per code.

Step flashing: 4x4 or 5x5 in pieces, at least 2 in under the slate and 4 in up the wall, lapped 2 in to the next piece.

Counterflashing: reglet at least 1 in deep with a 1/4 in gap at the bottom for movement; sealant alone is not a substitute.

Chimney cricket: required for chimneys wider than 30 in (some codes 24 in); slope at least 1/4 in per foot.

Valley metal: copper or lead-coated copper, 16–20 in wide, W-shaped center, extending at least 6 in under the slate on each side.

Compatibility: copper, lead-coated copper, lead, and stainless are standard; avoid bare galvanized and aluminum near slate.

Condition survey: map broken/slipped slate, check flashing, and assess deck and underlayment from the attic before planning repairs.

Slate ripper: a long flat bar with a hook used to cut or pull the nails of a broken slate without damaging adjacent slate.

Replacement: slide the new slate up into place from below the slate above, then secure with a nail or slate hook; match exposure and size.

Slate hooks: copper or stainless hooks sized to slate thickness, driven into the deck above the repair and bent over the slate head.

Matching slate: match thickness, size, color, and grade; salvaged slate from the same roof is preferred and should be sorted to blend.

Flashing first: failed flashing causes more water damage than broken slate; inspect step flashing, valleys, and chimney counterflashing first.

Moss and lichen: remove with a soft brush and approved cleaner; avoid pressure washing and harsh chemicals.

Snow retention: add snow guards or snow fences where snow slides are a hazard; anchor into the deck, not just the slate.

Replacement threshold: widespread broken slate, failed underlayment, sagging deck, or more than ~20–25% damage usually justifies full replacement.

OSHA fall protection: 6 ft trigger for construction (1926.501); guardrail, safety net, or personal fall arrest system required; anchors rated (commonly 5,000 lb).

Ladder safety: 4:1 angle, 3 ft above the landing, secured; use standoffs to protect gutters and slate.

Roof jacks: required for steep-roof work; seated on the deck, not on slate, and spaced per manufacturer.

PPE: fall harness and lanyard, hard hat, safety glasses, cut-resistant gloves, and dust protection when cutting slate.

Silica: cutting slate produces respirable crystalline silica; use wet cutting or dust collection and respiratory protection per OSHA 1926.1153.

Code references: IRC Chapter 9 and R905, ASTM C406, ASCE 7, and local amendments; check the locally adopted edition.

Ice barrier: required in cold climates; extend from the eave to at least 24 in inside the interior wall line per code and manufacturer instructions.

Fire rating: slate is Class A noncombustible, but the assembly rating depends on deck and underlayment; verify the rated assembly.

Historic preservation: the Secretary of the Interior's Standards apply; in-kind replacement and retention of original material are preferred.

Slate is metamorphic rock from shale; fissility (splitting into thin sheets) is the key property for roofing slate.

Thickness grades: 3/16 in, 1/4 in, 3/8 in, and 1/2 in; thicker slate requires more headlap and adds dead load.

Standard sizes: roughly 6x8 in to 14x24 in; larger slate requires thicker material and greater headlap.

ASTM C406 grades: S1 (uniform, no harmful minerals), S2 (some variation), S3 (limited use); types classify weathering behavior.

Durability: hard, low-absorption slate can last 75–200 years; soft, high-absorption slate fails faster and may stain.

Color: carbon (black/gray), hematite (red/purple), chlorite (green); pyrite weathering causes rust stains.

Weight per square: ~800 lb (3/16 in), ~1,000 lb (1/4 in), ~1,500 lb (3/8 in), ~2,000 lb (1/2 in).

Ordering: 1.10–1.30 times roof area plus 10–20% waste; keep matching stock for repairs.

Fasteners: copper or stainless steel nails/hooks; galvanized and electroplated copper stain slate and shorten service life.

Exposure = slate length − headlap; 3 in headlap for slate up to 3/8 in, 4 in for 1/2 in and thicker.

Nail placement: two nails per slate, 1–1.5 in from side edges and about 1 in above the butt line of the slate above.

Snug, not tight: slate must hang on the nail and move with thermal expansion; tight-hung slate cracks.

Side lap: minimum 3 in; offset joints at least 3 in between courses to avoid cross joints.

Starter course: double layer at the eave, set in mortar or on a cant strip to shed water off the fascia.

Rake and eave overhang: 1–1.5 in at rakes, 1–2 in at eaves, so water clears fascia and enters gutters.

Hip and ridge: lay from the bottom up, alternating sides, each piece overlapping the one below; use saddle slate or metal ridge.

Valleys: open metal valleys 16–20 in wide with slate cut back 3–4 in from the centerline; closed valleys are woven.

Ventilation: balance intake and exhaust (e.g., 1 sq ft net free vent area per 150 sq ft of attic floor with vapor barrier).