NRCA Slate common mistakes
NRCA Slate · Understand the traps and how to avoid them.
Treating all slate as interchangeable and specifying or installing a single generic slate type regardless of the roof's slope, exposure, and expected service life, without checking grading, absorption, or ASTM classification.
Candidates memorize 'slate is a natural stone' and skip the material-selection variables: commercial vs. clear grading, ASTM C406 service-life classification, water absorption, and modulus of rupture. They assume any slate will perform on any roof.
What to do instead
Study slate selection as a decision matrix: match slate grade and thickness to slope, climate freeze-thaw cycling, and required service life; know that ASTM C406 sorts slate by expected durability and that absorption and strength testing underpin that ranking. Practice choosing slate for a low-slope porch versus a steep main roof and justify the choice in writing.
Confusing the several slate-related ASTM standards (C406 roofing slate, C120 test methods for slate, C629 slate for structural purposes) and citing the wrong one for a given property.
The numbers look alike and candidates memorize them shallowly rather than learning what each standard actually governs.
What to do instead
Build a one-page reference table mapping each standard to its scope (material classification, physical test methods, structural use) and self-test by writing the standard number from the scope description, not the reverse.
Installing slate with the exposed headlap too small or relying on the slate's butt edge to set course exposure, producing leaks and wind-driven rain infiltration.
Candidates confuse headlap (overlap between courses) with sidelap and with exposure, and they assume the visible exposure is the controlling dimension. In reality headlap at the top of the slate is what sheds water, and it must be maintained at the minimum for the roof slope.
What to do instead
Learn the geometry explicitly: exposure equals half the difference between slate length and required headlap. Practice computing exposure for several slate lengths and slopes, and remember that steeper roofs can use less headlap while low slopes need more. Check headlap at the top of each course, not the butt.
Fastening slate with nails driven too high (above the nail hole centerline or through the headlap of the slate above) or overdriven flush, so slates hang loose, crack, or cannot be replaced individually.
Candidates focus on getting the slate on the roof quickly and treat nailing as a minor step. They also don't realize that in a proper installation each slate is hung on two nails and the next course covers the nail heads, which is what makes individual slate replacement possible later.
What to do instead
Memorize the fastening rule: two nails per slate, driven at the nail hole centerline or slightly above it, with the nail head left just proud of the slate surface so the slate hangs rather than being pinned. Practice on a mock-up and inspect from below to confirm no overdriving and that each slate is independently removable.
Omitting or under-specifying underlayment, or using a single layer of building paper where the slope, ice-dam exposure, or slate weight demands a heavier or double-layer system.
Candidates think the slate itself is the waterproofing and treat underlayment as optional. They also don't connect underlayment selection to slope, eave ice damming, and the fact that slate is not a watertight membrane.
What to do instead
Study underlayment as a slope-driven requirement: steeper roofs permit lighter underlayment, lower slopes require heavier or double layers, and eaves in cold climates need ice-dam protection. Learn the accepted underlayment types for slate (asphalt-saturated felt in appropriate weights, self-adhering membranes at eaves) and be able to specify a system by slope.
Detailing flashing generically-using the same step flashing, valley, or chimney treatment as for asphalt shingles-without accounting for slate thickness, the need for concealed flashing under each course, and open or closed valley geometry appropriate to slate.
Candidates transfer shingle knowledge to slate. Slate is thicker and installed in discrete courses, so flashing must be woven or stepped with the slate, and valleys must be built to shed water over a long service life.
What to do instead
Learn slate-specific flashing details: step flashing integrated course by course, chimney and wall flashings with proper counterflashing and reglet, and valley construction (open metal valley or woven) matched to the roof design. Draw each detail from memory and label the lap dimensions and material gauges.
During repair, replacing a broken slate with a mismatched size, thickness, color, or grade, or using a slate that is too thick or thin for the surrounding courses, so the repair reads as a patch and may fail early.
Candidates grab whatever slate is on the truck and don't inventory the existing roof's slate dimensions, colors, and weathering. They also skip checking that the replacement slate's headlap and exposure match the surrounding courses.
What to do instead
Make a repair protocol: measure and photograph existing slates, match length, width, thickness, color, and grade, and verify the replacement sits at the same exposure. Keep a salvage pile of removed slates for matching. Practice the slate hook or nail-and-hook replacement method so the new slate locks into the course above.
Diagnosing a leak by replacing slates at the visible stain rather than tracing water back up-slope to its actual entry point, and failing to check flashings, valleys, and penetrations first.
Candidates treat the water stain as the leak location. On slate roofs water travels along the underlayment and rafters, so the entry point is often well above and lateral to the stain.
What to do instead
Adopt a systematic leak-investigation sequence: start at penetrations and flashings, then valleys, then field slates, working up-slope from the stain. Use hose testing in stages and inspect from the attic where possible. Document the actual entry point before removing any slate.
Working on a slate roof without a written fall-protection plan, relying on roof brackets or a rope alone, or walking the slate field in a way that cracks slates and creates a fall hazard.
Candidates underestimate how slippery and fragile slate is and assume 'roofing is roofing.' They also don't plan anchor points, staging, or debris control before starting.
What to do instead
Learn the hierarchy: guardrails and personal fall-arrest systems with engineered anchors, roof jacks and staging for work areas, and a plan for material handling. Walk on the slate over the laps where it is supported, use roof ladders or hooks, and inspect for cracked slates after work. Know the applicable OSHA fall-protection thresholds for roofing work.
Citing code requirements from memory without distinguishing which code or standard governs-confusing model codes, ASTM material standards, and manufacturer instructions-and failing to verify local amendments.
Candidates memorize a few numbers and assume they apply everywhere, ignoring that jurisdictions adopt different code editions and amendments, and that manufacturer instructions can be more stringent than code.
What to do instead
Build a code map: which model code governs roofing, which ASTM standards define slate material, and where local amendments commonly differ. Practice answering 'what governs' questions by naming the hierarchy (code, then standard, then manufacturer instructions, then project specifications) and always note that the local authority having jurisdiction controls.