Roof Deck, Underlayment and Flashing
Domain 3 - Roof Deck, Underlayment and Flashing
This domain tests what holds the slate up and what keeps water out before any slate is laid.
Deck
Slate needs a solid, stiff deck. Traditional practice is nominal 1 in. boards - tongue-and-groove or shiplap, boards no wider than 8 in. - laid with fitted joints. Sheathing panel decks work if thick enough for the load. Gaps allow movement but must not be so wide that slate is unsupported, and the deck must not deflect under slate weight and foot traffic.
Underlayment
Minimum 30-lb asphalt-saturated felt (ASTM D226 Type II) or an equivalent synthetic; self-adhering membrane at eaves where ice barriers are required (IRC R905.1.2). Model-code tables (IRC Table R905.1.1(2)) set the number of layers by slope; slate in the 4:12 to 6:12 range generally gets two layers, steeper roofs one. Lap shingle-fashion so water always sheds over a lap, with side laps staggered and laps increasing as slope decreases.
Flashing
Match flashing life to slate life: 16- or 20-oz copper (ASTM B370), lead-coated copper, terne or stainless. Never let copper touch aluminum or bare steel - galvanic corrosion and run-off staining follow. Step flashing at sidewalls rises at least 3-4 in. above the roof, one piece per course. Counterflashing seats in a reglet at least 1 in. deep and overlaps the base flashing at least 3 in. Valley metal gets at least 12 in. each side of the centerline.
Chimneys
Chimneys 30 in. wide or more need a cricket upstream. Base flashing laps the slate at least one course; counterflashing covers the top of the base flange.
Worked example
A sidewall runs up a 7:12 roof. Step-flashing pieces are bent 4 in. high and 4 in. wide, one per slate course, each lapped 2 in. over the piece below. The bend must finish at least 3 in. above the finished slate so the top of the flashing stays above the waterline of the two courses above it.
Traps
Fastening through valley metal; aluminum against copper; underlayment lapped backwards; skipping the ice barrier; smearing mastic where flashing height is short.