Roof Deck, Underlayment, Flashing, and Waterproofing
Deck and underlayment
Sheathing must be adequate for the span: minimum 5/8-inch lumber or wood structural panels sized from the code tables for the rafter spacing, gapped about 1/8 inch at panel edges, with clips where required. Fastening for 1/2- to 1-inch panels on 24-inch supports is typically 8d common at 6 inches on panel edges and 12 inches in the field. Over-driven or missed fasteners are the most common sheathing rejection.
Underlayment: one layer of No. 15 asphalt felt or listed equivalent at 4:12 and steeper; two layers from 2:12 up to 4:12 under shingles. Ice-and-water membrane runs from the eave to at least 24 inches inside the interior wall line, and in valleys where the code or manufacturer requires it.
Flashing is the real waterproofing
- Step flashing at sidewalls, one piece per shingle course, lapped - never replaced by a bent apron or caulk.
- Base flashing at least 4 inches up the wall and 4 inches onto the roof, with counterflashing cut into masonry or siding.
- Apron flashing at headwalls; valley flashing at least 24 inches wide; drip edge at eaves and rakes.
- Layering rule: at the eave, underlayment laps over the drip edge; at the rake, the drip edge laps over the underlayment.
- A cricket is required behind chimneys wider than 30 inches.
- Kick-out flashing wherever a roof edge dies into a wall - the leading cause of hidden rot on Massachusetts re-roofs.
Worked example
A dormer sidewall meeting a lower roof, shingles woven without step flashing and sealed with caulk. Correct repair: cut back the shingles, install individual step flashings interleaved with each course, and terminate with a kick-out at the bottom of the run.
Traps
Re-shingling over corroded step flashing, using ice barrier as a substitute for valley flashing, and reversing the eave/rake layering rule.