Roof Deck, Underlayment, and Substrate Preparation
Deck condition and thickness
Tile is one of the heaviest roof coverings, so the sheathing must be sound, flat, and thick enough for the span. Code sets a minimum wood structural panel thickness for tile, and many manufacturers plus NRCA call for 3/4 in. nominal (23/32 in.) panels; always check the specific code edition and the tile manufacturer's span table. Replace delaminated, rotted, or thin panels, refasten loose ones, and limit live-load deflection (often L/360) so brittle tile does not crack. Correct framing before the underlayment goes down; tile will not hide a wavy deck.
Underlayment by slope
- 4:12 and steeper: one layer of No. 30 felt or an approved equivalent.
- 2.5:12 up to 4:12: two layers, lapped in shingle fashion.
- Ice-dam membrane from the eave to at least 24 in. inside the exterior wall line where ice damming is a history.
- Side laps at least 2 in., end laps 4 to 6 in., upper layer over lower layer so water always sheds.
- Fasten with cap nails or plastic-capped fasteners; keep the laps aligned over framing where possible.
Metal drip edge typically goes on first at the eave with underlayment lapping over it, while at rakes the drip edge laps over the underlayment. Ventilation openings and any battens or counter-battens are set out before tile laying begins.
Worked example. A 30 square roof at 3:12 needs two layers, so 60 squares of felt. One roll of No. 30 felt covers about 432 sq ft, or roughly 4.3 squares, so 60 divided by 4.3 equals 14 rolls minimum; order 15 or 16 for laps and damage.
Traps. One layer on a 3:12 roof; upper layer tucked under the lower; skipping the ice barrier at cold-climate eaves; nailing through felt into thin air; covering a wet or dirty deck.