Roof Underlayment

Roof underlayment is the sheet material rolled or stuck onto the roof deck before the tile, shingle, or panel goes on. The 2021 International Residential Code requires it in Section R905.1.1 and names four standards: ASTM D226, D1970, D4869, and D6757. For clay and concrete tile, Section R905.3.3 sends you to that same list.

Here is the part buyers get wrong: concrete tile is the wear surface and sunshade, not the waterproofing. As InterNACHI puts it, most roof coverings "are not waterproof, but water-resistant," and are meant to sit over a membrane. Wind-driven rain blows under and between tiles, and the underlayment carries it to the eave. PNNL's Building America Solution Center lists the common choices under tile: a self-adhered peel-and-stick membrane over the deck, or two layers of No. 30 felt.

Heat shortens its life. Hot, dry climates accelerate the loss of volatiles from bituminous underlayment, so a Phoenix roof bakes the felt brittle while the tile above is still sound. That mismatch produces a repair Arizona owners know: a lift and relay. A roofer pulls and stacks the tile, strips the failed underlayment, lays new, and resets the same tile. You pay for labor and membrane, not a new roof.

Slope changes the spec. Under Section R905.3.2, tile needs a slope of at least 2 1/2 in 12, and from there up to 4 in 12 the code requires double underlayment. Water moves slowly on a shallow pitch and backs up under the tile, so those planes get two layers. For example, a low-pitch entry porch tied into a steeper main roof gets a different build.

Underlayment is the roof's version of the water-resistive barrier in a wall, the hidden layer doing the water control while the visible material takes the weather. When you compare roofing bids, the tile line tells you little. Read the underlayment line, and ask how many layers go down.