What goes under shingles — the asphalt roof assembly from the deck to the ridge cap
What’s under the shingles, eave to ridge: roof deck, drip edge, an ice-and-water shield that must reach at least 24″ inside the warm wall line, underlayment lapped over it, a starter course, the overlapping field shingles, and the ridge cap. Each course covers the nails of the one below.
What this diagram shows
Cross-section up a roof slope showing the layers of an asphalt shingle roof from the deck to the ridge. From the bottom up: the wood roof deck (sheathing), a metal drip edge at the eave, an ice-and-water shield membrane that runs from the eave to at least 24 inches inside the exterior wall line to stop ice-dam leaks, synthetic underlayment lapped over the top of the ice barrier, a starter course along the eave that seals the first row, the field shingles installed in overlapping courses so each course covers the nail line of the one below, and a ridge cap that closes the peak. The ice barrier extending past the wall line is dimensioned because the IRC requires it in cold climates.
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