Underlayment Installation

Underlayment (called underlay in the UK, Ireland, Australia and New Zealand) is a sheet laid over the deck or rafters beneath the main roof covering. It sheds water that gets past the covering, protects the deck during construction and, in some systems, adds wind resistance. Traditional products are asphalt-saturated felt; modern options include synthetic sheets, breathable membranes and self-adhered polymer-modified bitumen.

General method

1. Start at the eaves and work upward, so each upper course laps over the one below.
2. Lap horizontal edges (headlaps) and vertical joints (endlaps or sidelaps) by the amount the manufacturer and code require. Canadian tile guidance, for example, gives 100 mm (4 in) headlaps and 150 mm (6 in) endlaps for underlayment beneath tile.
3. Fasten only as much as needed to hold the sheet flat, with the fasteners the manufacturer specifies.
4. Line valleys with a separate centred strip before the field underlayment, and trim field courses to overlap it.
5. Turn underlayment up walls and around penetrations so it integrates with step and apron flashings.

Low slopes

Under the IRC, asphalt shingles are limited to slopes of 2:12 (about 9.5 degrees) or more, and slopes between 2:12 and 4:12 require two layers of underlayment or another approved method. Building America guidance repeats this requirement.

High-wind practice

US guidance based on the IBHS FORTIFIED programme gives three ways to seal the deck: a self-adhered membrane over the whole deck, tape over all deck seams covered with underlayment, or two layers of No. 30 felt.

Drip edge

Metal drip edge at eaves and rakes keeps water off the fascia and supports the shingle edge. The order in which drip edge and underlayment are installed varies with the code, the manufacturer and programmes such as IBHS, so installers follow the governing instructions. Some local codes require drip edge only when the shingle manufacturer does.

Tiled and slated roofs in Europe

On batten-hung roofs, underlay is draped over the rafters and held by counter-battens and battens. In the UK, BS 5534 requires underlays to be classified for wind uplift resistance, and its 2018 amendment set a maximum drape of 15 mm between rafters.