Once limited to building paper, underlayment now includes self-adhering rubberized sheets that replace metal flashing in some cases. Thus you can base your choice of underlayment on climate, composition, and position on the roof.
Weather-resistant underlayment. Traditionally, underlayment has been 36-in.-wide, 15-lb. or 30-lb. felt paper used as a weather-resistant layer with several purposes. It keeps sheathing dry till shingles are installed, serves as a backup layer when water gets under shingles or flashing, and separates sheathing and shingles and so prolongs shingle life. (Without underlayment, shingle asphalt can leech into wood sheathing, or resins in sheathing can degrade the shingle.) Heavy — duty, 30-lb. felt paper is often specified in high — wear, high-water areas such as eaves and valleys. A standard roll of 3-ft.-wide, 15-lb. felt paper covers roughly 400 sq. ft.; the same-size roll of 3-ft.-wide, 30-lb. paper covers only 200 sq. ft. because it’s roughly twice as thick.
Because unreinforced lighter grades of building paper (15 lb.) tear easily and wrinkle when wet, some types are now reinforced with fiberglass. Moreover, all asphalt-impregnated building papers dry out and become less water resistant when exposed to sunlight, so the sooner they’re covered by shingles, the better. Building paper was never intended to be an exterior membrane.
When installing building paper on a sloped roof, have a helper and work from the bottom up. As you roll the paper out, it will tend to slide down the roof, so be sure to unroll it straight across the roof. The first course of paper should overlap a metal drip-edge nailed along the eaves. Align the paper’s lower edge to the lower metal edge and unroll the paper, stapling as you go. Staples are only a temporary attacher, to help you keep the paper from bunching or sliding. Then nail the underlayment, using the tabbed roofing nails (also called Simplex® nails) shown on p. 57. Along roof edges and where the ends of the building paper overlap, space nails every 6 in., insetting them 1 in. from the edge. Elsewhere, drive nails in a zigzag pattern, spacing them 12 in. to 15 in. apart.
For steep-slope roofs (4-in-12 or steeper), overlap horizontal courses of building paper 2 in.
Overlap ends of seams (end laps) at least 6 in. To prevent water backup on low-slope roofs (less than 4-in-12), building codes often specify two plies of 36-in.-wide underlayment, with horizontal seams overlapped at least 19 in. and (vertical) end seams overlapped 12 in. Check your local code to be sure, for it may also specify self — sticking waterproof shingle underlayment along the eaves.
If you’re installing wood shakes or slate, use shake liner: 18-in.-wide rolls of 30-lb. building paper alternated between roofing courses.
Waterproof shingle underlayment (WSU).
WSU is a heavy peel-and-stick bituminous membrane that protects roof areas most likely to leak because of concentrated water flows in valleys, ice dams at eaves, or high winds at eaves and rake edges. Many building codes also specify WSU in lieu of building paper where asphalt shingles are installed on low-slope roofs. In addition to being self-adhering, WSU also self-seals around nails, making it a truly waterproof membrane.
Installing WSU along valleys is vastly easier and superior to lining them with unwieldy roll roofing or aluminum flashing. And with WSU,
there’s no need to trowel on 3-ft.-wide swaths of roofing cement between 30-lb. building-paper layers to protect eaves from ice-dam damage. Snap a chalkline to position the WSU. Then unroll and cut the membrane to length, align it to the chalkline, peel off its release-sheet backing, and press the material down. Most manufacturers recommend rolling it once it’s down.
WSU is more easily installed with two people, but if you’re working alone, fold the WSU in half, lengthwise, and temporarily staple one edge of the membrane to a chalked guideline. Peel off the release-backing from the unstapled half of the WSU, and flop the adhesive side of the membrane over onto the sheathing. Finally, yank free the stapled edge, peel off the backing from the second half, and stick it to the sheathing.
Because WSU is self-adhering, you needn’t nail it; you need to use staples only to keep the sheet from sliding around before sticking it down.
WSU comes in 9-in. to 36-in. widths. Brand names suggest usage, for example, CertainTeed Winterguard™, W. R. Grace Ice & Water Shield™, and GAF Weatherwatch™.
