How to Apply Underlayment to a Roof
Installing roofing felt, or "tar paper," is an important step when roofing a building. Here's how to do it.
Before applying roofing, you'll need to cover the sheathing with roofing felt, also often called "tar paper." Most local codes call for using 30-pound felt. Some roofers prefer to attach felt underlayment with 1-inch roofing nails or special nails with plastic washers, but most codes allow staples, which are easier to drive. For the lower portion of the roof—especially the part that overhangs the eaves and is susceptible to ice dams—it is a good idea to apply self-stick waterproof shingle underlayment (WSU), also called ice guard.
Underlayment, flashings, and shingles all work together and must be installed in the correct order. If you lay the felt perfectly straight, you can use its lines (instead of horizontal chalklines) to align the shingles.
Do not use felt as a temporary protection against rain: If it gets wet it will wrinkle, making it harder to shingle. If you need to temporarily protect a roof, cover it with plastic sheeting or a tarp.
When working with a helper, expect to spend several hours to install drip edge, WSU, and roofing felt on a medium-sized roof. Prep for the project by sweeping the sheathing clean of all debris and making sure there are no nails or splinters poking up.
What You Need
- Tin snips
- Hammer stapler
- Utility knife
- Tape measure
- Chalk Line
- Drip edge
- Waterproof shingle underlayment (WSU)
- Roofing felt
- Staples or roofing nails
Step 1: Apply and Attach Drip Edge
Apply drip edge to the eave. Cut drip edge flashing at an angle at the corner, press it lightly against the eave end (not too hard or you will misshape it), and drive a nail near the end. Use a chalkline or align it by eye to make sure it is straight, then drive roofing nails every foot or so to fasten it to the sheathing.
Step 2: Overlap and Cut Drip Edges
Where drip-edge pieces meet, overlap them by at least 2 inches. Where the drip edge meets a valley or a hip, cut the pieces with overlapping tabs. Press the tabs firmly to make crisp corners with no gaps and attach with roofing nails.
Step 3: Align Felt
Snap a chalkline to help you align the WSU or felt. The WSU should overhang the drip edge by 1/2 inch, so snap a line that is 35-1/2 inches above the drip edge. Check at several points to make sure the overhang is consistent; it does not have to be perfect, but you may need to realign the drip edge.
Step 4: Roll Out WSU
Using the chalkline as a guide, roll out about 12 feet of WSU smooth and free of creases and bubbles. Often you can reroll it, then unroll it while peeling off the paper backing. On a steep roof, you can adhere the bottom half first, then pull off the backing from above. Avoid creases and stay aligned with the chalkline. Press down to tightly seal the WSU.
Step 5: Apply WSU at Valley
No matter which valley method you use, for extra protection apply WSU at a valley. Snap chalklines on both sides for guides. Roll out and smooth the WSU and drive a few nails on one side to hold it temporarily. Fold over the other side and peel off the paper backing. Press into place, remove the nails, and peel off the paper on the other side.
Step 6: Stay Paralell
Overlap felt courses at least 4 inches. Snap a chalkline 32 inches above the top edge of the WSU or use the lines on the felt as guides. If you use the lines as guides for installing shingles, measure up from the drip edge to confirm that you stay parallel. Roll out the felt, eliminating creases or bubbles, and drive at least three staples every foot.
Step 7: Apply Felt
Apply felt right up to the ridge or to no more than 4 inches below it. (Rather than cutting the felt horizontally to fit, overlap it by more than 4 inches.) Apply felt on the ridge so it overlaps at least 8 inches on each side. Eliminate creases and bubbles.
Step 8: Attach Where Roof Meets Wall
Where the roof meets a wall, lap the felt up 4 inches if possible. Pry away the siding and slip the felt under the siding. In some situations, you may have to temporarily remove the siding and replace it after installing the underlayment, shingles, and flashing.
How to Prep a Hip Roof
Where you meet a hip, apply felt up to the hip and cut the pieces so they fit snugly. Then apply a vertical course of felt along the hip that overlaps on each side.
How to Multitask
When the weather promises to stay dry, many roofers prefer to install underlayment as they go. Apply one or two courses of WSU or felt, install the shingles, then apply more felt when you come within 6 inches of the top edge.