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Applying underlayment

MATERIALS: Eave drip cap, rake drip cap, underlayment, ice-darn barrier underlayment, hot-dipped galvanized roofing nails, roofing cement TOOLS: Safety glasses, claw hammer, tin snips, chalk line, roofing knife, hand roller, tape measure, pry bar

In roofing, underlayment means rolls of 15- or 30-pound roofing felt (felt soaked in asphalt). Check with the manufacturer of your shingles and use the underlayment they specify. Underlayment is a must—-not an option—for a good roof. Using the wrong materials can void the warranty. Here’s why: The roof is a system of layers designed to keep out water. The shingles do the basic job of shedding and channeling rain. The underlayment is a final barrier to moisture penetration. Application begins at the bottom of the roof and works its way up. Each strip overlaps the previous one by a few inches. Any water flowing down from the top of the roof is directed over the seam instead of into it.

In cold climates, heat leaking from the house often melts snow on the upper part of the roof; the water refreezes when it reaches the colder eaves, and the resulting slush and water (called an ice dam) can be forced up under the shingles. Proper insulation and venting help prevent this (see pages 432-439), but you should also install an ice-dam barrier underlayment over the section of roof covering the first 2 feet of the attic and in the valleys. It applies much like roofing felt but is heavier and self-adhesive.

Measure the slope of your roof with a level and tape measure. If the slope is between 2 and 4 inches per foot, apply a double layer of roofing felt at the bottom, and overlap courses by 19 inches instead of by the amount shown here. For a roof with a slope of less than 2 inches per foot, use roll roofing. Applied correctly, underlayment offers a degree of protection in case it should rain before you shingle. And if the roof has problems, you’ll have peace of mind that comes from knowing that its underlayment is solid.

USE THE RIGHT EDGING - The metal trim along the eave (bottom) of the roof is different from that which runs up the side of the roof. The bottom trim is called gutter apron and directs water into the eave trough. Drip or roof edge is narrower and runs along the side of the roof to support the ends of the shingles. Underlayment goes over the gutter apron and under the roof or rake edge (see Step 7).

1 NAIL A STRIP OF EAVE DRIP EDGE ALONG THE BOTTOM OF THE ROOF. If you need more than one strip, overlap the ends of neighboring strips by 2 inches. Use tin snips to miter the end that will butt against the drip edge covering the rake, or edge, of the roof (see Step 7).

2 UNROLL ROOFING FELT, ALSO CALLED UNDERLAYMENT, ALONG THE BOTTOM EDGE OF THE ROOF. The felt should overlap the eave drip edge by about 3/8 inch. Staple or nail the felt in place. If you nail, use 1-inch hot-dipped galvanized roofing nails, even though they will poke through the sheathing below.

3 ROLL OUT THE NEXT COURSE OF ROOFING FELT, overlapping the existing paper or ice-dam barrier by U inches. Fasten the felt with staples or with hot-dipped galvanized roofing nails.

4 WORK YOUR WAY UP THE ROOF DECK WITH ROOFING FELT, OVERLAPPING COURSES BY U INCHES. When a roll runs out, start a new one, overlapping the ends by 12 inches. Roll felt across valleys from both sides, extending it 36 inches on both sides of the valley. WHEN YOU COME TO AN OBSTRUCTION, CUT A HOLE IN THE FELT that will slip over it if possible. Otherwise roll the roofing felt up to the obstruction, then resume the course on the other side. Cut a patch extending 12 inches on each side of the obstruction, fit it over the obstruction, and nail it into place.

5 AT RIDGES AND HIPS, WRAP 6 INCHES OVER THE TOP AND NAIL OR STAPLE IN PLACE. Staple every 3 inches along the edge. NOTE: If you are installing roll roofing cut a strip of felt 12 inches wide, snap a chalk line 6 inches on each side of the ridge or hip; spread a 2-inch-wide strip of roofing cement just inside the lines. Set the underlayment so that the outside edges anchor in the roofing cement.

6 IF THE ROOF HAS A DORMER OR SIDEWALLS, START AT THE SIDEWALL. Tuck the roofing felt under the siding to create an unbroken seal at the roof and wall joint.

7 NAIL A STRIP OF DRIP EDGE OVER THE RAKE EDGE THAT COVERS THE UNDERLAYMENT, STARTING AT THE BOTTOM AND WORKING TOWARD THE RIDGE. Overlap joining strips of drip edge by 2 inches. Miter the ends of drip edge where they meet at eaves. (Rake and eave drip edges are different. Make sure you get the proper amount of each kind.)

Preventing ice-dam damage - In areas of the country with rough winters, roofers often install an ice-dam barrier. (This heavy-duty, self-adhesive, waterproof sheet of underlayment prevents melting snow from getting under the shingles when it runs into ice at the bottom of the roof). To be effective the ice dam has to extend at least 26 inches up the outside of the roof, measuring from the interior attic wall. (The eaves must be covered but exclude them when you measure.) Measure up the roof from inside the attic to determine how many courses of ice-dam barrier to lay down.

Apply the ice-dam barrier along the bottom edge of the roof. Ideally the strip should run the length of the roof with no cuts. For longer runs, cut the barrier into manageable lengths. Peel off about 2 feet of the backing on the underside to expose a contact adhesive. Put the sticky side face down on the roof, overhanging the drip edge by about 3/8 inch. Press the barrier onto the roof with your hands, nail across the top every 18 inches, and roll the edges with a hand roller. Work your way across the roof, peeling off the paper, pressing, nailing, and rolling as you go. Overlap seams by 6 inches, nail every 6 or 8 inches, and roll the seam with the hand roller. (The manufacturer may also recommend applying the barrier along the rake edges of the roof; if so, follow the directions for installation.)

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