When a porch roof intersects the main roof at a right angle, a valley is formed. Shingles can be laid across a vallev in different wavs.
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A woven valley is formed by weaving shingles across the valley, alternately overlapping from the main roof onto the porch roof, then vice versa. When properly done, a woven valley is watertight. Let each course of shingles overlap the valley by at least 12 in. and keep nails 8 in. or more from the centerline of the vallev.
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An alternative to a woven valley is a closed, or cut, valley. To create a cut valley, let all shingle courses from the main roof lap across the porch valley by at least 12 in. Don’t alternate back and forth from the main roof to the porch roof. After the shingles are laid on the main roof, lay the shingles on the porch roof
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Install step flashing at roof-wall intersections
Use metal step flashing to waterproof the intersection where a roof butts into a wall. Usually made from aluminum or copper, metal step flashing is bent to form a series
of elongated, L-shaped pieces that ere lapped over each other in successive shingle courses as well as upward along the wall. The step flashing we used on this house is 10 in. by 6 in., and each leg is 3 in. wide.
The illustration at left shows how step flashing is installed on each course of shingles that runs into a wall. (Its also used where shingles meet a chimney or a skylight curb.) Each time a regular shingle is laid covvn, a step shingle is placed under it—on the part of the regular shingle that will be covered. Lap step shingles by about 2 in. One nail above the tar strip should be enough to hold each step shingle in place. As each successive course of shingles is nailed in place, a portion of the step shingle on the roof is covered. Later, when the wall is covered with siding or stucco, the vertical leg will be covered as well.
