Valleys are notorious for developing leaks. In downpours, torrents from two planes of the roof meet. Often they are dammed by debris. These factors can cause water to go under nearby shingles, resulting in leaks. There are three ways to roof valleys: open metal, closed-cut, and woven. All three work well, but check which is preferred by your local building department. Regardless of the method you choose, do not drive nails within 6 inches or so of the valley center or you will provide an unwanted pathway for water.
A strip of waterproofing shingle underlayment, or WSU, provides extra protection and is an essential component of woven and closed-cut valleys. It is also a good idea for an open metal valley.
Chimney flashing is fairly complicated. If your chimney did not leak before and you can use the old flashing pieces as templates for the new, you should have no problems. However, if you have no templates, you may want to call a professional roofer.
With underlayment and flashing installed, apply shingles up to one side of the flashing. Do not drive nails into the flashing. Then install shingles on the other side. Again, do not drive nails through flashing. You'll have to overlap shingles as you go, much like weaving.
Some roofers cut the shingles parallel to the flashing's center ridge. Others start with a 3-inch-wide gap at the top and widen the gap by 1/8 inch per running foot as it runs downward. Check with your inspector to see which method is preferred in your area. Snap chalklines for the cuts.
Place a piece of sheet metal under the shingles to make sure you will not damage the flashing. Use a utility knife with a hook blade to cut the shingles along the chalkline. You may find it helpful to use a straightedge as a guide.
Snipping off the corners of adjacent shingles (roofers call it dubbing) adds a measure of protection against water being channeled under shingles. Lift up shingles and use a hook blade to cut all the pointed unexposed ends. Make each cut about 2 inches from the point.
Caulk twice between shingles and flashing. Using roofing cement in a caulk tube, insert the tube's tip all the way under the shingles and run a continuous bead. Then hold the nozzle 2 inches back and apply a second bead nearer the edge.
Lift up each top shingle and apply roofing cement to adhere the shingles to each other. This is important both to seal out water and to attach the shingles because there are no nails.
Roof one side of the valley, running the shingles across it. Stop nailing 6 inches from the center. Lay one- and two-tab shingles as you near the valley so full-size shingles will run across the valley. Shingle the other side of the valley the same way. Snap a chalkline 2 to 3 inches past the valley center on the top layer of shingles.
Cut the top layer of shingles along the chalkline. Slip a piece of sheet metal flashing under the top layer of shingles to protect the bottom layer in the valley while you cut the shingles with a hook blade.
Dub-cut the top-layer shingles (see Step 4 in Open Metal Valley above). Apply two beads of roofing cement to attach the top-layer shingles to the bottom-layer shingles. Also apply cement to attach the top-layer shingles to each other (see Steps 5 and 6 in Open Metal Valley above).
All woven pieces must be full-size shingles that lap onto the other side of the valley by at least 1-1/2 tabs. The two sides of the valley must be shingled at the same time: Shingle a course on one side, then the other.
Attach the subsequent courses. Continue working on alternate sides to weave shingles across the valley. Do not drive nails within 6 inches of the valley center.
Use single- or double-tabbed shingles as needed. Every shingle that crosses the valley should be full-size and should lap the valley by at least 8 inches. To achieve this, you will need to cut and attach single- or double-tabbed shingles just to the left or right of the overlapping shingles.
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