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Because of the additional amount of water that valleys get they are more susceptible to leaks. Debris or snow can pile up there as well and slow down the water flow off of the roof. For these reasons we have to make sure that the valleys have extra protection.
For my project, I started by putting a strip of metal flashing along the valley right on top of the sheathing. This will also give additional support if someone happens to be on the roof and steps in the valley. We don't want anything to pucture the layers of water protection there.
Next, I installed a layer of 36" wide roll roofing along with some roofing cement to seal it down good. The idea is to make a wide area around the valley that will keep the water from getting to the sheathing.
After the roll roofing, I put 15 pound roofing paper on the entire area as usual and started laying shingles with the porch side (to the left in the photo). I started at the gable end and continued with shingles until I was well past the valley, making sure to not nail in the valley and penetrate my flashing below.
As you can see in the picture, after the porch side is done, I did the house side and overlapped the valley again. The only difference here is that I won't put any nails past the valley. Instead, once the shingles are all down, I'll make a chalk line straight down the valley and then cut the top layer of shingles along this line. I believe that this technique is called a closed-cut valley.
Note: Cutting shingles on a valley or even along the rim of the roof is easier with a roofing blade for your utility knife. This is a razor-sharpe blade just like the others except that it's in the shape of a small hook. Very handy when cutting shingles from their face side.
Whatever you call it, when I was finished it matched the upper gables and the rest of the roof. That's a good thing when you're just renovating a portion of the house.
Trim is next!
-Peter
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