Cindy from Ajax, Ontario writes: We have an L shaped house, and our roof is in need of re-shingling (we are considering asphalt shingles). Where the two roof slopes meet in the centre of the "L" there is a major valley, quite long, with steep pitch. This valley is currently an open valley, made of granular asphalt similar to shingle material, and it is worn away and leaking badly. Different roofing contractors have given us differing advice as to what type of valley they would install. Some say a closed valley (made up of the shingles overlapping) with an ice/water shield underneath is the best protection against leakage and ice dams. Others swear by an open metal valley, because the metal will never "wear out", and a 30 lb. tar paper underneath should be sufficient. Although we actually prefer the "look" of the closed valley, we are concerned that the granules on the surface of the shingles will wear away rapidly in the valley. What is your opinion on the more durable type of valley? Your advice would be greatly appreciated.
The closed valley is probably the best "looking" arrangement, but definitely not the most long lasting and trouble free. The problem is that all the wear is in the valley and that will wear the inter woven shingles. Also there is differential movement between the two roofs, and that will tear at a closed valley.
The attached graphic shows a valley that is rarely done in all of this detail, but is the best way to make a valley. The metal is bent into a "W". This does two things. It gives the flashing room for expansion and contraction without pulling at the nails or other sealants. It causes water to jump over the valley, and not be forced under the shingles on the other side, a common problem when the two roofs are not at the same pitch. Note also that the edge of the valley is rolled over. The roll serves two purposes. It allows for the use of clips, eliminating all nail holes in the material that is supposed to shed water. It also provides a mechanical barrier to water which may try to flow under the shingles. Not so obvious in this illustration, but important for all valleys, is that it should be more open at the bottom than at the top, to allow snow and ice to slide down without getting stuck. Most roofers will consider this overkill, but it sure does work.
Click to see a photo and description of the water jumping a 'W' Flashing.