What are open valley flashings?

expert EXPERT April 26, 2007

Open valleys have flashing material exposed to the elements along the length of the valley. Valleys should be 4 to 6 inches wide at the top. Valleys should widen out at a rate of about 1/8 inch per foot as you move down the valley.

There are two reasons the valleys should widen at the bottom. 1. Lower sections of the roof see more water than upper sections. Lower sections of valleys also see more water than upper sections. 2. Valleys that widen as they descend are less likely to hang up snow and ice. Valleys of uniform width are more prone to ice damming. Typical Materials Open valley flashings for asphalt shingle roofs are typically roll roofing or metal.

Roll roofing valley flashing has two layers. There is typically an 18-inch-wide piece laid with the mineral surface down. The second layer is a 36-inch-wide piece of roll roofing with the mineral surface facing up. The flashings are installed before installing the shingles. Open valleys laid with overlapped individual shingles are very likely to leak. This is poor practice.

Metal valley flashings are typically 24 inches wide. Some authorities recommend that they extend at least 4 inches out under the roofing material at the eaves. We believe this leaves very little room for error. We prefer to see them extend out further. On very long valleys, the valley flashings may have to be wider than 24 inches because they widen about 1/8 inch per foot. Many roofers use 15-pound felts under metal valley flashings, others use roll roofing or Ice and Water Shield. Some roofers also use 18-inch-wide strips of Ice and Water Shield to extend the width of metal flashings on either side.

Individual pieces of metal valley flashings should be no longer than 10 feet. This is because the expansion and contraction, caused by changes in temperature, may cause buckling. When the sheets are lapped, the upper piece overlaps the lower piece by at least a foot. Ideally, plastic cement is used to seal the joint.

The same overlap technique is used on roll roofing. Where the roll is cut, the upper piece overlaps the lower piece by a foot and the two sections are cemented together.

Many roofing authorities specify that no nails should go through metal valley flashings. The elegant approach is to bend the outside edges of the metal over to form a 1/2 -inch return. Metal cleats are then hooked onto this return and it is the cleat that is nailed into the decking. This provides a flashing detail with no nails through the flashing. Furthermore, the 1/2 -inch return on the edges stops water that may drive up under the shingles and along the flashing. The cleats are usually spaced about every 12 inches along the valley length.

This treatment is very unusual and is almost never seen with asphalt shingles. In practice, the metal flashings are usually nailed near the edges. Using a layer of Ice and Water Shield under the flashing (base flashing) is helpful. The nail hole in the metal can leak but the Ice and Water Shield will not. Ice and Water Shield is a self-healing material. If a nail is driven through it, it won't leak around the nail.

Shingles covering the valley flashing are often held in place with plastic asphalt cement rather than nails to avoid nailing through the valley flashings. Most of these details won't be visible; you'll have to look for resultant leakage.

Metal valley flashings often have a one-inch upstand or splash diverter in the center (metal sticks up perpendicular to the centerline of the valley).

This inverted v shaped flashing detail helps prevent water driving down from one side of the roof, under the shingles on the other side of the valley. Upstands are more important on -

• steep roofs
• roofs that have one side of the valley fed by a much larger surface than the other
• or valleys where one side is much steeper than the other

No matter what type of valley flashing material is used, there should be no exposed nail heads in the valleys.

Clipping the corners off the top of the shingles where they enter the valley discourages water from running along the top of the shingle and getting into the roof system. It is good practice to cut the points.

Joints between adjacent wood shingles and shakes should not break into the valley. Good roofers set aside the widest shingles for use at the valleys to avoid this problem.