Valleys are formed when two sloping roof planes meet at an angle. The sloping planes direct water toward the valley, concentrating the drainage along the joint and making it especially vulnerable to leakage.
Roof Valley Centerline
As a result, one of the most important concentrating installation details for good roof performance is proper valley flashing. Consult the appropriate roofing manufacturers for recommendations on a particular application. To install underlayment in a valley, first center a 36″ wide strip of the appropriate underlayment in the valley and secure it with only enough nails to hold it in place or place a 36″ wide strip of self-adhered membrane directly to the deck. Then trim the horizontal courses of felt underlayment applied on the roof to overlap the valley strip at least 6″. In all valleys the underlayment or self-adhered membrane should be tight in the valley but not creased or wrinkled following application of the underlayment, roofs with multiple planes require the construction of one of four types of valleys: open, woven, no cut or closed out. Regardless of the type of valley used,
How valleys are formed cont…
it must be smooth, unobstructed, of sufficient capacity to carry water away rapidly and capable of withstanding occasional backing up of water. Follow application of the underlayment, roofs with multiple planes require the construction of one of three types of roof valleys: open valley, woven valley or closed cut valley.
The recommended flashing material is a 26-gauge galvanized metal or an equivalent corrosion resistant, non-staining material (check with shingle manufacturer and local code). The width should be no less than 24″, but local building codes or shingle manufacturer may require a greater width.
Roof valley construction is an important part of the roofing process.
If the valley is covered with a minimum 36” wide self-adhered membrane that complies with ASTM D1970, adhered directly to the deck the valley is ready for the metal valley flashing. If the valley is covered with felt underlayment and additional later of #50 or heavier valley underlayment is required before the metal flashing is applied. Center the strip in the valley, securing the non-self-adhered materials with only enough nails to hold it in place. Nail the strip along a line 1” from the edges, first on one edge all the way up, then on the other while pressing the flashing strip firmly and smoothly into the baller. Laps should be 12” and cemented.
After the underlayment has been secured, install the recommended corrosion resistant metal in the valley. Secure the valley metal to the roof deck without puncturing, with roofing nails spaced 8”-12’ apart. Overlaps should be 12” and cemented. The valley will be completed with shingle application.