Essentials

Roofing contractors who install asphalt shingle roof systems know proper valley construction is critical to attaining weathertight systems. Because of the volume of water and lower slope along valley lines, valleys particularly are vulnerable to leakage. A clear, unobstructed drainage path is important for valleys to quickly carry away water.

There are three basic types of valleys for asphalt shingle roof systems: open, closed-cut and woven. Deciding which valley type is appropriate depends on several factors, such as limitations because of a roof system's slope, climatic and geographic factors, and the possibility of debris accumulation from surrounding foliage or vegetation.

These general valley types only are to be constructed after the necessary layer(s) of underlayment and any specified valley lining material have been applied to a roof deck.

Valley underlayment

Regardless of what valley type is used, valley underlayment should be installed before valley construction. An additional full-width, 36-inch (914-mm) sheet of No. 15 or No. 30 asphalt-saturated underlayment felt or polymer-modified bitumen underlayment, base sheet or ice-dam protection membrane should be centered in a valley. Valley underlayment sheets should be secured with only enough fasteners to hold them in place until the balance of valley materials are applied. It also is important to avoid placing fasteners near a valley center. Generally, underlayment fasteners should be kept a minimum of 8 inches (203 mm) from a valley's center.

Open valleys

Open valleys typically are lined with corrosion-resistant sheet metal, and asphalt shingles are held back from a valley's center, leaving a clear, unobstructed path for water runoff.

NRCA recommends valley metal be a minimum of 18 inches (457 mm) wide. For easier installation and to control thermal expansion and contraction, NRCA suggests valley metal pieces be no longer than 10 feet (3 m).

NRCA also suggests valley sheet metal be formed with a splash diverter or rib in the valley center. A center rib can be beneficial in valleys where adjoining roof areas are of unequal slope because a rib helps prevent "wash over" of runoff. A center rib should be a minimum of 1-inch (25-mm) high.

Open valleys can be used effectively on lower-sloped roof systems and are advantageous in locations where debris from surrounding foliage can settle and accumulate in valleys.

In areas prone to snow and ice accumulation or regular freeze-thaw cycling, open valley construction should be enhanced by adding self-adhering modified bitumen underlayment or tapering valley metal so the valley is wider at the low point than the high point.

Closed-cut valleys

In closed-cut valleys, shingles on one side of a valley are installed across a valley and extended at least 12 inches (305 mm) onto the adjoining side. Then, shingles from the other side are cut about 2 inches (51 mm) short of a valley center line; valley tabs may be sealed with asphalt roof cement.

NRCA recommends shingles first should be installed on the side of a valley with the shortest distance to the ridge (the side that will experience the lowest volume of runoff). The cut shingle side of a valley should be on the side of a valley that has the longest distance to the ridge or steepest slope.

This method offers a partially open design for drainage and relatively durable valley construction. It is preferred by many roofing contractors because it can be used with most asphalt strip shingle types.

Woven valleys

A woven valley occurs when asphalt shingles from adjoining roof areas are interlaced to form a closed-cut valley. This valley type generally is limited to strip shingles on roof systems where valley slopes are at least 4-in-12 (18 degrees). This method creates a durable valley; however, it may hamper runoff in areas where heavy accumulations of debris or plant growth may collect in a valley.

In addition, NRCA does not recommend the use of woven valleys with laminated shingles because woven valleys can create a relatively thick buildup of material. The resultant valley surface also may be irregularly sloped, which could interfere with drainage.

Individual locking-type shingles cannot be used with woven valleys because nails are required for each tab, which would require placing nails at or near a valley center.

For more information about valley construction, refer to The NRCA Roofing and Waterproofing Manual, Fifth Edition; call NRCA at (847) 299-9070 or fax (847) 299-1183; or click here.

Joan P. Crowe is an NRCA manager of technical services.