9.3 Valleys: Linings, Open Metal, Closed-Cut & Woven Valleys

Code Requirements for Asphalt Shingle Valleys (R905.2.8.2)

Valley linings are installed according to the manufacturer's instructions before applying shingles. The permitted linings:

1. Open valleys lined with metal: lining at least 24 inches wide, made of a corrosion-resistant metal from Table R905.2.8.2.
2. Open valleys lined with roll roofing: two plies of mineral-surfaced roll roofing meeting ASTM D3909 or ASTM D6380 Class M. The bottom layer is 18 inches wide and the top layer at least 36 inches wide.
3. Closed valleys (covered with shingles): one ply of smooth roll roofing meeting ASTM D6380, at least 36 inches wide, or either of the linings above.

Self-adhering polymer-modified bitumen underlayment meeting ASTM D1970 may be used in place of the lining material. This is the usual choice on Michigan roofs, because valleys hold snow and ice.

Table R905.2.8.2 Valley Lining Metals

Open Metal Valleys

The shingles stop short of the centerline, leaving the metal exposed. Typical manufacturer details:

• Install the lining (usually D1970 membrane), then the metal, which is often preformed with a center diverter rib to keep water from one slope from running across and under the shingles on the other.
• Fasten the metal near its outer edges only, with clips or nails, as the metal and shingle makers direct. Do not nail through the open channel.
• Snap two chalk lines for the shingle edges. Manufacturers commonly start about 6 inches apart at the top (3 inches each side of center) and widen them about 1/8 inch per foot toward the eave, because the valley carries more water lower down. Follow the specific instructions.
• Clip the upper corner of each shingle that ends at the valley so water is directed into the channel. Bed the edges in asphalt cement where instructed. Keep nails away from the metal channel.

Open metal valleys shed snow and debris well and suit laminated shingles, slate, and tile.

Closed-Cut Valleys

The most common asphalt shingle valley. Typical manufacturer steps:

1. Install the lining.
2. Lay shingles on one roof plane (often the lower-slope or smaller plane) across the valley and at least about 12 inches onto the adjoining plane. Keep nails at least 6 inches from the centerline.
3. Apply shingles on the other plane over the first, then snap a line about 2 inches back from the centerline on that side.
4. Trim along the line, clip the upper corners, and bed the cut edges in asphalt cement if the manufacturer calls for it.
5. Protect the lower layer while cutting. Slip a scrap of metal under the line so the knife does not cut the shingles or lining.

Woven Valleys

Shingles from both planes are interlaced across the valley, course by course. This works with flexible 3-tab shingles. Many manufacturers do not permit weaving laminated shingles, because the thick layers bridge the valley and trap water. Check the instructions. Keep nails out of the valley center.

Other Roof Coverings

Valley Troubleshooting

• Leaks near the bottom: often water overshooting a closed valley on steep-to-shallow intersections, a missing kick at the eave, or ice damming. Check that the ice barrier and valley lining run under the eave detail.
• Leaks along the middle: nails too close to the centerline, cut shingles not clipped, or debris dams.
• Cracked shingles in a woven valley: usually laminated shingles bent too sharply, or cold-weather weaving.