Description: Valley flashings are usually categorized as open or closed. The former is visible when completed, the latter is not. The main difference is that for open valleys, long sheets of 16 oz. (minimum) copper are cleated to the sheathing and underlayment before the shingles are applied. Adjacent sheets of copper are lapped a minimum of 8". The sheets are nailed at the top only with copper or bronze nails.
Closed valleys are constructed during shingle installation by inserting copper flashing squares between successive layers of shingles. These flashing squares are folded on the diagonal. The recommended square dimensions for slate and shingle roofs are shown in Table 18.104.22.168.
|Square Size, Inches Roof Slope|
|Slate Size, Inches||6" or greater |
|less than 6" |
|12||9 x 18||9 x 24|
|14||10 x 18||10 x 24|
|16||11 x 18||11 x 24|
|18||12 x 18||12 x 24|
|20||13 x 18||13 x 24|
Cant strips are used to raise the shingles, thus breaking contact with the copper surface which minimizes line corrosion.
If slate or tile is used for the roof covering, 20 oz. copper is recommended for valley flashing.
Special Conditions: The details shown are for roof intersections resulting in valley slopes of at least 4-1/2" per foot.
A. Open Valley The detail shows a typical open valley flashing for a shingle or slate roof. Two different cants are illustrated. The cant strip can also be constructed as shown in Detail D. The shingles or slate must lap the flashing at least 6".
B. Equal Slopes - Unequal Water Flow Where unequal water flow is expected, a baffle, 1-1/2" high, should be installed as shown to prevent water of higher velocity from forcing its way past the opposite edge of the valley flashing. The baffle can also be constructed as shown in Detail D.
C. Closed Valley Intersecting roofs using a closed valley must have the same slopes so that the shingle butts line up at the valley intersection. For roof pitches of 6" or more per foot the flashing extends at least 9" under the roof covering on each side. For roof pitches less then 6" per foot the flashing extends at least 12".
The lower edge of the flashing should be held 1/2" short of the butt line of the slate, tile or shingle in the succeeding course. The upper edge is fastened to the sheathing with copper or bronze nails.
D. Unequal Slopes This condition requires a baffle for the same reason as Detail B. It can be constructed as shown in either detail. This detail also shows a different cant strip. Other methods of raising the shingles away from the copper are shown in Details A and B.
For additional information on copper roofing valley details see: Roofing Systems - Details.