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Shingling a valley is the most complex part of a roof shingling project, as it involves the shingling of one roof line and tying it into another. Shingling a valley can be done in a number of ways. Shingling valleys can be done by the open valley approach or the closed valley approach.

Shingling a valley in the open valley approach involves forming a channel for the water to flow down, where as shingling a closed valley involves completely shingling over the valley.

Shingling Open Valleys

With open valleys, shingles overlap into a flashing material channel that allows water to run through. The width of the flashing material should be a minimum of 4-6 inches wide near the top of the valley, and widen as it approaches the roof eve, or if you desire a consistent open valley width the channel should be wider and consistent in width throughout the entire length of the valley.

When shingling an open valley with metal flashing, an Ice and Water shield should first be applied to the valley. Ice and water shields seal around nails that puncture them and thus prevent leaks.

The metal flashing should then be installed over the Ice and Water shield. Metal flashing does expand and contract with temperature so it is best to install the metal flashing in the roof valley in sections that do not exceed 10 feet in length. The metal flashing sheets should be installed so that they overlap each other by several inches, and with the upper piece resting over the bottom piece. Roofing cement should be used to seal the two sections of metal flashing together and nails should be installed along the edges of the metal flashing. Again the Ice and Water shield will prevent any water leaks.

When shingling an open valley using rolled roofing, an 18” wide section of rolled roofing should first be installed into the valley, followed by a 36” wide section. The 18 inch wide section should be placed into the valley with its mineral surface laid face down, and the 36 inch wide section should be placed on top of it with its mineral side facing up. In addition, and similar to metal flashing, the top piece should overlap the lower piece and should be sealed with a roofing cement.

Finally, after trimming the roofing shingles near the valley to evenly expose the flashing, roofing cement should be applied on the underside of the bottom edges of the shingles near the valley. This will help prevent water from sliding up and underneath them.

Depending upon the types of shingles being installed you may want to work your way into the valley or out of it. With 3-tab shingles it is best to work your way into the valley. With laminated shingles you can work your way out of the valley. In the ensuing sections I am referring to shingling with 3-Tab shingles.

Basically shingling a closed valley involves shingling a roof plane through the valley and up over and into the other roof plane by approximately 12 inches (a full tab with 3-Tab asphalt shingles). As you work your way up the valley the shingles passing through the valley are woven into place. This is accomplished by tying in and weaving into the valley both roof planes simultaneously. To do this, one shingle of one roof plane over laps the other roof plane, and then you move up a tier and alternate the process. This creates the weaving pattern.

It is important that during the weaving process, that no nails are installed within 6 inches of the valley and that the shingles are pushed snuggly into the valley. If the shingles are not properly pushed down into the valley bridging will occur such that an air pocket is created underneath the valley. When someone steps on this air pocket the shingles could be cracked or damaged. Ideally shingling closed valleys should be done on warm days so that the asphalt shingles are more malleable.