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Q.What is the minimum slope allowed for asphalt shingles? I have heard that asphalt shingles can be applied on a roof with a pitch as low as 2/12, as long as a rubberized asphalt eaves membrane is installed under the shingles.

A.Corresponding Editor Paul Fisette replies: The CABO One and Two Family Dwelling Code permits the installation of asphalt shingles on roofs with a slope as low as 2/12 if the shingles are double-coverage, self-sealing versions.

Shingles that are not self-sealing must be hand-sealed with asphalt roofing cement. This low-slope application requires a double layer of #15 felt underlayment, cemented together, for a distance extending from the eaves up to a point 24 inches inside the interior wall line of the building. For the rest of the roof slope, the felt underlayment must be installed with a 19-inch top lap and a 12-inch side lap, providing double coverage of the felt layer.

Asphalt shingle manufacturers will warrant shingles installed on roof slopes as low as 2/12 when these special installation procedures are followed. Self-sticking rubberized asphalt products like Grace Ice & Water Shield are acceptable but expensive substitutes for #15 felt.

Having said this, I would never install shingles on a 2/12 roof, especially in an area where snow falls. In fact, I have had numerous problems in northern climates with asphalt shingles on 3/12 roofs. As a result of this experience, I would limit the application of asphalt roof shingles to roofs that are 4/12 and steeper. The National Roofing Contractors Association recommends that asphalt shingles be installed only on roofs with slopes that are 3/12 and greater.

I believe that if you have to depend on felt paper, asphalt cement, or Ice & Water Shield to block water after it gets past the primary roof covering, you have the wrong roof system. Shingles get brittle with time, and at the reduced slope of a 2/12, a slight curl at the end of the shingle line shunts water backwards under the leading edge of the overlapping shingle. When you have snow or ice sitting on the roof and melting, you don’t have a prayer at keeping the water out. Felt paper and Ice & Water Shield are emergency backups, not a plan for everyday protection.