9.10. Dormers

Description: The conditions for flashing the junction of a dormer and roof are similar to those for chimney flashing. The two methods described for chimney flashing can be applied. The first is based on using copper flashing squares as base flashing inserted between successive courses of roofing. The second uses a single copper runner flashing under the roofing material. In both cases the dormer cap flashing is simpler than chimney flashing because the wall is of light weight construction. The upper edges of the base flashing are installed under the siding or shingles.

Dormer sill conditions vary from typical window sills in that the flashing is extended into an apron which laps over the roofing.

Copper flashing used at dormers should weigh a minimum of 12 oz. Aprons must be at least 16 oz. copper.

9.10A. Section at Apron Flashing

This section shows a typical sill condition where the flashing remains visible. It extends over the roofing material to provide positive drainage. The lower edge is then cleated at 12" O.C. maximum, and the upper edge is inserted into the sill drip and nailed to the sheathing at 3" O.C.

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9.10B. Dormer to Roof Junction

This detail illustrates the first method described above, using individual copper flashing squares woven between the shingles, tile, or slate. The flashing extends at least 4" onto the roof and 8" up the wall. The joint between the copper apron and base flashing is soldered.

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9.10C. Apron Flashing - Alternate

This section shows the use of separate sill and apron flashing covered by trim. The vertical portion of the flashing is hidden. The vertical piece is nailed to wood blocking or nailable sheathing at 3" O.C.

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9.10D. Dormer Flashing - Alternate

A second method is illustrated here. The flashing is attached to the roof deck before the roofing material is installed. The roof portion of the flashing is formed with a hooked edge and cleated to the sheathing at 12" O.C. maximum. Joints in the flashing are lapped 6" in the direction of flow. A cant strip is formed in the roof flashing, as shown.

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