Skylights in a Flat Roof

Q:

Is it possible to install a skylight in an almost-flat roof (2-pitch) that has already been covered with an EPDM membrane?

Chris Ermides, Beacon, NY

A:

Mike Guertin, author of “Leak-Free Skylights” (FHB #204, pp. 40-43), replies:

You can install a skylight on just about any roof slope. Very-low-slope and flat roofs present the added challenge of ensuring that the skylight surface is set at a minimum slope. Most major residential-skylight manufacturers require a slope of 3-in-12 or 4-in-12. To install a skylight in a lower-sloped roof, you have to install a site-built or prefabricated curb that elevates one end of the skylight. The skylight mounts on top of the curb, which is basically a wooden box made of 1x material. In snow country, design the lower edge of the skylight curb to be at least 6 in. off the low-slope roof surface, and elevate the top edge to meet the minimum-slope requirement or greater.

You can use the existing membrane to flash the curb into the roof. Before cutting a hole for the skylight, make an X-cut in the membrane so that each flap can run up the sides of the new curb. Let about 6 in. lap onto the sides, and trim off the excess. Stretch and bond double layers of uncured EPDM to create corner patches. Splay the patch material about 4 in. onto the roof-deck surface and 6 in. up the side of the curb. Cut a piece of EPDM to wrap around and cover the walls of the curb fully, starting about 1 in. above the roof surface. Adhere the piece around the perimeter of the curb and the EPDM.

Patches Seal The Corners

When installing a skylight on a roof with less pitch than is required by the manufacturer, add a sloped curb, and flash it to the roofing material. In the example here, EPDM roofing membrane continues up all four sides of the curb, leaving the corners unprotected. Overlap the corners with two layers of EPDM or molded outside corners.

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