How to Flash Around a Skylight

Flashing around a skylight ensures a watertight seal.

Skylights have become an almost standard feature in the modern home to bring in additional natural light or enhance a room's ventilation. Available in a variety of shapes and sizes -- domed or flat, fixed or operable -- with clear or opaque acrylic panels, there is a skylight design for almost any application or need. Whether adding a new skylight unit or replacing the roofing around an existing unit, the proper installation of the flashing is essential to insuring a watertight seal and years of maintenance-free functionality.



For any specific skylight dimension, prefabricated flashing units are available and should be used to minimize seams and maximize proper drainage at the perimeter of the skylight. Refer to your skylight manufacturer's product catalog or a local sheetmetal shop for flashing configurations for your specific application.

Things You'll Need

• Tape measure
• Hammer
• Metal snips
• Nail set or punch
• Caulk gun
• Exterior grade silicon caulk
• Continuous "L" or side flashing
• 2 prefabricated end flashings with corners
• 1-inch roofing nails

Instructions

1. Verify that the roofing felt extends to the edge of the skylight curb on all sides. Position the lower-end flashing assembly, or "sill flashing," against the skylight curb. Keep the flashing base flat against the roof surface on top of the roofing felt. The vertical part of the flashing should sit flat against the curb and reach within 1/2 inch of the top of the curb frame, but not extend above it. Secure the flashing in place with roofing nails through the top edge and into the skylight curb.

2. Measure the length of the skylight curb's side. Cut a piece of continuous "L" flashing to this length, minus 2 inches. The piece should overlap the sill flashing installed in Step 1 by a minimum of 3 to 4 inches, and should be held back from the lower vertical corner of the curb at least 1 inch.

3. Lay a bead of silicon caulk on the vertical and horizontal surfaces of the sill flashing where the side flashing will overlap it. Put the side "L" flashing piece cut in Step 2 in place, keeping the horizontal base beneath the adjacent roofing felt. Slip the flashing into place, applying pressure at the overlap to spread the silicon caulk. Secure the flashing in place with roofing nails through the top edge and into the skylight curb. Make certain the flashing piece sits flat against the curb and roof surface when nailed.

4. Repeat Steps 2 and 3 for the flashing on the opposite side of the skylight curb.

5. Place a bead of silicon caulk on the vertical and horizontal surfaces of both side flashings, where the top end flashing will overlap them.

6. Position the upper end flashing assembly, or "head flashing," against the upper length of the skylight curb. Make certain both corner pieces overlap the side flashing pieces installed in Steps 2 through 4 by a minimum of 3 to 4 inches. Slide the horizontal base of the flashing beneath the roofing felt above the skylight curb. Apply downward pressure on the flashing where it overlaps the side flashing to spread the silicon caulk. Secure the flashing in place with roofing nails through the top edge and into the skylight curb. The flashing is now complete and ready for the installation of the skylight unit.

References

• Millard Lumber: Installing Skylights