Skylight Flashing Installation

A skylight or roof window creates a large opening in the roof plane that must be sealed on all four sides. The flashing approach follows the same logic as a chimney: water from above must be diverted around the unit, water at the sides must be shed course by course, and water at the bottom must be carried back out onto the roofing.

Most modern units are installed with a manufacturer's flashing kit matched to the roof covering (low-profile kits for shingles and slate, deeper kits for profiled tiles). Following the kit instructions is usually a condition of the product warranty.

Typical components

• Sill (apron) flashing. Fitted first at the bottom of the unit and laid over the course below.
• Side flashings. Either step flashings woven into each course, as at a sidewall, or continuous side pieces for tile.
• Head flashing. At the top, tucked under the roofing above so water is carried around the unit.
• Underlayment collar. Many manufacturers supply a self-adhered or pleated underlay collar that seals the frame to the roof underlayment before the metal flashings go on.

General sequence

1. Frame the opening and fasten the unit to the deck with the supplied brackets.
2. Seal the frame to the underlayment with the collar or a self-adhered membrane, lapped so water flows over it.
3. Install the sill flashing over the course below.
4. Shingle up the sides, inserting step flashings with each course; each piece extends onto the roof and up the frame.
5. Install the head flashing and continue the roofing above it.
6. Leave a clear gap between the roofing and the head flashing so debris can wash out.

Considerations

Canadian guidance on roof penetrations warns against exposed fastener heads and recommends sealing any that remain with compatible sealant. Wide skylights may need a small diverter or cricket above the unit, as with chimneys. Condensation on the inside of older skylights is sometimes mistaken for a leak.