Modern skylights are highly reliable, and flashing is the main reason why. When skylights were first copied here in North America, 30 years after their introduction in Europe in 1941, too many of them relied on caulking and tar to keep water out. That was a recipe for failure that never happened in Europe, and it gave skylights a bad name in North America.
As I’ve explained before, the best time to cut skylight holes in your roof is after the 2×8 roof boards go down, but before the insulating roof panels are applied. We tried cutting through both panels and roof boards initially, but this approach didn’t work as well as it should. Don’t bother trying. So, before your skylight installation campaign begins, your roof insulation should be down and a weatherproof membrane applied on top. Begin work by cutting an X-shaped hole through the membrane where it goes over the skylight openings, then trim this excess off 3 inches or so back from the edge, as you can see in the video up next.
VIDEO: Skylight Installation
Most skylights are anchored with nails driven through some kind of flange around the perimeter of the skylight frame. That’s the way the VELUX skylights we installed are fastened, but rather than relying only on the short, roofing-type nails supplied in the skylight kit, we opted to go with 3 1/2”-long roofing nails. These pass through the 7/16” OSB sheathing on the insulating panels, biting into the 2×8 roof sheathing underneath the foam. This provides a much better hold than short nails through the OSB alone.
VIDEO: Skylight Vent Action
Skylights Save Energy and Boost Comfort
Besides admitting wonderful light, openable skylights are key to making this cabin comfortable during hot weather. Ventilation is the reason why. Immediately after opening the skylights, you can feel upward air movement in all parts of the cabin, even when there’s little breeze outside. Hot air does rise, but having openings in the top of the roof lets that hot air go right outdoors, drawing fresher, cooler air in through windows in all lower floors. The effect is just like the draw on a big chimney. You can feel the cooling air move quietly, quickly and in large volumes.
The best skylights are always installed after ice and water shield underlay is applied, but before shingles go on, followed by seal-sticking flashing, then metal flashing interwoven with the roof shingles as they go on.
This creates an interlocked connection between flashing and shingles, resulting in a reliably waterproof surface. All this is why it makes sense to wait to carry your skylights up onto the roof and anchor them there when you’re already set up with roof jacks and roof boards to install shingles at that level.
