Installing a Skylight

Air, light and beauty: Real-world tricks for the best-ever roof window installations.

Written and Photos by: Steve Maxwell

Every time I enjoy the look and feel of a room enhanced by a good roof window, I think of Villum Kann Rasmussen. He was the Danish inventor and businessman who developed the first modern roof window system in 1941. His passion for adding light, air and beauty to otherwise dark interior spaces has since spread around the world.

Today’s best roof windows are reliable, surprisingly beautiful and extremely well made. Modern flashing systems and roof membranes make it easy to create leak-free installations, though that doesn’t mean total success is a slam-dunk. There’s more to a great roof window than simply keeping water out.

It’s one thing to have access to great roof window hardware and flashing that’s reliable by design, but quite another to create an installation with maximum aesthetics, great energy performance and freedom from wintertime condensation. For this happy outcome, you need to take some rather boring planning steps, coupled with specific hands-on know-how and construction details that are unique to the roof window game. Just don’t be fooled. Not all tricks of the roof window trade are found in the instruction pamphlet that came in the package.

Big-Picture Planning

Roof windows are all about improving the ambience of indoor spaces, but they can also make the outside of your home surprisingly ugly if their size, shape and location aren’t chosen with care. The best way to make sure your new skylights don’t make your house look silly from the street is to cut 1⁄4-inch-thick OSB templates to the same size as your proposed skylights. Place these on your roof and take a look from the ground. Are they a reasonable size? How does the placement look? Too high? Too low? Start with the largest template size you think you might want, and then whittle them down until you have the size that looks best.

Be aware that different roof frame designs have an effect on the size and shape of roof windows that are available. Old-style roofs framed with rafters, for instance, leave your options wide open. Rafters can be cut and re-framed in many ways, allowing you to choose roof window size and shape based on aesthetic grounds only. At the opposite end of the scale are roofs framed with trusses. They can’t be cut or modified easily on site without affecting their strength. In cases like this, you’re restricted to narrow roof windows meant to fit between undisturbed trusses spaced 16 or 24 inches on centre.

As you mark out the precise location for your roof windows, you need to think about maximizing the transmission of light and fresh air into the space you’re enhancing. A simple hole in the roof with straight sides lets in far less light and air than a roof window installation with flared sides. Flares are harder to build, but they’re essential. Don’t take short-cuts here.

If your roof window is going into a finished attic or cathedral ceiling where the roof structure is eight to 16 inches thick, aim to angle the top of the opening so it’s horizontal to the floor, and the bottom of the opening so it’s perpendicular to the floor. Both sides should flare out at about 45 degrees. If you need to build an extended light tunnel to pass through an unfinished attic, begin to flare the openings as close to the glass as feasible.

Layout and Framing

Right about now you’ll find it handy to mark things out on the underside of your roof sheathing and rafters in full scale using a black marker, tape measure, carpenter’s square and four-foot level. This way you can be sure you’re leaving enough room for all the critical framing details. It’s safest to start with a piece of cardboard that’s the size of the opening required for your roof window (typically 11⁄2 inches longer and wider than the window itself). You’ll also need to mark where to cut rafters so that double horizontal headers can be installed to span their ends.

As you plan, keep this in mind: for every rafter you cut while framing, you need to add another full-length rafter sistered to the uncut ones on each side of the new opening. Make this additional rafter as long as possible while still being able to shoehorn it in on the top of the wall and the peak of the roof for support. Use construction adhesive and three-inch deck screws to secure the additional rafter to the existing one. Many roof window installations require two rafters to be cut, so doubling up on each side of the opening re-establishes proper strength.

Since you’ll be working in close quarters, you’ll find it difficult and dirty to swing a hammer and pound nails. That’s why it makes much more sense to join all your additional roof framing using deck screws. They’re stronger than nails and much easier to drive, especially when you’re using an impact driver to torque them down. Also, as added insurance, support the ends of the horizontal headers using double joist hangers in addition to screws. They’re cheap, quick to install, and take part of the roof load off the screws.

Insulation Details

A crucial issue in our cold, Canadian climate is insulation of the light tunnel that connects the ceiling to the roof window. Long tunnel or short, without enough R-value and resistance to air infiltration, condensation will build up on the inside face of the light tunnel, causing big problems that are annoyingly difficult to remedy after the fact.

A key ally in your insulation work is foam. With a high R-value and resistance to air infiltration, you can’t beat foam for roof window work. Apply rigid sheets of two-inch-thick extruded polystyrene foam to the insides of your light tunnel frame (one or two layers), seal the gaps with spray on polyurethane foam, and then apply drywall right over top. If your light tunnel is short, as in an attic or cathedral ceiling installation, build your flared sides with plywood and drywall, and then fill the tapered edges of the opening by injecting low-expansion poly foam into the cavities through 1⁄2-inch diameter holes drilled every 12 inches. When you’re done, patch these injection holes and paint.

At first glance, roof window installation seems easy. That’s why subtle details are often overlooked. Many installations fall short of their potential. But address the details that go beyond the obvious, and you’ll enjoy the kind of ambiance, fresh air and reliability that Mr. Rasmussen had in mind when he brought roof windows to the world.