How to install a skylight

A skylight installation can be tricky and complicated. You can have issues with anything from controlling dust to protecting floors to laying out a flared shaft ‒ all depending on the house, the roof and the room beneath. Plus, weaving the skylights and flashing into the existing roof system without tearing out and destroying shingles requires proper materials and techniques. Here is how to install a skylight.

Skylight plan

To install a skylight, start by planning a safe route to the opening location inside and the roof outside. Create a stable work area, using ladders and other methods appropriate for the ceiling height and roof pitch. The unit installed here, a Velux VS606, is about 4 feet by 4 feet and it’s heavy.

The skylight must be positioned between the eaves and roof ridge for optimum aesthetics. Some skylights have shafts, others ‒ like the ones I installed on this project ‒ are drywalled or trimmed flush along the rafter bottoms.

Instructions for the VS606 say to do a rough layout on the shingles and then cut a small exploratory hole from the outside in. The reason for this is so the skylight placement lines up with the roofing material layout. And on an installation in a ceiling 8 or more feet away from the living space, this makes sense.

However, this skylight was being installed in an occupied attic and due to knee walls below and a soffit above ‒ not to mention the fact that the unit was literally in the room like a window (you can see out of it) ‒ inches up or down made a huge difference. For the best possible position, we did the layout on the interior first and then drilled the exploratory hole to the outside.

The unit fit between two rafters, with part of a center rafter requiring removal. To position the unit up and down where the homeowner wanted it, I used a stick the same length as the skylight’s height (46¼ inches) so we could imagine the sight lines out once the skylight was in place. We got the position just right and marked it.

Next, I struck a level line across the top of the rough opening and marked the header locations. I drove nails into the rafters on the pencil line to hold my tape measure and pulled down to mark the bottom of the opening. With all four corners mapped out, I used a combination square to mark the cut layout and lines.

With all four corners mapped out, I used a combination square to mark the cut layout and lines.

Header installation

We cut the new framing for the header installation. To add a flare to the opening, Velux says to install a 2×6 flat next to the rough opening line and nail the header behind it. On the front edge of the 2×6 add another 2-by beveled 30-degrees as a nailer for drywall or trim.

Roof penetration

The last step inside is to pre-set a 3-inch deck screw in each corner of the framed opening. Don’t puncture the roof surface until you’re ready to take the skylight install to completion.

Before climbing to the roof to cut the hole, make sure you know how to use your ladders properly and either install roof brackets and a walk board or tie-off with a proper safety harness ‒ or both.

After the center rafter is cut, I use a flat bar and hammer to pry it free.

On the roof, snap lines between the screws you pre-set. Then remove the screws.

To get a square cut on the rafters, I laid it out on the sides and bottom of each rafter with a 7-inch Stanley combination square. I started the cut with my circular saw. I can get the blade right on the line and get a nice, low-impact cut. Then, I finished the cut with my recip saw. Avoid removing entire shingles. Instead, cut the shingles back 2 inches from the hole on the sides and 3 inches on the top and bottom to expose the roof decking.

After the center rafter is cut, I use a flat bar and hammer to pry it free. This also enables me to prep the job in bad weather. Or, if I have to leave for some reason, the roof system is still in tact. The last step for framing the header is installing joist hangers. The roof decking is likely to fall inside in one large piece, so make some provision on the inside for this.

Prep the skylight

Remove the glass from the skylight and bend the mounting tabs back. Then open the flashing package and set it aside.

Velux recommends a self-adhesive membrane be installed in all areas with severe weather. Starting at the bottom of the opening, I pried the shingles up and tucked 5-inch strips of Ice and Water Shield under the shingles on top of the asphalt paper. On each side, I tucked a piece under the shingles and over the bottom piece of membrane. When I reached the next layer of asphalt paper, I slid the Ice and Water Shield under it directly over the roof deck.

At the top, I tucked a piece of membrane directly over the roof deck, and then folded it over the side pieces. Once the pieces were installed, I reached under the shingles and peeled the backing off, enabling the membrane to stick to the roof system. This was frustrating, but it worked. If water does manage to get under the shingles and the asphalt paper by the skylight, it will be caught by the Ice and Water Shield and have a way to flow out.

Install the skylight

Set the skylight in position centered in the opening. Fasten it to the roof deck with the supplied nails.

Starting at the bottom of the opening, I pried the shingles up and tucked 5-inch strips of Ice and Water Shield under the shingles on top of the asphalt paper.

Place the skylight in the opening. On some roofs, it makes sense to pass the frame out the opening from the inside. Set the skylight in position centered in the opening. Fasten it to the roof deck with the supplied nails.

Then, working from the bottom up, wrap the frame with Ice and Water Shield. I added “patches” to cover the corners where I had cut the Ice and Water Shield. Re-install the glass.

Install the flashing kit according to manufacturer instructions. Be sure to get the head flashing under all the layers of shingles above the skylight opening or you’ll have a leak for sure.

Working it under all those shingles and asphalt paper is tricky, but doable. Carefully pry up the shingles and nails with a flat bar and slide the head flashing underneath.

Screws from the flashing package are aluminum and won’t stick to your magnetic driver bit holders. When I’d lost several, I used ⅜-inch lath (or Teks) screws as replacements and sealed the tops with roof cement to prevent leaks and corrosion.

Installing a skylight can be tough on shingles. Have a tube of roof cement at hand to make any needed repairs.

Testing for leaks

If you’re going to have a leak, its best to know about it after you install a skylight but before you take the ladders down. I ran a garden hose full-blast and drowned each window for five minutes before coming down from the roof. Not a drop got through and they are still dry.

–By Mark Clement, remodeling contractor and host of MyFixitUpLife.