How to Install Skylights in a Flat Roof

Installing a skylight on a flat roof might seem an unlikely task. Concerns about drainage would urge the cautious homeowner to avoid the idea. However, it is simple to frame a curb mounting that elevates and angles the skylights properly. The project requires knowledge of proper roofing techniques and safety principles. Improper installation can lead to water leaks which will damage the frame and ceiling of your building. The benefits of natural lighting are worth the extra effort.

Step 1 – Plan Design

Determine where you wish to place the skylights. Look at the model you wish to install. Consider factors such as the maximum opening angle and the opening mechanism as well as size and tint. Look for skylights with a pre-soldered lip for ease of installation. You will need to design a mounting curb that raises the skylight at least 4 inches from the roof surface and tilts it at a 15-degree angle. The inside dimensions of the curb will be the same as the manufacturer’s specified dimensions for the skylight opening. Build the curb by framing together 2x6 boards. The curb must last a long time so avoid any actions that would lead to cracking or warping. Avoid cutting through knots and drill pilot holes for nailing the frame together. Brace two opposite corners of the curb until it is installed so that it retains its shape.

Step 2 – Measure and Mark

Use your chalk line, tape, and level to mark the outline of the skylight on the ceiling and roof.

Step 3 – Cut Roof

First, cut away the ceiling beneath the location of the skylight. Remove insulation and wiring. If you will have to cut through ceiling rafters, brace them temporarily until the skylight is framed. Scrape off the roofing material at the skylight site and cut open the roof with a saw. Drop plumb lines from each corner of the hole to verify that it is in the right spot.

Step 4 – Frame Skylight

Install header joists perpendicular to the existing roof joists at around the skylight hole. If necessary, frame the light tunnel with 2x6 boards. Install the mounting curb above the opening.

Step 5 – Install Skylight

Lay the skylight onto the frame and screw or nail it into place. Cover the shaft with roofing material by slipping tar paper under the old shingles and then laying new shingles over the paper.

Step 6 – Install Flashing and Insulation

Install sheet metal flashing over a water and ice barrier around the perimeter of the skylight. Use a single piece of flashing in the front and step flashing up the sides of the curb. Install rigid insulation inside the light tunnel to prevent condensation and heat loss.