Installing a skylight in a flat roof

It may seam quite impossible to install a skylight on a flat roof, mainly because of the water drainage. But this can be achieved with some advanced skills and with the proper techniques and equipment. The idea is to frame a curb mounting that elevates and angles the skylights.

Some of the tools and materials needed for the job are:

  • 2×6 boards
  • Roofing tar paper
  • Water and ice membrane
  • Shingles or rolled roofing
  • Level
  • Square
  • Crowbar
  • Tin shears
  • Drill
  • Polyurethane caulk
  • Hammer
  • Screwdriver
  • Knife
  • Chalk line
  • Plumb bob
  • Measuring tape
  • Circular saw
  • Reciprocating saw
  • Keyhole saw
  • Stainless steel
  • Extension ladder
  • Dust mask
  • Safety goggles
  • Gloves
  • Extension cord


First of all determine where do you wish to place the skylights. You will need to consider the maximum opening angle and the opening mechanism. For the ease of installation, look for skylights with a pre-soldered lip. You will need to design a mounting curb that will need to elevate the skylight by 4 inches and angle it at a 15 degrees angle. You can build the frame by framing together 2×6 boards. Avoid cutting through knots and drill pilot holes for nailing the frame. This will prevent any cracking and will make the frame last longer.

Measurements and cutting

Mark the outline of the skylight on the ceiling and roof. Use a chalk line, tape and level.

The cut away the ceiling beneath the skylight. You will need to remove any wiring and insulation. Brace ceiling rafters temporarily until the skylight is framed. Then get rid of the roofing material from the site and cut the roof with a saw. Use drop plumb lines to assure that the spot is correct.

Framing the skylight

Next you will need to install header joists perpendicular to the existing roof joists at around the skylight hole. Frame the light tunnel with 2×6 boards. The mounting curb will need to be installed at the opening.

Installing the skylight

Place the skylight onto the frame and then fix it with nails. Use roofing material to cover the shaft. Slip tar paper under the old shingles and then lay new shingles over it.

Flashing and insulation

The final thing that you will need to do is install sheet metal flashings over the water and ice barrier around the perimeter of the skylight. Use one piece of flashing in the front and step flashing up the sides of the curb. Prevent heat loss by installing a rigid insulation inside the light tunnel.

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