How to Build a Flat Garage Roof

car in a garage with a flat roof
  • 16-40 hours
  • Intermediate
  • 1,500 2,800
What You'll Need
5/8th inch Plywood to cover the roof
2 scaffolding planks
The correct amount of timber for beams
Sufficient roof covering for roof size plus the overlaps
Metal ties to fix the Timber in place.
EMPD rubber membrane to cover the roof
ISO board to cover the roof
Glue to stick it to the plywood
Extra 2 x 4 wood to raise the roof where required
Rip saw

A flat garage roof is far easier to build than one with a pitch. Taking into consideration that you will be working at some height, it is advisable to use at least two ladders and to have a helper, as lifting the timber into place will take two sets of arms. Building a flat garage roof requires less covering than that of a pitched roof, but you would probably use the same amount of timber, if not slightly more. One thing to remember is that, although you call it a flat garage roof, it’s not entirely flat. It does have a slight raise to it to prevent it from pooling water. This is achieved by raising the roof in 1/8th increments every foot. To do this you use pieces 2’ x 4’ cut into sections and increasing by 1/8th inch every section up the beam. If you want drainage on both sides of the garage, do the same on both sides and meet in the middle.

Step 1 - Preparation

Place the beams in place. As you are building a flat garage roof you will require beams to be closer together to enable them to take any additional weight that snow might give to the roof. You will need to attach them tightly to the wall area. This you do by tying them down to the walls by using the wall ties and making sure the nails are hammered fast. You don’t want your roof flying off at the first high wind.

Step 2 - Sloping the Flat Garage Roof

construction work

Once your beams are firmly attached to the walls it is time to prepare them for the plywood. Evenly space the increasing sized 2 x 4 pieces of wood—the ones that increase by 1/8th inch every foot. Every beam must be in succession to the previous beam and the following beam. If the roof is going to slope to both sides, you need to do alternate sides to keep it even.

Step 3 - Cover the Beams with Plywood

Nail plywood across the entire beam structure to make a fit. Leave an expansion joint between plywood sheets. For this, you would use 4-inch nails and nail them into the beams. Cover the entire area with ISO board and fasten to the plywood. The ISO board is a snug fit, unlike the plywood.

Step 4 - Sweep the Boards

Clean up any dust or sawdust that might have collected on the boards by sweeping it clear.

Step 5 - Apply the Membrane

Roll out the glue over both the ISO and the membrane on the flat garage roof and allow for drying time. Once dry, adhere to the membrane in place carefully so as to have no bubbles.

Should you require it you can place downspouts and gutters to remove any water runoff from your flat garage roof.