DIY Network

How to Slope and Waterproof a Roof

The DIY to the Rescue crew offers step-by-step instructions on how to give a flat roof a declining slope and waterproof it.

More in Remodeling

give flat roof declining slope and waterproof it
  • Time


  • Price Range

    $2,500 - $5,000

  • Difficulty



Step 1: Cut the Face Board and Trim the Joists

The deck must first be cut on the front and at the joists to allow for a slope. As a general rule, the slope should be about a 1/4" per foot. (This deck is 10', so the slope was 2-1/2".)

Cut down the face board using the pitch line as your guide. Using a circular saw, remove the 2-1/2" that will give the new roof a declining slope. By cutting this board first, you have an accurate guide for cutting down each of the perpendicular joists (Image 1).

Again, using the trimmed front board as a guide, snap a pitch line along each joist from the house end down to the front board (Image 2). Continue this process until all the joists have been trimmed (Image 3).

Step 2: Install the Roof Decking

Once all the joists have been trimmed, install the roof decking, which in this case, is made up of oriented strand board (Image 1). Start by laying the first board parallel to the house; groove in, tongue out.

Using galvanized roofing nails, face the boards to the joists below, nailing every 8" or so. Make certain the flat ends always land on a joist so they can be nailed down securely (Image 2).

Install the end joints of each adjacent panel with a minimum spacing of 1/8" between them to allow for expansion and contraction (Image 3).

Step 3: Apply the Aluminum Flashing

Once all the panels are nailed in (Image 1), the next step is applying the aluminum flashing, all the way around the roof. Using standard roofing nails, attach the strips of aluminum flashing directly to the new roofing material (Image 2).

Step 4: Waterproof the Area

After the flashing is applied, waterproof the area with a single sheet of synthetic rubber (Image 1). Roll this out evenly covering the entire surface and then trim it to size before gluing it down.

After making sure the rubber is positioned evenly along the front edge and it has been smoothed out completely, the rubber is ready to be attached to the roof.

First, fold half of the sheet back onto itself (Image 2). Next, using stand paint rollers, apply contact cement directly to the roof and the flashing (Image 3).

After 15 minutes the cement will become tacky and the rubber can be rolled back out over the roof. Smooth out any bubbles or kinks as you go (Image 4).
Repeat the process on the other half and trim up the outside edges.




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