How To Slope A Flat Roof

Knowing how to slope a flat roof will create a better run off surface. Flat roofs can cause puddles to form from water and snow, which can be very damaging to your roof. Adding a slope is a great solution that is far more dependable than any other option.

To slope a flat roof, you will need:

  • Roof trusses
  • Tape measure
  • Level
  • Particle board
  • Roof tiles
  • Nails
  • Roofing nails
  • Nail gun
  • Hammer
  • 4x4s
  • 2x4s
  • Work goggles
  • Work gloves
  • Hand saw
  • Angled metal brackets
  • Roof Cement
  • Roofing Paper
  1. First, you will need to measure the length and width of your roof.  This will determine how long you will need your roof trusses to be. Each truss will need to be placed about 18” apart.  Divide the total length of your roof by 18 to determine the number of trusses you need.
  2. Select roof trusses that are slightly wider than the width of your flat roof.  This will allow proper overhang to ensure proper run off.  Generally, 3” to 6” is required on each side.
  3. Begin the slope installation process by installing 2×4 around the entire perimeter of the flat roof. These should be nailed down by applying one nail to every twelve inches.
  4. Install a truss at each end of your roof.  Measure the space between the two end trusses, and divide it by the number of roof trusses you have purchased to determine exactly how far apart the need to be. 
  5. Measure the space between the trusses again.  This will determine how long you will need to cut the 4×4 boards to act as braces.  Cut the 4×4 boards for the measured size. 
  6. Install the 4×4 boards between the trusses.  You will need two 4×4 between each truss. This is generally connected at the center of each 2×4 in the truss.  two angle brackets to each side to brace the 4×4 onto both 2x4s connected at the center of the truss. 
  7. Begin at one end of the roof, and lay the particle board beginning at the middle of the truss.  Nail the particle board every 12” down each truss.  You will want to nail into every truss under each board. 
  8. Begin to install the particle board on the opposite side of the center corner.  This piece of particle board should overlap the first installed board.  This will prevent water leakage.  Continue to install the particle board in this manner over the entire roof.
  9. Begin applying roofing paper.  Apply a thin layer of roofing cement over the particle board, and evenly press down the roofing paper.  This will act as a water proof backing for your roof tile. Overlap each layer as you apply.
  10. Begin to install roof tiles to your new sloped roof. Start at the bottom corner of the roof and work your way up.  Do not leave any space between these tiles, and continue to install by layers.  Complete each layer, overlapping each tile with the one below.  Offset each tile to misalign the breaks in the tiles.
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