How to Fix a Concave Flat Roof

Fixing a concave flat roof as a do-it-yourself project can save a home owner the costs of hiring a contractor. Most flat roofs are not truly flat. Some are convex, or higher in the center. Others are concave. A concave flat roof angles downwards towards the center, typically to a drain. Both designs allow water to drain instead of pooling on the roof. When repairing a concave roof, the key is to keep the slope the same. This is achieved through replacing one piece of the roof base at a time to keep the original shape.

Drill
Push broom
Claw hammer
3/4-inch plywood
3-inch nails
Utility knife
Roofing felt
Heavy-duty staple gun and staples
Measuring tape
Fiberglass mesh
Roofing cement
Trowel
Roofing tar
Paint brush
Gravel

Instructions

- 1
  Locate the area in need of repair. From the top floor of the interior, attempt to locate leaks, sags or dark spots on the roof. Mark the spot by drilling up through the roof to make a location hole. Get a general idea where the area is on the exterior by locating guide points such as windows, vents or chimneys.
- 2
  Sweep away any gravel and debris from the roof. Reserve the gravel for use after the repair.
- 3
  Pull up the roofing in the damaged area. Use a claw hammer to remove the layers of old roofing and pry up the old piece of plywood sheathing.
- 4
  Place a new piece of 3/4-inch plywood in place of the removed piece. Attach the plywood with 3-inch nails to the exposed trusses every 6 to 8 inches. The piece may not appear flat. It must flush to the edges of the surrounding plywood to maintain the concave lay of the roof.
- 5
  Cover the new plywood with roofing felt. Cut the felt approximately 6 inches wider each direction to completely cover the area. Use a heavy-duty staple gun and staples every 3 to 4 inches around the edges to secure in place.
- 6
  Mix roofing cement according to the manufacturer's directions and apply to the repair area. Smooth a thin layer over the felt area with a trowel.
- 7
  Cut a piece of fiberglass mesh to cover the cement area. Use a utility knife to cut the mesh. Sink the mesh into the roofing concrete before it dries.
- 8
  Apply a second thin layer of roofing cement. Smooth with a trowel and allow to dry for at least six hours.
- 9
  Apply a layer of roofing tar with a paintbrush. Overlap the surrounding area slightly to seal.
- 10
  Cover the area with gravel. Use a push broom to smooth gravel over the repaired and surrounding areas.

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