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How to Repair a Flat Roof

Working on a flat roof is a little safer than a pitched roof. Flat roofs only have very slight slopes. However, falling the edge remains a hazard, and you should exercise caution. Wearing a fall prevention harness is appropriate. Ideally, you should wait for a sunny, dry day to follow the directions for how to repair a flat roof. Many problems only require a patch that you seal into place. Finding the actual leaks can sometimes be more challenging than fixing them.

Flat roofs have a membrane product adhered to the surface that keeps out water. Cracks can develop in the membrane as well as holes where water pools in a dip in the surface. How to repair a flat roof involves sealing cracks or gaps with fiberglass mesh, membrane patches, and caulk or roofing cement.

How to Repair a Flat Roof Directions

For finding the leak:

  1. If you see water stains on the ceiling, measure its distance from the two nearest walls.
  2. Climb onto the roof and apply these interior measurements to the exterior.
  3. Examine the approximate area where the stain is appearing underneath.
  4. If you don’t see any damage above the stain, search up slope from the stain.
  5. Any obvious holes or cracks are the source of leaks.
  6. Also plan to patch any areas that look worn out because, if they are not leaking now, they will.
  7. Check the edges of the roof as well for places where the membrane may be peeling back.

For fixing small cracks or holes:

  1. If the work area is wet, dry it thoroughly with some old cotton towels.
  2. Sweep the roof clean.
  3. Cut open blisters and bubbles that may have formed on the membrane with a utility knife.
  4. Trim them close to the membrane where it remains adhered to the roof deck.
  5. Scrape off any other damaged roof material.
  6. Do not cut into any roofing that remains sound.
  7. Spread 1/8 inch of roofing cement over the repair area with a trowel.
  8. Smooth the cement at least 6 inches beyond the perimeter of damage.
  9. Cut a piece of fiberglass mesh to fit the repair area.
  10. Lay it into the roofing cement.
  11. Spread more roofing cement over the mesh until you can no longer see the mesh.
  12. Sprinkle gravel into the fresh cement. The small stones protect the patch from sun damage.
  13. If you saw cracks along the edges of the roof, fill them with caulk.

For patching a membrane:

  1. For larger leaks, how to repair a flat roof calls for patching the membrane.
  2. You may buy a piece of roofing membrane or a roof repair kit.
  3. Some membrane materials require heating with a blow torch to activate their adhesive.
  4. Select a membrane product that matches your existing membrane.
  5. Consult the manufacturer’s directions for specific information about sticking it to the roof.
  6. Cut and trim away damaged membrane.
  7. Spread 1/8 inch of roofing cement into the exposed repair area.
  8. Squish cement under edges of undamaged roofing material.
  9. Cut a membrane patch to fit the repair area.
  10. Attach the patch.
  11. Spread more roofing cement over the patch.
  12. Cut another patch that is larger than the first patch by 6 inches in all directions.
  13. Attach this patch over the first patch.
  14. Spread more cement over the second patch and smooth it even with the rest of the roof.
  15. Press fresh gravel into the wet cement.

For resealing the entire roof:

  1. An old flat roof may need you to apply a fresh layer of sealant over the entire surface.
  2. Sweep the roof clean and make sure that the surface is dry.
  3. Working with a paint roller, apply 1/2 inch of bitumen primer over the roof.
  4. Consult the manufacturer’s directions for drying time. It should be about 20 minutes.
  5. Working in sections, unroll a strip of fiberglass mesh across the width of the roof.
  6. Cut the mesh as necessary to fit.
  7. Fold back the ends of the mesh and weight them down with boards or buckets on the roof.
  8. Once you have one row of mesh in place, stir bitumen roof sealant until smooth.
  9. Dip a paint broom into the sealant and work in one direction as you spread it.
  10. The sealant layer needs to be about 1/2 inch thick.
  11. Unfold the mesh and cover it with more sealant.
  12. Press the mesh into the sealant.
  13. Spread more sealant over the mesh.
  14. Repeat the process until the entire roof has mesh embedded in sealant and completely covered.
  15. About 12 hours later, apply a second layer of sealant.
  16. After allowing that coat to dry, inspect it for dips, gaps, or other imperfections.
  17. Fill them in with more sealant.
  18. For the final step, paint on a reflective coating to delay sun damage.

Tools & Materials

  • Tape measure
  • Caulking gun
  • Broom
  • Old towels
  • Utility knife
  • Trowel
  • Scissors
  • Handheld blow torch
  • Paint roller
  • Paint broom

Get Help From Our Expert Roofers

When you get how to repair a flat roof right, you’ll preserve the structure and avoid leaks. For professional flat roof repair, you can trust PJ Fitzpatrick. Our goal is always 100% customer satisfaction. Ask us for a roof repair estimate today.

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