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:
- If you see water stains on the ceiling, measure its distance from the two nearest walls.
- Climb onto the roof and apply these interior measurements to the exterior.
- Examine the approximate area where the stain is appearing underneath.
- If you don’t see any damage above the stain, search up slope from the stain.
- Any obvious holes or cracks are the source of leaks.
- Also plan to patch any areas that look worn out because, if they are not leaking now, they will.
- Check the edges of the roof as well for places where the membrane may be peeling back.
For fixing small cracks or holes:
- If the work area is wet, dry it thoroughly with some old cotton towels.
- Sweep the roof clean.
- Cut open blisters and bubbles that may have formed on the membrane with a utility knife.
- Trim them close to the membrane where it remains adhered to the roof deck.
- Scrape off any other damaged roof material.
- Do not cut into any roofing that remains sound.
- Spread 1/8 inch of roofing cement over the repair area with a trowel.
- Smooth the cement at least 6 inches beyond the perimeter of damage.
- Cut a piece of fiberglass mesh to fit the repair area.
- Lay it into the roofing cement.
- Spread more roofing cement over the mesh until you can no longer see the mesh.
- Sprinkle gravel into the fresh cement. The small stones protect the patch from sun damage.
- If you saw cracks along the edges of the roof, fill them with caulk.
For patching a membrane:
- For larger leaks, how to repair a flat roof calls for patching the membrane.
- You may buy a piece of roofing membrane or a roof repair kit.
- Some membrane materials require heating with a blow torch to activate their adhesive.
- Select a membrane product that matches your existing membrane.
- Consult the manufacturer’s directions for specific information about sticking it to the roof.
- Cut and trim away damaged membrane.
- Spread 1/8 inch of roofing cement into the exposed repair area.
- Squish cement under edges of undamaged roofing material.
- Cut a membrane patch to fit the repair area.
- Attach the patch.
- Spread more roofing cement over the patch.
- Cut another patch that is larger than the first patch by 6 inches in all directions.
- Attach this patch over the first patch.
- Spread more cement over the second patch and smooth it even with the rest of the roof.
- Press fresh gravel into the wet cement.
For resealing the entire roof:
- An old flat roof may need you to apply a fresh layer of sealant over the entire surface.
- Sweep the roof clean and make sure that the surface is dry.
- Working with a paint roller, apply 1/2 inch of bitumen primer over the roof.
- Consult the manufacturer’s directions for drying time. It should be about 20 minutes.
- Working in sections, unroll a strip of fiberglass mesh across the width of the roof.
- Cut the mesh as necessary to fit.
- Fold back the ends of the mesh and weight them down with boards or buckets on the roof.
- Once you have one row of mesh in place, stir bitumen roof sealant until smooth.
- Dip a paint broom into the sealant and work in one direction as you spread it.
- The sealant layer needs to be about 1/2 inch thick.
- Unfold the mesh and cover it with more sealant.
- Press the mesh into the sealant.
- Spread more sealant over the mesh.
- Repeat the process until the entire roof has mesh embedded in sealant and completely covered.
- About 12 hours later, apply a second layer of sealant.
- After allowing that coat to dry, inspect it for dips, gaps, or other imperfections.
- Fill them in with more sealant.
- For the final step, paint on a reflective coating to delay sun damage.