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How to Patch a Leaking Rubber Roof

This Old House general contractor Tom Silva repairs a leaking flat roof with a new, long-lasting patch

In this video, This Old House general contractor Tom Silva shows a foolproof way to build a leak-free shower.


1. Locate the source of the leak on the roof, then use a utility knife to slice through the rubber roof around the suspected trouble spot.

2. Pull off the old rubber roofing from the cut area.

3. Unscrew and remove the large metal washers holding down the fiberboard substrate.

4. Cut away the old substrate from the roof using a utility knife and a framing square.

5. Measure and cut a new piece of substrate from high-density fiberboard.

6. Set the new substrate patch into place and secure it with two washers and screws.

7. If necessary, build up the height of the new substrate by covering it with strips of rubber roofing.

8. Cut a piece of rubber roofing approximately 12 inches wider and longer than the repair area.

9. Lay the rubber patch over the area and trace around it, marking its perimeter onto the roof.

10. Lift off the patch and brush membrane cleaner over the outlined repair area on the roof and onto the underside of the rubber patch.

11. Use a 3-inch paint roller to apply rubber-to-rubber contact cement to one half of the outlined repair area, and to one half of the underside of the rubber patch. Wait about 10 minutes for the cement to dry.

12. Set the rubber patch into place, aligning it with the outline marked earlier onto the roof.

13. Lightly press down the rubber patch, being careful not to stretch the rubber.

14. Now fold back the un-glued half of the rubber patch and apply contact cement to the remaining half of the roof and patch.

15. Wait 10 minutes for the cement to dry, then press down the second half of the patch.

16. Apply a bead of lap sealant around the perimeter of the rubber patch.