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How to Patch a Roof Leak

The hard part about patching a leak in your roof is finding the leak itself. Once you’ve accomplished that, repairs are often relatively simple.

Leaking Roof, Roof Repair iStock

Have you ever woken up in the middle of the night to the steady of water, and you discover that the water is coming from a spot on the ceiling and not from any faucet? Chances are you have a leak in your roof. Find a bucket, place it under the drip, and try to go back to sleep. Tomorrow you can investigate.

How to Find a Roof Leak

If you have an accessible attic, that’s the place to start to look for the leak. As most roofs are pitched, where the leak appeared on your ceiling and where it entered the roof are probably two different places.

Look for Wet Spots on Sheathing or Rafters

Bring a flashlight and use it to detect any shiny or wet spots on the underside of the roof sheathing or rafters. If it’s an older leak, there may be rotted wood or dark mold growing around the site. If the rafter bays are insulated, you can also look for wet areas on the insulation batts.

If you’ve managed to pinpoint the leak’s location, it’s time to get a ladder and find its cause. It may be obvious—missing or torn shingles, for instance, or exposed fasteners. Check the flashing around vent pipes, dormers, and roof-pitch transition points.

Check for Holes or Tears in the Flashing

Cracks, tears, and holes in the flashing material are all likely suspects. Also, check for places where leaves and twigs may have built up and prevented water from draining away. It’s also a good time to assess the general health of the roof. Leaks can often be symptomatic of an old roof that needs to be replaced.

If the roof is flat or low-pitched, look for penetrations or blocked drains. Another trouble area may be indicated by puddles—low spots from which the water can’t drain. While you’re walking around, feel for soft spots under the roof surface that may indicate damage caused by water.

If you have trouble finding the leak, you might want to try to reproduce it by soaking different spots with a garden hose or bucket of water. Have someone on the inside watch out for telltale signs and call when they appear.

How to Fix a Roof Leak

Assuming you’ve found the leak, now onto the repair. Certain roof types, such as slate, tile, and galvalume (metal), should be repaired by a roofing contractor who has experience with that particular material. Also, roofing contractors have the necessary equipment (ladders, scaffolding, fall protection) to tackle really steep or challenging roof slopes, so if you’re not experienced, give them a call.

Repair costs can vary from less than $100 for a simple asphalt repair to $5,000 or more for slate, tile, or metal roofs.

Materials Needed

That said, if you’re going to do the work and you have asphalt shingles that need to be replaced, you’ll need the following:

It’s a good idea to coordinate your repairs with the outside temperature, if possible. Cold shingles become brittle, and hot shingles tear easily.

  1. To remove an old shingle, start by working the edge of the flat bar under the exposed edge to break the adhesive seal.
  2. Now slide the bar further under the shingle to find the nails. (There are typically eight nails in each shingle, four located at the sides and just above the vertical tab slits, and four across the very top of the shingle.)
  3. Try to center the nail head in the notch of the flat bar and, with a hammer, drive the bar under it to force it up. Try not to tear the surrounding shingles. You’ll have to repeat the procedure to loosen the nails at the very top of the shingle above as well.
  4. Remove the old shingle and slide the new shingle in its place. Holding up the shingle above, nail it just above its line of black adhesive.
  5. Apply a bead of sealant beneath the leading edges of any neighboring shingles that you may have loosened. There also may be places such as the ridge caps where roofing nail heads are exposed and present a potential source for leaks.
  6. Give the heads a coat of sealant.

Fixing Leaks in the Vent Boots

The flashing for plumbing vent pipes is often a source of leaks; the rubber boot around the pipe can develop cracks or tears. If the aluminum flashing is not leaking, it may be possible to replace just the boot. Remove the old boot and slide the replacement over the vent pipe.

  1. To replace the flashing, you’ll need to determine the proper diameter of the boot. (You may be able to find flashing units with adjustable boots like this one).
  2. Once you’ve found a new unit, use a flat bar to carefully pry up the exposed front edge of the old flashing, remove any nails, and lift the base and boot off the pipe.
  3. Apply a generous bead of roof sealant to the underside of the new flashing unit; slide it over the pipe and under the shingle course just above the pipe.
  4. Replace any nails, sealing all exposed heads—or use nails with neoprene washers.

Repairing the Flashing

There are lots of places where flashing—the copper, lead, or aluminum sheets that cover seams and transitions—can go wrong. In some cases, the repair is only a stopgap until the flashing can be replaced with the rest of the roof. If the counter flashing around your chimney is leaking, use roof sealant to repair cracks or glue individual pieces back down.

If you find that the source of the leak is in the step flashing along a dormer, a sidewall, or around a skylight, there are two options. The first is to replace the flashing, which involves the removal of shingles and siding, applying new flashing, and then re-installing shingles and siding. It’s an involved procedure, but sometimes a necessary one.

The second option is to repair the damaged flashing with a judicious application of roof sealant. The trick here is to keep the sealant’s surface relatively smooth–big beads or lumps of sealant can actually divert water and prevent it from draining properly. Nails used to secure flashing should not be exposed; if they are exposed, apply sealant to their heads.

Flat Roof Membranes

Before the advent of plastic membranes, flat roofs were covered by layers of tar paper or soldered sheet metal that was sealed with tar.

These days, a substrate of high-density fiberboard is attached to the roof with screws and big washers, and then covered with a rubberized material, usually made from EPDM (ethylene propylene diene terpolymer), that’s glued down to the roof deck.

How to Repair a Leak in the Membrane

To repair a big leak in the membrane, you’ll need EPDM or a similar material that matches the membrane on your roof, some rubber-to-rubber contact cement, acetone, a paint roller, a utility knife, a paintbrush, roof sealant, and a caulk gun.

  1. First, cut away the membrane around the leak to determine if the substrate is water damaged. If so, cut away enough membrane so that you can remove all the damaged fiberboard.
  2. Cut a replacement piece and screw it into place using the same screws and washers, shimming with the same material, if necessary, to make the new piece flush with the surrounding membrane.
  3. Next, cut a new piece of membrane that extends beyond the original by several inches. Lay the new patch over the damaged area and mark its outline.
  4. Use the acetone to clean both the new and old membrane, then apply a coat of contact cement across the back of the patch and to the area outlined.
  5. When the cement has dried, position the patch over the outlined area and, starting at one end, press the patch down onto the membrane, smoothing the surface as you go.
  6. Finish by applying a bead of sealant around the perimeter of the patch.