How to repair flat roof flashing and seams

Where do flashing and seam leaks start?

Flashing is anything that seals the roof where it stops, turns or gets interrupted: at walls, curbs, pipes, skylights, drains and the roof edge. Seams are where two pieces of membrane overlap. Both are places where materials meet and move at different rates, and both are where flat roofs most often leak. Facility-roofing guidance points to penetrations, perimeters (where flexible membrane meets rigid sheet metal) and seams as the predictable trouble spots.

How do you fix open seams on asphalt roofs?

On modified bitumen and roll roofing, seams fail when the lap stops bonding and the edge starts to lift. IKO’s repair guidance for mod bit and built-up roofs is to cut away the unbonded edges and cover the area with a compatible coating and reinforcement. On mod bit, patches can also be heat-applied or set in a cold adhesive.

For a homeowner, the practical version is a three-course repair:

1. Trim loose, curled material back to where the lap is still bonded. Don’t tear it back further.
2. Clean and dry the area and brush off loose granules.
3. Apply sealant about 1/8 inch thick (Henry’s Pro-Grade spec, about 125 wet mils), extending at least 2 inches past each side of the seam.
4. Press in polyester fabric, 4 to 6 inches wide, with no wrinkles.
5. Cover the fabric completely with a second layer.

Choose the “cement” by what comes next. Asphalt flashing cement is fine if the roof will stay uncoated. If you plan to coat, use the coating maker’s sealant, such as acrylic for an acrylic coating or silicone for silicone. That way there’s no weathering wait and no bleed-through.

Torches: many mod bit roofs were torch-applied, and torch repairs are common in the trade. Open flame on a roof starts fires, especially at walls, wood blocking and under edges. Leave any torch work to a licensed, insured roofer.

How do you fix seams on EPDM, TPO and PVC?

EPDM seams are typically taped or glued. When an edge lifts, the fix is a cover strip. Holcim Elevate’s instructions for its EPDM tapes call for scrubbing the membrane, priming it with an EPDM primer such as its QuickPrime Plus, letting the primer flash off, then applying the tape and rolling it firmly with a narrow silicone roller. Carlisle sells a low-VOC primer for the same job. EternaBond’s RoofSeal tape is another option that lists EPDM and TPO. It should be applied over a cleaned surface, with the company’s primer recommended on porous or granulated surfaces. Keep asphalt off EPDM entirely.

TPO and PVC seams are heat-welded, and so are their repairs. GAF says weathered TPO and PVC must be scrubbed and wiped with the right membrane cleaner before welding, and any membrane contaminated with asphalt, adhesive or grease should be cut out and replaced. An industry repair manual from the Western States Roofing Contractors Association adds that some aged membranes need special techniques before they’ll take a weld. Leave these seams to a roofer, and if your roof is still under warranty, use a contractor the manufacturer approves.

How do you repair wall and curb flashing?

Wall flashing has two parts. The base flashing is membrane turned up the wall. The counterflashing is metal, or a termination bar with sealant, that covers its top edge. Many “wall leaks” are actually the top of the base flashing opening up behind loose metal.

IKO’s sequence for built-up roofs is a good model for any asphalt roof: remove the counterflashing as needed to expose the base flashing, apply a compatible coating and reinforcement, then reinstall or replace the metal. In practice:

• Refasten first. Re-secure loose counterflashing and termination bars, replace stripped or rusted fasteners, and reseal the top edge with a sealant made for that joint.
• Reinforce cracks. Three-course cracked base flashing, carrying fabric and sealant several inches up the wall and onto the roof.
• Check the wall above. Cracked stucco, open parapet caps and failed window seals can feed water behind perfect flashing. In Florida, give stucco parapets and walls a close look, since wind-driven rain can get in through them.

Curbs for AC units, hatches and skylights follow the same logic, with extra attention to the corners, where the membrane folds and is most likely to split.

What about pipe boots, pitch pans and drains?

Pipe boots are pre-formed collars around vent pipes. Rubber boots crack in the sun. Many can be replaced with a matching boot for your membrane; on asphalt roofs, three-course around a sound boot’s base.

Pitch pans (pitch pockets) are metal boxes around odd-shaped penetrations such as bundles of AC lines, filled with grout and topped with sealer. NRCA calls them a non-preferred detail that can be a constant maintenance problem, and notes that most roof system manufacturers carve them out of their warranties. NRCA’s guidance is to fill the pocket partway with non-shrink grout, then top it off with a pourable sealer compatible with the roof. When the sealer has shrunk below the rim or pulled away from the pipe, clean it and top it off so it sheds water away from the penetration.

Drains and scuppers fail where the membrane meets the drain body or the scupper box. Keep them clear, since a clogged drain makes water rise over flashings that were never meant to be underwater. Henry’s coating instructions seal at least 12 inches around a drain opening before coating. Cracked drain clamping rings and scupper re-flashing are roofer work.

Why does edge metal matter so much in a hurricane?

The roof edge takes the highest wind uplift, and when edge metal lets go, wind gets under the membrane and can peel it back. Research into wind loads on edge metal in the late 1990s was prompted by perimeter failures in Hurricanes Hugo and Andrew, and the International Building Code now requires low-slope metal edge systems to be tested to the ANSI/SPRI ES-1 standard.

For a homeowner, that means:

• Check edge metal before hurricane season (it runs June 1 to November 30) and after every big blow. Look for lifted sections, loose fasteners and open joints.
• Refasten loose metal, but leave replacement to a roofer. Replacement edge metal has to meet the code’s tested-design requirements, so hire a licensed roofer and check permit requirements with your local building department.
• Read our hurricane prep guide for the full pre-storm checklist.

When is it time to call a roofer?

• The repair needs a torch, a hot-air welder or edge-metal replacement.
• Flashing is failing along most of a wall, or seams are opening in many places. That points to an aging roof, not a one-off defect.
• The deck is soft, or you suspect wet insulation.
• The roof is under a manufacturer’s warranty that requires approved contractors.

Florida’s building code also limits how much of some older roofs can be repaired in a 12-month period before the whole roof section has to be redone. Ask your local building department how it applies before a large repair. Our guide on hiring a flat roofer covers licensing and insurance checks.

Safety: flashing work puts you at the roof edge and on walls. Use fall protection near unprotected edges, keep the ladder tied off and extended 3 feet above the roof, and wear gloves; cut sheet metal is sharp. Don’t stand on skylights, AC units or gutters.