How to Repair a TPO Roof: Cleaning, Welded Patches, Open Seams, Punctures and Weathered Membrane
A good TPO repair is a clean surface, a round welded patch and a probed edge. A bad one is caulk over dirt. Here is the difference and when repair will not work.
- Professional
- 1 to 3 hours per repair
- Updated
The short answer
To repair a TPO roof, find the actual leak, scrub the area with TPO cleaner and let it dry, cut a patch of matching TPO with rounded corners that extends at least 2 inches past the damage, and hot-air weld it all the way around, with a finished weld at least 1-1/2 inches wide. Probe the edge. If weathered membrane will not weld, use a manufacturer-approved coating or recover.
TPO roofs fail in a few predictable ways: an open seam, a split at a corner or flashing, a puncture from dropped tools or foot traffic, a cracked pipe boot, and eventually an aged membrane that has become brittle. Most are repairable with a welded patch, provided the membrane is clean, dry and still weldable.
The second of those conditions is the one that is most often skipped. Dirt, algae and years of weathering leave a film on TPO that prevents fusion, and a patch welded over it looks perfect and peels within a season. The correct repair starts with cleaning, tests the weld on the actual roof and probes the edge, exactly like new work. For the welding method, see how to hot-air weld TPO seams.
Before repairing anything, find the real leak. Water travels under membrane and along insulation, so the stain inside the building is rarely under the hole. See how to find a leak on a flat roof.
Tools
- Hot-air hand welder with 1-1/2 inch nozzle
- Silicone roller
- Rounded-tip probe
- Scissors or hook blade utility knife
- Stiff nylon brush or scrub pad
- Marker and tape measure
- GFCI-protected power supply
- Clean cotton rags
Materials
- Matching TPO membrane for patches, same type and colour
- TPO membrane cleaner
- Round T-joint patches
- Membrane-compatible sealant for terminations only
- Manufacturer-approved liquid flashing or coating, for membrane that will not weld
Safety first
- Roofs that are leaking may have wet or weakened insulation and soft decking. Probe ahead of you when walking and keep off areas that spring or feel spongy.
- Wet membrane is slippery, and electric welders and wet roofs do not mix. Work only on dry membrane with GFCI protection.
- Tie off near edges and hatches. A patch repair is small work, but the roof is the same height as any other job.
Step by Step
Step 1: Confirm it is TPO and find the actual leak
Check the roof records, a label on a flashing or a manufacturer's marking on the membrane. If in doubt, ask the manufacturer or supplier to identify a small sample. A patch of the wrong type will not fuse. Note the thickness and colour of the existing membrane as well.
Leaks on a flat roof usually start at details: seams, T-joints, penetrations, corners, drains and edges. Examine those first, then the field. Wet insulation spreads water far from the entry point, and a leak that appears at a light fixture may originate at a drain ten feet away.
Step 2: Clean the area thoroughly
Weathered TPO carries dirt, algae, oxidation and sometimes the chalky residue of its own surface. Scrub an area at least 3 to 4 inches bigger than the patch in every direction, using the manufacturer's cleaner and a clean pad or stiff nylon brush. Wipe with clean rags, changing them often so dirt is not spread around.
Let the membrane dry completely. Any moisture between the patch and the roof steams and prevents fusion. Never use solvents the manufacturer has not approved, since some leave a residue that stops welding.
Tip: After cleaning, run a hand over the membrane: if it still feels chalky or leaves a film on your glove, scrub again.
Step 3: Prepare the damaged area
Cut away torn, frayed or lifted material with scissors or a sharp knife so the edges are clean. For a puncture, leave it as a small clean hole or cut a neat round. A hole that opened over wet insulation means the wet material has to be dealt with. Patching over wet insulation traps water and the membrane blisters.
For a failed seam, open the seam far enough to find sound, clean membrane on both sides, then prepare a cap strip rather than trying to reweld old membrane that has gone dirty or brittle.
Step 4: Cut a round patch that extends well past the damage
Use unreinforced or reinforced TPO as the manufacturer specifies, of the same type as the roof. Round the corners with scissors, as a sharp corner lifts and peels. The patch is usually at least 6 inches across for a small puncture, and larger in a wider area. The finished weld must be at least 1-1/2 inches wide all around, so the patch must be large enough for that beyond the edge of the damage.
Lay the patch over the area, trace its outline on the membrane with a marker and clean inside and around that outline once more. Repairs for seams are cap strips, typically 6 inches wide, centred over the open seam.
Step 5: Make a test weld on the roof
Conditions on the day matter. Cut a small scrap, place it on a clean spot of the roof near the repair and weld it with the settings you plan to use. After cooling, peel it. A good weld tears the membrane. A patch that peels clean off the existing roof means the old sheet will not weld, however clean it looks.
If the test weld fails, re-clean and test again. If it fails twice on a clean surface, see the section below on membrane that will not weld.
Step 6: Weld the patch all the way around
Tack the patch in position, then weld the edge. Keep the nozzle about half under the patch and half on the existing membrane so both surfaces reach fusion temperature together, and follow right behind with the roller. A squeeze-out bead along the edge is the sign of fusion.
Work steadily and do not hover. A scorched ring around the patch is a failed weld. A patch over a seam or T-joint is placed so that it covers the junction and extends past the cold spot. For a split at a corner or flashing, follow the methods in how to flash TPO walls, curbs and corners.
Step 7: Probe the repair and watch it
A rounded probe along the whole patch edge will find any spot that did not fuse. Mark, reweld and probe again. Check the surrounding membrane for any other damage: a patch often reveals a second problem nearby.
If the roof leaked into the building, note where the water appeared and check that spot after the next heavy rain. A repaired roof that still leaks has another entry point, which is when a more systematic approach is needed, such as flood testing.
When Old TPO Will Not Weld
Some membranes, especially older TPO that has been outside for many years, will not fuse with a new patch even when cleaned. The surface has oxidized, lost plasticizers or picked up contaminants from the roof. A test weld tells you. If a clean test weld peels away cleanly, hot-air welding will not hold on that sheet.
Options at that point are limited and the manufacturer should be consulted. Some offer a bonding primer, a liquid-applied flashing or a coating system approved for aged TPO. Others recommend a recover with a new membrane over a cover board, or a full replacement when much of the roof has aged. A sealant-only repair on membrane that will not weld is temporary, and should be treated that way.
Common Repair Types
- Puncture: clean, round patch extending at least 2 inches past the hole, welded and probed.
- Open seam: clean, reweld if the membrane is sound or cover with a 6 inch cap strip.
- T-joint leak: round patch over the junction with rounded corners.
- Cracked pipe boot: replace the boot rather than patching. See how to flash TPO penetrations.
- Blister: a blister over a fully adhered sheet may be cut, vented, dried and patched, but check for moisture in the insulation first.
Common Mistakes
- Not cleaning. Dirt and oxidation prevent fusion, and the patch peels within a season.
- Caulk over a leak. Sealant hides a problem for a few months and then fails, often pushing water further into the system.
- Patching over wet insulation. Trapped water destroys the insulation and blisters the membrane, and the roof keeps leaking.
- Square patch corners. Sharp corners lift and peel, so the patch opens at its corners first.
- Wrong membrane type. A PVC patch on TPO, or the reverse, does not fuse and lifts.
- Skipping the test weld. Old membrane may not fuse at all, and a failed patch looks like a good one until it rains.
When to Call a Professional
A small puncture on a roof you know is TPO can be a careful repair for someone with a welder and experience, but finding a leak, judging whether old membrane will weld and dealing with wet insulation are professional calls. A patch on the wrong cause only moves the water.
Call a roofer if the roof has multiple open seams, blisters over a wide area, ponding water, or stains on the ceiling that keep returning. Widespread problems usually mean the membrane is near the end of its life, and the question becomes recovering or replacing rather than patching. See roof repair and roof replacement.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Flat Roofing and Torch On service or the towns we cover.
How to Repair a TPO Roof: Questions
How do you patch a hole in a TPO roof?
Clean the area with TPO cleaner and let it dry, cut a patch of matching TPO with rounded corners that extends at least 2 inches beyond the hole, then hot-air weld it around its full edge so the finished weld is at least 1-1/2 inches wide. Roll it with a silicone roller and probe the edge after it cools.
Can you use roof cement or caulk on a TPO roof?
Only as a short-term emergency measure. Asphalt roof cement can damage some membranes and does not bond to TPO for long, and general caulk cracks and peels. A lasting repair is a welded patch of matching membrane. Where welding is impossible, use a product approved by the membrane manufacturer, and treat it as temporary.
Why will my old TPO roof not weld?
Weathered TPO can carry a film of dirt, algae and oxidized surface material that prevents fusion, and some aged membranes have lost enough of their surface layer that a new patch will not bond even when cleaned. A test weld on the roof tells you. If it peels cleanly off, the options are a manufacturer-approved coating or a recover.
How long does a TPO repair last?
A clean, correctly welded patch on sound membrane can last as long as the roof, because it is a fused joint. A repair made over dirt, over wet insulation or with sealant alone may fail within a season. The quality of the cleaning, the test weld and the probe check decides how long a repair lasts.
What causes a TPO roof to leak?
Most TPO leaks start at details, not in the field: open seams, unwelded T-joints, cracked pipe boots, split corners and failed terminations at walls and edges. Punctures from foot traffic and dropped tools are the next most common, followed by ponding that ages the membrane. Finding the true source before patching is essential.
Related Guides
All TPO and PVC Roofing guides|The full How-To Library
This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
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