Pros find a commercial roof leak by mapping where water shows up inside, then checking the roof upslope at seams, flashings, drains and penetrations. If that fails, they use infrared or impedance scans to map wet insulation, electronic leak detection to pinpoint holes in the membrane, and cores or a controlled water test to confirm.
If you have water on the floor and the last patch did not fix it, the problem is usually not the patch. It is that nobody found the real source. This guide explains how roof leaks travel, what to write down before the roofer shows up, which tests find what, and what a good leak report should show you.
OneRoof Solution has worked on commercial and industrial roofs in California since 2008 (CA License #930054). We offer 24/7 emergency dispatch from our Napa and Visalia offices and cover the whole state. We do not do homes.
Why the drip is rarely under the hole
Water does not always fall straight down. Once it gets through the roof, it can run along the deck, soak through insulation and follow seams before it finds a way into the building.
The Canadian Roofing Contractors Association tells roofers to start right above the leak, but not to stop there. Water may be moving inside the roof, so the whole area, nearby walls and rooftop equipment should be checked. Roofers should also look upslope, because water can travel from higher up. On steel decks, the grooves in the metal act like gutters and can carry water a long way before it drips (CRCA).
Here is a simple example (an illustration, not a real job). A drip shows up over a packing line. The roof right above it looks fine. Twenty feet upslope, a cracked seal at the base of a rooftop unit lets water in. The water runs down the deck grooves and drops out at a fastener over the line. Patch the roof over the drip and the leak comes back with the next storm.
Step 1: What you can do before the roofer arrives
These notes help the roofer find the source faster. They follow the questions the CRCA tells roofers to ask building staff.
- Write down when it leaks. Note the date, how hard it rained, and how long after the rain started the drip began. Note if it keeps dripping after the rain stops.
- Note the wind. If it only leaks in wind-driven rain, and only from one direction, the cause is often a wall, parapet or flashing, not the flat part of the roof.
- Mark the spot. Put the drip location on a floor plan and take photos of the ceiling, the stain and anything nearby, like drains, vents or ducts.
- Rule out other water. Plumbing, condensate lines and HVAC equipment can look like a roof leak.
- Protect people, product and equipment. Move or cover what you can. At food plants, follow your own food safety steps.
- Stay off the roof. Do not send untrained staff up, and never in the rain. Leave the roof to a trained crew.
Step 2: The visual inspection (where most leaks start)
A trained roofer starts at the spot above the leak and works out from there in a set pattern. The CRCA lists what they look for: cracks, blisters, soft spots and fasteners that have backed out. Pressing on a spot that hides a hole will often push out air or water.
Common leak sources on a flat roof:
- Seams and laps where two sheets of roofing meet.
- Flashings at curbs, walls and parapets (the pieces that seal the roof where it turns up a wall).
- Drains and scuppers, especially if they are clogged and water sits around them.
- Penetrations like pipes, conduit, vents, anchors and HVAC stands. The CRCA calls these some of the most common leak points, and an IIBEC paper says penetrations and vertical turn-ups are the source of a large share of leaks (IIBEC).
- Parapet caps and walls, where cracks let water in above the roof.
- Ponding areas, where water stands for days after rain.
Rooftop equipment and service traffic
Food plants, cold storage buildings and busy warehouses often have a lot of rooftop units, process exhaust and service crews walking the roof. Every unit is a set of curbs, pipes and flashings, and every service visit is a chance for a dropped tool or a cut in the membrane. On these roofs, check the equipment areas first. See the roofing for food production and cold storage we do.
Step 3: Leak detection test methods compared
When a visual check does not find the source, there are several tests. Each one finds something different. IIBEC says it plainly: no one method gives all the answers, and a mix of methods is usually needed (IIBEC). ASTM D7053 is the standard guide for finding the cause of leaks on low-slope roofs (ASTM D7053).
| Method | What it finds | What it cannot do | Best use | Standard |
|---|---|---|---|---|
| Infrared (thermal) scan | Likely wet insulation under the roof, seen at night as warm or cool patches. Can be done on foot or from the air. | Does not find the hole itself, or say where water got in. Needs the right weather. | Mapping how much of the roof is wet | ASTM C1153 |
| Electrical impedance (moisture) scan | Areas that are wetter than the rest, under the membrane. Does not need a temperature swing. | Not accurate on EPDM or membranes with conductive additives. Metal near the surface can throw it off. | Moisture mapping when infrared weather is poor | ASTM D7954 |
| Nuclear moisture meter | Moisture at single points, read on a grid. | Reads only a small spot at a time, so the grid may miss things. | An older method you may still see in bids | NIST Tech Note 1146 (overview) |
| Electronic leak detection (ELD), low voltage | The actual hole in the membrane, even very small pinholes. | Needs a wet roof surface and a conductive deck right under the membrane. Insulation or cover board under the membrane can block it. Not for black EPDM. | Pinpointing holes | ASTM D7877 |
| Electronic leak detection (ELD), high voltage | The actual hole in the membrane, on a dry roof, including at penetrations and wall turn-ups. | Same deck limits as low voltage. Not for black EPDM. | Pinpointing holes, including around details | ASTM D7877 |
| Controlled water test | Reproduces the leak to confirm where water gets in. Water is run on one area at a time, low to high, with someone watching inside. | Slow, and can cause more damage if not controlled. | Confirming a suspected entry point | ASTM D7053 (diagnostic water testing) |
| Core cuts | A small sample that shows if insulation is wet and what the roof is made of. | Only shows one spot, and must be patched properly. | Checking what a scan shows | Used to verify infrared per ASTM C1153 |
Sources for this table: IIBEC, The Leak Stops Here; IIBEC Technical Advisory 018 on ELD; ASTM C1153; ASTM D7954; ASTM D7877; NIST Technical Note 1146.
A few things most leak guides leave out:
- Infrared depends on the weather that night. IIBEC lists four things that help: a big difference between inside and outside temperature, a fast change in temperature in the hours before, plenty of sun that day, and low wind. Without them, the scan is much less useful. So a “clean” infrared scan is not proof that your roof is dry.
- Infrared and impedance need water to find. If the roof has a hole but the insulation under it has dried out, these scans may show nothing.
- ELD does not work on every roof. It needs a membrane that does not conduct electricity, sitting right on a deck that does. Black EPDM conducts electricity, so standard ELD cannot test it without special primers and equipment (IIBEC Technical Advisory 018).
- Flood testing is not a roof test. Holding water on a roof for a day or more (ASTM D5957) is meant for waterproofing over occupied space, not for roofing systems, according to IIBEC.
How we approach it
We start with your notes and a full visual inspection, then pick the tests that fit your roof and its membrane, and show you what we find. Our Rainshield Roof Care Program also includes infrared scans and scheduled inspections to catch wet areas early.
Infrared roof scan cost and what the report should include
We have not found a reliable, current price range for infrared roof scans that we can stand behind, so we are not quoting one here. We quote every roof after we see it. What moves the price:
- Roof size
- Roof access (ladders, hatches, lifts, or a drone)
- Night work, since infrared scans are done after dark
- How many core cuts are needed to confirm the findings
- Drone scan or walk-on scan
- How detailed the written report is
What a good leak or moisture report shows:
- A roof plan with the wet areas marked
- Infrared images next to normal photos of the same spots
- Core results (what was found, and how wet)
- The likely cause or causes of the leak
- The square footage of wet material
- A clear recommendation: repair, recoat, recover or replace
That square footage matters. GAF uses 25% of the roof area with wet material as the point where a full tear-off may be considered (see our guide to roof coating vs replacement).
When the leak means repair, recoat or replace
A single leak at a flashing is usually a repair. But if a scan shows a lot of wet insulation, the leak is a sign of a bigger problem. Wet insulation has to be cut out and replaced before any coating or new roof layer goes on top. Covering it traps the water. See our recent reroof projects.
Watch your warranty. If your roof is under a manufacturer warranty, read the terms before anyone works on it. Carlisle’s Golden Seal warranty, for example, asks the owner to report a leak within 30 days. It can be voided by repairs or rooftop additions made without Carlisle’s written OK, or by not taking reasonable care of the roof (Carlisle warranty PDF). Terms vary, so check yours, and use a contractor your manufacturer authorizes. We are certified or authorized applicators for Carlisle, GAF, Duro-Last, Versico, Henry, CertainTeed and American Standard Coatings.
How to stop repeat leaks
- Inspect twice a year and after big storms. Carlisle’s care guide asks for roof checks at least every six months, ideally spring and fall, and after heavy rain, high winds or hail (see our guide to how often a commercial roof should be inspected).
- Keep drains and scuppers clear.
- Control roof traffic. Keep a log of who goes up and why, and ask service crews to report any damage.
- Log every leak. Date, weather, location and what was fixed. Over time, the pattern points to the cause.
Our Rainshield Roof Care Program is built for this. It starts with a full inspection to set your baseline, adds advanced scoring and infrared scans, keeps every detail of your roof in a secure digital roof passport, and sends you an easy-to-understand report with photos and recommendations after each visit.
FAQ
How do you find a leak in a flat commercial roof?
Start inside: mark where the water shows and note the weather. Then a roofer checks the roof above and upslope at seams, flashings, drains and penetrations. If the source is still hidden, infrared, impedance or electronic leak tests narrow it down, and a core cut or water test confirms it.
Why is my roof leaking only in heavy rain?
Heavy or wind-driven rain can push water into spots that stay dry in light rain, like walls, parapets, flashings and clogged drains where water backs up. Note the wind direction when it leaks. It helps find the source.
Can an infrared scan find a roof leak?
It finds wet insulation, not the hole. It shows where water has spread, which helps narrow the search. It also needs the right weather on the night of the scan, so a clean scan does not prove a dry roof.
How much does an infrared roof scan cost?
It depends on roof size, access, night work, drone or walk-on, how many cores are cut and how detailed the report is. We quote every roof after we see it.
What is electronic leak detection on a roof?
It uses a small electric current to find holes in the roof membrane. Where the current gets through, there is a hole. Low voltage needs a wet roof. High voltage works on a dry roof. Both need a conductive deck right under the membrane.
Does electronic leak detection work on EPDM?
Not with standard methods. Black EPDM conducts electricity, so IIBEC says it cannot be tested accurately without special primers and equipment made for it.
What is a roof moisture survey and do I need one before recoating?
It is a scan, usually infrared or impedance, checked with core cuts, that maps wet insulation under the roof. Yes, get one before recoating. Wet areas must be replaced first, and if a lot of the roof is wet, replacement may make more sense.
Will a leak void my roof warranty?
A leak itself usually does not, but how you handle it can. Many warranties require you to report leaks quickly (Carlisle asks for 30 days) and to use an authorized contractor. Unapproved repairs can void the warranty.
Leak back again? Book a leak investigation.
We trace it to the source, show you photos and test results, and tell you straight whether it is a repair or a bigger problem. We offer 24/7 dispatch, and we respond fast.
Need a crew out for a known leak? Request a leak repair visit.
24/7 emergency line: (559) 697-6252. Main office: (855) 762-7663. Napa office: (707) 440-2351. Email: sales@1roofsolution.com.
Stop repeat leaks with scheduled inspections and infrared scans.