The short answer
Start inside: mark the drip, note when it leaks and look for staining that shows how far water has run. On the roof, the source is usually upslope of the drip at a flashing, seam, drain or penetration. Water travels under membrane and along the deck, so the hole is often well away from the stain. Leave the roof walk to someone with fall protection.
On this page
A drip in the middle of a warehouse floor tells you there is a leak. It rarely tells you where. On a flat commercial roof, water can enter at one point, run under the membrane or along the deck, and drop through a fastener hole or seam in the ceiling 20 or 30 feet away. Finding the real entry point is most of the job.
Start inside the building
You can learn a lot without setting foot on the roof.
- Mark the drip. Put tape on the floor and the ceiling grid, and photograph it with the date.
- Lift a ceiling tile nearby if you can do it safely from a ladder. Look at the underside of the deck for wet streaks, rust trails or water sitting in the flutes of a steel deck. The trail usually points uphill toward the source.
- Note the pattern. Does it leak every rain, only in heavy rain, only with wind from one direction, after snow melts, or when the air conditioning runs? Each points somewhere different.
- Check what is above. Use a roof plan or walk the floor below to see whether the drip sits under or near a rooftop unit, skylight, pipe, drain or wall.
What the pattern tells you
| When it leaks | Likely source |
|---|---|
| Every rain, light or heavy | A hole, open seam or failed flashing |
| Only in heavy rain | Ponding water reaching a weak detail; partly blocked drain |
| Only in wind-driven rain | Edge metal, wall flashing, coping or a gap in the wall above the roof |
| After snow, in a thaw | Ice at drains or scuppers backing water up |
| When cooling runs, dry weather | Condensate line or drain pan on a rooftop unit, not the roof |
Where flat roofs usually leak
From experience across the industry, the same details fail again and again:
- Flashings at rooftop unit curbs, walls, skylights and pipe boots.
- Seams and laps that were poorly welded, taped or sealed, or have opened with age.
- Drains and scuppers, blocked with leaves or loose at the clamping ring.
- Perimeter edge metal lifted by wind. FEMA’s post-hurricane advisories also point to poorly attached rooftop equipment and its loose panels tearing membrane.
- Punctures where service crews walk, kneel or drop screws, often around HVAC units.
- Pitch pans, the filled boxes around odd-shaped penetrations, where the filler has shrunk.
How a roofer traces it
A commercial roofer will usually:
- Walk the area above and upslope of the drip, checking every detail by eye and by probing seams.
- Use a moisture meter or take cores to see whether insulation is wet and how far it has spread.
- On larger roofs, use an infrared scan at dusk on a sunny day: wet insulation holds heat and shows up warm.
- Sometimes run a water test, flooding one area at a time while someone watches below.
Why you should not go up yourself
Wet membrane is slippery, skylights can give way, and the edge is often unprotected. OSHA’s fall protection rule for construction requires guardrails, nets or personal fall arrest near unprotected edges six feet or more above a lower level. The National Roofing Contractors Association likewise advises that people without the experience and equipment should not walk roofs. Your part is the inside work above; the roof walk belongs to a crew that is equipped for it.
What finding it costs
Most leak repairs on low-slope roofs cost $300 to $1,100, averaging about $700, according to national cost guides from January 2026. If the leak has soaked the insulation, expect more. Our repair cost estimator gives a range for your roof, and the leak triage checklist covers the first hour.
When you are ready, send us the ZIP code of the building and we will look for a commercial roof repair specialist who covers it.