A stain on a ceiling tile tells you that water got in. It rarely tells you where. On a flat or low-slope commercial roof, water that slips past the membrane can run along the top of the deck, soak into insulation, follow a joist or a deck flute, and drop out wherever gravity and a gap in the deck finally let it through. The drip over the break room may start well across the building, near a rooftop unit nobody has looked at in years.
The good news is that flat roofs leak from a fairly short list of places. Almost every leak we trace on Midlands commercial buildings starts at a transition, an opening or a seam, not in the middle of an otherwise sound sheet of membrane. Knowing that list helps you describe the problem accurately, judge whether a repair actually addressed the cause, and avoid paying for the same leak twice.
Why the Leak Inside Is Rarely Under the Hole Outside
Before looking at sources, it helps to understand why tracing is hard. A low-slope roof is a sandwich: membrane on top, then insulation board (often several layers), sometimes a cover board, then the structural deck. Each layer gives water a new path.
- Steel decks carry water along their flutes, so a leak can show up a long way down the length of the flute from where it entered.
- Insulation acts like a sponge. Water can spread through wet board for weeks before enough collects to drip, which is why some leaks appear a day or two after the rain stops.
- Slope moves water toward drains, so an entry point is often uphill from the interior stain, not directly above it.
- Condensation can mimic a leak. Warm, humid interior air meeting a cold deck or uninsulated duct can drip on dry days. If it "leaks" when it has not rained, condensation belongs on the suspect list.
That is why a good leak investigation starts on the roof at the most likely sources nearest the stain and works outward, rather than patching whatever spot is directly overhead.
Seams and Laps in the Membrane
Every roll of membrane has to join the next one, and those joints are a common entry point on any system. How they fail depends on what you have:
- TPO and PVC seams are heat-welded. A weld that was too cold, contaminated with dirt or moisture during install, or stressed by building movement can open along a short section while the rest of the seam looks fine.
- EPDM seams rely on seam tape or adhesive. As the rubber ages and shrinks, those laps can peel back, especially at corners and where seams meet flashings.
- Modified bitumen and built-up roofs leak at lap edges where granules have worn away, where the asphalt has dried and cracked, or where blisters have formed at the laps.
Seam failures are often invisible from a walk-by. A roofer will probe welded seams with a dull tool to find voids and lift test lapped seams by hand. If you are unsure which membrane you have, the differences are laid out in our comparison of TPO, EPDM and PVC roofing systems.
Penetrations: Pipes, Conduit and Supports
Anything that passes through the roof is a hole the roofer had to seal around. Plumbing vents, gas lines, electrical conduit, exhaust stacks and equipment supports all need a flashing detail, and each type of detail has its own weak point:
- Pipe boots crack or split at the top clamp as they age in the sun.
- Pitch pans (sealant-filled pockets around odd-shaped penetrations) shrink and pull away from the pipe, leaving a gap that fills with water.
- Unflashed add-ons are a frequent culprit: a new conduit run or gas line added by another trade, sealed with a blob of caulk instead of a proper boot.
- Solar racking and sleepers add dozens of attachment points at once. If your building has panels, each attachment point deserves the same scrutiny as any other penetration.
A useful habit: keep a log of every trade that has worked on the roof. Leaks that start shortly after a new installation, a cable run or a plumbing change usually trace back to that work.
Rooftop Units, Curbs and Equipment
HVAC units sit on curbs, and curbs are one of the most leak-prone areas on any commercial roof. The membrane has to turn up the curb sides and be terminated and counterflashed at the top, and the corners are tricky to flash well. Common failures include open curb corners, flashing that has slipped down the curb, rusted or loose counterflashing, and gaps between the unit and the curb cap.
Rooftop equipment also attracts foot traffic. Service technicians drop screws, set tools on the membrane, drag panels and leave cut-off pieces of duct behind, all of which puncture membranes. Condensate lines that discharge onto the roof can wear one spot and keep it wet. When a leak sits near a unit, the first question is often when it was last serviced.
Skylights and roof hatches follow the same logic as curbs, with the added issue of failed glazing seals. Our guide to skylight leak causes and fixes goes into those in detail.
Drains, Scuppers and Gutters
Drainage points are where all the water on the roof is trying to go, so a small defect there sees far more water than the same defect in the field. Typical problems:
- Drain bowls and clamping rings: the membrane is clamped into the drain bowl by a ring. Loose or missing bolts, a cracked bowl, or membrane that has pulled out of the clamp lets water bypass the drain and run into the building around it.
- Scuppers: the opening through the parapet needs its own sheet-metal box and flashing. Open corners on that box, or sealant that has failed at the joint, let water into the wall.
- Clogged drains and gutters do not leak directly, but they back water up until it reaches a seam, flashing or penetration that was never meant to sit underwater.
That last point connects to standing water generally. If water stays on your roof long after a storm, read our article on when ponding water on a flat roof becomes a problem.
Parapets, Coping and Walls
Some "roof leaks" are really wall leaks. Parapet walls are exposed on both sides and on top, and water can enter through them in several ways before it ever reaches the roof membrane:
- Coping joints: the metal cap on the parapet is installed in sections. Open joints, failed joint covers or loose cleats let water into the top of the wall.
- Masonry: cracked mortar joints, porous brick and failed caulk around wall penetrations let water soak down inside the wall cavity and come out at the ceiling line.
- Wall-to-roof transitions where a lower roof meets a taller wall need counterflashing set into the wall. If that counterflashing is missing or only caulked to the surface, water runs behind the base flashing.
Leaks that appear along an exterior wall, especially during wind-driven rain from one direction, point strongly toward the parapet or wall rather than the field of the roof.
Flashing Terminations and Edges
Wherever the membrane stops, it has to be terminated: fastened with a termination bar, tucked under counterflashing, or wrapped into edge metal. These terminations depend on sealant and fasteners that weather faster than the membrane itself. A termination bar with failed sealant, edge metal that has lifted in wind, or base flashing that has pulled away from a wall all give water a direct path behind the roof.
Perimeter edges also take the brunt of wind uplift. A loose edge does not always leak right away, but it can let wind get under the membrane and turn a minor problem into a large one in the next storm.
How to Help Find the Source Faster
A roofer will find the leak, but the building's staff often hold the clues that make it quick. When you call, it helps to have:
- Photos of the interior stain or drip, with its location on a floor plan if you have one.
- When it leaks: every rain, only heavy rain, only with wind from a certain direction, or a day after rain stops.
- Any recent rooftop work: HVAC service, new equipment, cabling, solar or sign installation.
- Prior repair history, including any spots that have been patched more than once.
Resist the urge to send maintenance staff up with a tube of caulk. The wrong sealant can be incompatible with the membrane, can void a manufacturer warranty and usually hides the evidence a roofer needs. A routine commercial roof inspection catches many of these sources before they leak, and a written roof maintenance program keeps drains, curbs and flashings on a regular check cycle.
If you are chasing a leak you cannot pin down, our team handles commercial roof repair across the Midlands, from single leak traces to full re-roofs. You can request a free commercial roof assessment and we will trace the source and explain what we find.