Commercial Roof Leaking? Find the Source Before You Patch

Why can't we find where the roof is leaking?

Because water does not fall straight down. It enters the roof at a seam or a penetration, travels along the deck, follows a purlin or a insulation joint, and drips wherever it finds an opening. On a large commercial roof, the entry point can be a long way from the ceiling stain.

That is why patching under the stain almost never works.

The symptom: the leak moves

Classic signs you are chasing rather than fixing: the stain appears only in certain storms, it shows up in a different tile after each rain, a repair "worked" until the wind came from another direction, or you have several patches in one area and water still gets in.

The problem: leaks are found by guessing

The usual approach is to look at the ceiling, go to the corresponding spot on the roof, and seal whatever looks suspicious. That misses the real entry point and adds a new patch edge, which becomes the next failure. Do that four or five times and the roof is harder to diagnose than when you started.

Meanwhile the water has been in the insulation the whole time. Wet insulation loses R-value, corrodes steel deck, and does not dry out on its own.

Where commercial roofs actually leak

• Penetrations. HVAC curbs, vent pipes, drains, skylights, conduit. The most common source by a wide margin.
• Seams and laps. Aging adhesive on EPDM, failed welds on TPO, open end laps on metal.
• Fasteners. On metal, backed-out screws with flattened washers.
• Flashings and terminations. Parapets, counterflashing, coping joints, and edge metal.
• Drains. Clogged or a failed drain bowl seal, holding water where it should not be.

The solution: diagnose the roof, then decide the scope

A proper leak investigation means walking the whole roof, not one section, documenting the condition of every penetration and seam, and probing for moisture in the insulation to map how far the water has already spread. Photos and a written report come out of that.

Then the scope follows the findings. Isolated damage on an otherwise sound roof gets repaired. A roof with widespread seam failure and a dry deck gets restored with a fluid-applied coating, which seals every joint at once instead of one at a time. A roof with saturated insulation needs that section replaced first, no matter what anyone wants to sell you.

Related: signs you need a roof repair, what happens during an inspection, and common flat roof problems.

Talk to a Conklin-trained contractor near you

No article can tell you what your roof needs. A leak that has been patched twice needs a real investigation, not a third patch. A contractor has to walk it, check the seams and penetrations, and look at the deck before anyone gives you a real answer.

White Roofing Systems coordinates Conklin-trained commercial roofing crews across the country. Call (309) 303-3128 or request a free roof inspection and we will route you to the contractor who covers your city. You can also browse the contractor directory by city and state or look through real before-and-after projects.