Flat Roof Leak Repair | How Pros Find and Fix Leaks on Commercial Roofs

How Do You Fix a Leak on a Flat Commercial Roof?

Patching the spot directly above the drip is the most common reason flat-roof leaks come back. Here’s the process we follow on every leak call as part of commercial roof repair in San Antonio, and what you can do right now. If you’re comparing options more broadly, see our work in commercial roofing in San Antonio.

What to Do Inside the Building Right Now

1. Move people, inventory and equipment out from under the leak. Shut off power to any wet electrical equipment.
2. Catch the water with containers, and use plastic sheeting to channel it away from stock.
3. Relieve a bulging ceiling by poking a small hole over a bucket, so water doesn’t spread across the ceiling grid.
4. Photograph everything: ceiling, damaged goods and the time, which helps any insurance claim.
5. Call a commercial roofer. Don’t send staff onto a wet roof.

Most Common Causes of Flat Roof Leaks

Source	What fails	How often we see it
Flashings	Wall, curb and edge flashing pulls away or cracks	Very common
Seams	Welds, tape or adhesive open up	Very common
Penetrations	Pipe boots, pitch pans, HVAC curbs, skylights	Very common
Low spots / ponding	Standing water finds weak seams and flashings	Common
Punctures	Dropped tools, foot traffic, hail	Common
Clogged drains	Water backs up over flashings	Seasonal

Leak sources: Waterproof Magazine, Roof Leaks: Pinpointing and Repairing.

Why the Leak Inside Isn’t Where the Hole Is

Water that gets through the membrane can travel along the insulation, the deck flutes and framing before it finds a gap in the ceiling. On a near-level roof it can move in any direction, and wet insulation spreads it further. That’s why the drip you see may be 10, 20 or more feet from where water actually enters.

How Roofers Locate Leaks | Infrared, Moisture Meters, Flood Tests

Method	What it does	Limits
Visual inspection inside and out	Traces stains to the roof and checks flashings, seams and penetrations	Always the first step
Infrared scan	Wet insulation holds heat; shows up warm on a night scan	Needs a dry, calm night; not for all insulation types
Capacitance or nuclear meters	Grid readings show where insulation is wet	Confirmed with core cuts
Electronic leak detection (EFVM, high-voltage)	Pinpoints the actual breach in the membrane	Needs a suitable deck or setup
Water / flood testing	Recreates the leak by wetting an area	Full flood tests add heavy load and can damage the roof; use with care

Methods and limits: Waterproof Magazine. Moisture surveys typically cost about $0.05 – $0.12 / sq ft (ThePricer, 2026).

What If Nobody Can Find the Leak?

Some leaks hide well. When the obvious spots check out, work through this list:

1. Keep a leak log: date, time, rain amount, wind direction, and whether the AC was running. Leaks only in wind-driven rain point to walls, coping or windows; leaks on dry days point to HVAC condensate.
2. Check non-roof sources: clogged condensate pans and lines, plumbing vents, parapet walls, windows and masonry above the roof line.
3. Look upslope and along the deck flutes from the drip; water can run 10 to 20 feet or more.
4. Run a moisture survey to find wet insulation, which usually sits near the real entry point.
5. Use electronic leak detection or controlled water testing of one small area at a time.

Should You Fix It Yourself or Call a Roofer?

Your team can handle the inside of the building and a clogged drain; anything on the membrane should go to a commercial roofer. Here’s where the line usually falls:

Your team can safely do	Leave to a commercial roofer
Move stock, catch water, protect equipment	Any cut, patch or weld on the membrane
Clear leaves and debris from drains and scuppers you can reach safely	Flashing, curb and pipe-boot repairs
Photograph stains and note when the leak happens (heavy rain, wind direction, AC running)	Finding the entry point and checking for wet insulation
Call your roofer and, if needed, your insurer	Work on a wet, steep or unguarded roof edge

Two reasons to hold back: most manufacturer warranties only cover repairs by an authorized contractor, and the wrong sealant on TPO, PVC or EPDM has to be cut out later, which makes the real repair bigger.

How Each Type of Leak Is Repaired

The repair depends on what failed. A proper repair always starts by cleaning and drying the area and cutting out anything wet underneath.

Leak source	Permanent repair
Open seam (TPO / PVC)	Clean, prep and re-weld with hot air; add a cover strip if the seam is aged
Open seam (EPDM)	Clean and prime, then new seam tape and a cover strip
Puncture or tear	Cut out damage, replace wet insulation, install a patch of the same membrane extending past the damage
Wall or curb flashing	Remove failed flashing, install new flashing and termination bar, seal the top edge
Pipe boot or pitch pan	Replace with a prefabricated boot or new pan and pourable sealer
Edge metal or coping	Re-secure or replace metal, then strip in with membrane
Clogged or low drain	Clear the drain, replace the strainer and clamping ring; correct low spots with tapered insulation
Modified bitumen blister or crack	Cut out, dry, and patch with matching cap sheet

Temporary Fixes vs Permanent Repairs

	Temporary	Permanent
When	During rain or an emergency	Once the roof is dry and the source is confirmed
What	Tarps, emergency sealants, temporary patches	Membrane-matched patch, re-flashing, seam welding, wet insulation replacement
Lasts	Days to weeks	The remaining life of the roof
Warranty-safe?	Only if the warrantor is notified	Yes, when done by an authorized contractor

Avoid smearing asphalt mastic on single-ply roofs as a “fix.” It doesn’t bond well and the contaminated area usually has to be cut out later, making the permanent repair larger. For urgent leaks, see emergency roof repair in San Antonio.

How Long Does a Flat Roof Leak Repair Take?

Most single-leak repairs are finished in a few hours to one day once the roof is dry. Larger jobs take longer:

• Seam or flashing repair: usually same day
• Puncture with wet insulation replacement: one to two days
• Several leaks plus a moisture survey: survey first (often a night infrared scan), repairs in the following days
• Rainy weather: temporary seal now, permanent repair once the surface is dry

How Much Flat Roof Leak Repair Costs

Repair	Typical cost
Small leak or puncture patch	$300 – $1,200
Flashing or sealant failure	$450 – $1,200
Curb or penetration flashing	$500 – $2,500 each
Seam reseal (50 – 200 linear ft)	$1,800 – $7,500
Wet insulation area	$1,500 – $5,000+
Emergency / after-hours premium	+25% – 50%

The Roofing Brief (2026); SuperSeal Roofing (Apr 2026).

Will Insurance Pay for the Leak Repair?

Usually only when a covered event caused it. Commercial property policies typically cover sudden damage from hail, wind or falling objects, plus resulting interior damage, minus your deductible. They typically exclude leaks from age, wear and lack of maintenance.

• Photograph the damage and note the storm date before cleanup
• Have the roof inspected and get a written cause-of-leak report
• Make temporary repairs to stop further damage and keep receipts
• Report promptly; see storm and hail damage roof repair

How to Keep the Leak From Coming Back

1. Fix the entry point, not the drip: confirm the source before patching.
2. Replace wet insulation around the repair so it doesn’t keep feeding water to the deck.
3. Inspect twice a year (spring and fall) and after hail or high wind.
4. Keep drains clear, especially before San Antonio’s spring storm season.
5. Control roof traffic: walk pads at HVAC units and a sign-in for contractors.
6. Track repairs on a roof plan: three or more leaks in different areas is a sign to price a coating, recover or replacement.

FAQs | Flat Roof Leaks

Why does my flat roof only leak in heavy rain?

Heavy or wind-driven rain raises water levels on the roof above flashings and seams that stay dry in light rain, or overwhelms clogged drains. It points to a weak flashing, an open seam near a low spot, or a drainage problem.

Can a leaking flat roof be repaired in the rain?

Only temporarily. Membranes need a clean, dry surface for a permanent weld or adhesive bond, so emergency crews tarp or apply a temporary seal and come back for the permanent repair when the roof dries.

Can a flat roof leak cause mold?

Yes. Wet insulation, ceiling tiles and drywall can grow mold within a few days. Replace wet ceiling tiles and insulation rather than letting them dry in place, especially in occupied spaces.

Why does the same leak keep coming back?

Usually because the repair was made above the drip instead of at the real entry point, or wet insulation was left in place. A moisture survey and a careful trace of the water path fix most repeat leaks.

Should I replace the roof if it leaks in several places?

Not automatically. Multiple leaks are often fixable, but if a moisture survey shows widespread wet insulation or more than about a quarter of the roof needs work, compare repair against coating, recover and replacement before spending more.

Schedule Leak Detection and Repair

We’ll trace the leak to its real source, check for wet insulation around it, and give you a written repair price before work starts.

• Fast response across San Antonio, including Westover Hills, roof repair in Alamo Heights and Leon Valley
• Repairs matched to your membrane to protect your warranty

Book your free roof inspection in San Antonio or see flat roofing in San Antonio.