How to Find a Leak in a Flat Roof
Finding the leak is 80% of the work — once you know where water is entering, the repair itself is straightforward. The challenge with flat roofs is that water can travel several feet along the membrane before finding a way through to the interior.
Indoor Detection
Look for water stains on the ceiling, damp spots on walls near the roofline, peeling paint, or mold growth. In the attic or crawlspace below the roof, look for wet decking, stained rafters, or damp insulation. Mark all indoor evidence — it narrows your search area on the roof. Remember: the actual entry point on the roof is usually 5-10 feet uphill from the interior stain.
Roof Surface Inspection
Check these high-probability areas first: Seams (where membrane sheets overlap — feel for lifted edges), Flashing (where the roof meets walls, curbs, and parapets), Penetrations (around every pipe, vent, HVAC unit, and drain), Drains (clogged drains cause ponding that finds weak spots), and Low spots (where ponding water has degraded the membrane). Over 80% of flat roof leaks are at flashing or penetrations.
The Garden Hose Flood Test
If you can't find the leak visually, use this method: have a helper watch inside from below. On the roof, use a garden hose to flood a small 4x4 foot section at the lowest point of the roof. Wait 15 minutes. If no dripping appears inside, move to the next section uphill. When your helper sees water, you've found the leak zone. Then narrow it down by testing smaller areas within that section.
Emergency Temporary Fixes
Need to stop a leak right now? These temporary fixes work in minutes and hold for days to weeks:
Option 1: Roof Sealant
Apply directly to the leak source. Use wet-rated sealant (Henry's Wet Patch) if the surface is damp. Cover a generous area beyond the damage. Lasts weeks to months.
Option 2: Roofing Tape
Self-adhesive waterproof tape applied over the leak. Clean and dry the surface first if possible. Press firmly to eliminate air bubbles. Good for seam separations and small tears.
Option 3: Tarp
For large areas or during active rain. Extend 4 feet past the damage on all sides. Weight with sandbags or cinder blocks. Never nail or screw a tarp to a flat roof.
Option 4: Bucket Inside
If you can't access the roof safely: place buckets to catch drips, protect furniture with plastic sheeting, and use towels around the drip zone. Schedule a roof repair ASAP.
Safety Warning
Never go on a flat roof during rain, high winds, or icy conditions. Wet flat roofs are extremely slippery. If the leak is during a storm, manage it from inside (buckets, towels, plastic sheeting) and repair the roof when conditions are safe.
Permanent Flat Roof Leak Repair
Once you've found the leak and have dry weather (24+ hours), do a permanent repair that will last 5-10+ years:
Find the leak from inside
Mark any water stains on the ceiling with a marker and note their locations relative to the room walls. If you have attic access below a flat roof section, inspect with a flashlight — look for wet roof decking, damp insulation, or water trails on rafters. Water can travel several feet along the deck before dripping, so trace the wet trail as far uphill as possible. The highest wet point is closest to the actual roof penetration.
Locate the source on the roof surface
On the roof, look 5-10 feet uphill from where the interior stain appeared. Focus on: seam edges (feel for lifted or separated sections), flashing around walls, curbs, and parapets, around all penetrations (pipes, vents, HVAC units, satellite dishes, drains), blisters or bubbles in the membrane, and any punctures or tears. If you can't find the source visually, do the garden hose test: have someone watch inside while you flood small 4x4 foot sections one at a time. Wait 15 minutes per section. When your helper sees dripping, you've found the zone.
Apply an emergency temporary fix (if needed)
If rain is coming and you need a quick fix: apply wet-rated roof sealant generously over the suspected leak area. Cover with self-adhesive roofing tape for extra protection. For larger areas: lay a tarp over the damage, extending 4 feet past the edges. Weight down with sandbags or cinder blocks (never use nails or screws — they create new leak points). These temporary fixes buy you days to weeks while you plan a permanent repair.
Prepare the area for permanent repair
Wait for at least 24 hours of dry weather. Sweep away all dirt and debris. Clean the membrane surface with the appropriate cleaner: EPDM cleaner for rubber roofs, isopropyl alcohol for TPO/PVC, or general roof cleaner for modified bitumen. Let dry completely. Cut away any blistered, torn, or deteriorated membrane in a clean rectangular shape extending 2 inches past the damage. Remove any old sealant or adhesive from the repair area.
Apply the permanent patch and seal
Apply the manufacturer-recommended primer to both the roof surface and patch underside. Let tack up per directions. Cut your patch to extend at least 6 inches beyond the damage on all sides. For EPDM: peel backing and press firmly from center outward. For TPO: heat-weld edges at 800-1000°F. For mod-bit: torch or cold-apply. Roll the entire patch with heavy pressure using a hand roller — there should be zero air pockets. Apply lap sealant around all edges. Allow 24-48 hours for full cure. Verify by flood-testing with a garden hose for 15 minutes.
Your flat roof leak is permanently repaired. Monitor during the next rain and inspect the patch twice yearly.
Best Products for Flat Roof Leak Repair
Henry HE208R Roofing Sealant
$8 – $15Professional-grade wet/dry roof sealant. Can be applied to damp surfaces in light rain. Excellent for emergency repairs and sealing around flashing.
- Applies to wet or dry surfaces
- Stays flexible
- UV resistant
- Works on all roof types
Peel & Seal Self-Adhesive Roofing Tape
$15 – $30Instant waterproof repair tape for flat roofs. Peel, press, done. Great for seam separations, small tears, and around flashing edges.
- Instant adhesion — no tools
- Aluminum top for UV protection
- Works on all flat roof types
- Permanent waterproof seal
Gaco GacoFlex Silicone Roof Coating
$120 – $180 (5 gal)Premium 100% silicone roof coating for full-roof leak prevention. Withstands ponding water. Apply over existing membrane to seal and protect.
- 100% silicone — ponding safe
- Reflects 80%+ UV
- Covers ~250 sq ft per bucket
- No primer needed on most surfaces
Flat Roof Leak Repair Costs
Flat Roof Leak Repair Costs (2026)
| Repair Type | DIY Cost | Professional Cost |
|---|---|---|
| Roof sealant (emergency) | $8 – $20 | $150 – $300 |
| Membrane patch (< 4 sq ft) | $20 – $75 | $200 – $600 |
| Flashing repair | $30 – $100 | $300 – $800 |
| Full roof coating (per sq ft) | $0.50 – $1.50 | $3 – $7 |
| Full replacement (per sq ft) | Not recommended | $5 – $10 |
Costs are national averages. For multiple leaks, a full roof coating is often more cost-effective than individual patches.
When a Leak Means You Need a New Roof
When to Call a Professional
- Multiple leaks appearing in different locations
- Roof deck (plywood) is soft or spongy when walked on
- Roof is over 20 years old with widespread membrane deterioration
- Previous patches keep failing or leaks return nearby
- More than 25% of the roof surface is damaged
- Ponding water doesn't drain within 48 hours (structural slope issue)
Pro Tip
Related Guides
Frequently Asked Questions
Written by
HomeRepairBase Editorial Team
Our team of home improvement experts and licensed contractors creates detailed repair guides, cost breakdowns, and troubleshooting tips to help homeowners tackle structural issues with confidence.