How is a flat roof leak patched?

A flat roof leak is patched by finding the actual defect first, then repairing or replacing the damaged membrane or flashing with materials that match the roof system. The location of the drip inside the house is a poor guide, because interior moisture reported as a roof leak can come from adjacent walls, windows or mechanical units.^[1] That search is most of the work, and it is why a patch done by guesswork so often fails.

Where flat roof leaks usually start

Flat roofs are covered by a continuous membrane, such as EPDM rubber, TPO, PVC, modified bitumen or built-up layers, with metal edging and flashings at the walls, vents and drains. Leaks tend to start at the weak points of that skin:

• cuts and punctures caused by tools or debris, and worn spots where people walk^[2]
• joints in metalwork and flashings where sealant has cracked or shrunk^[2]
• areas where water stands. A small cut or puncture in a well-drained roof may cause minor damage, but the same puncture in standing water may allow significant water infiltration into the insulation, deck and building contents^[2]
• seams, drains and penetrations, where a membrane meets something else and the sealant and metalwork do the work

What a roofer does to patch it

The sequence a competent roofer follows is the same whatever the material.

Trace the leak. The Department of Defense's roofing standard says to locate the interior leak damage by measuring from a reference point on the roof, and to keep investigating adjacent building elements when no defect is obvious at that spot.^[1] Expect a roofer to look at the ceiling stain, the roof above it and the walls, windows and rooftop equipment nearby.

Identify the roof system. Repair methods differ by material. The same standard sends the repairer to separate sections of the NRCA repair manual for low-slope roofs for built-up, modified bitumen, thermoplastic and thermoset membranes,^[1] so a patch that suits one will not suit another. A roofer who cannot say what your roof is made of is not ready to patch it.

Repair the defect, not just the stain. Depending on the roof, that can mean a membrane patch bonded or welded over a cut or open seam, new flashing at a wall or vent, or fresh sealant at metal joints. On that last point, the manufacturer's guide says that if cracking or shrinkage of the sealant is observed, the roofing professional should remove the sealant and replace it with new sealant.^[2] On a coated membrane, the manufacturer's guide says the coating should be maintained to recover areas that have blistered, peeled or worn through.^[2]

Deal with the cause. Ponding is both a cause and a multiplier of leaks. The same manufacturer guide says roofs should drain completely within 24 to 48 hours, and that removing leaves and other debris often improves drainage.^[2] If water keeps standing, a patch only buys time.

When a patch is the wrong answer

The Department of Defense's standard says a roof that continues to leak despite repeated repair efforts should be scheduled for replacement.^[1] If your roof has been patched several times, ask for a written opinion on how much life the rest of it has. Our guide to flat roof replacement costs in Virginia covers that side.

Also check for a warranty before anyone cuts into the roof. For roofs under warranty, the standard says the warrantor should be contacted for recommendations before repairs.^[1]

When to call a licensed pro

For every flat roof leak. Patching a flat roof is not a job to teach, and it is not a good one to try. The standard notes that the use of hot asphalt, torches and solvent-containing materials is common in roof repair work and misuse can create problems, and recommends fall protection and at least two people on the job.^[1] A flat roof edge is still an edge, and wet membranes can be slippery.

In Virginia, the specialty of roofing contracting covers the installation, repair, removal or improvement of materials that form a watertight, weather resistant surface for roofs and decks.^[3] DPOR says that if the work is valued at $1,000 or more, a valid Virginia contractor's licence is required for the licence category in which the contractor is to work.^[4] Check the licence before you pay a deposit.

While you wait for the roofer, the safest thing you can do is keep water off your belongings and out of the ceiling; what to do when a roof starts leaking lists the first moves. To see what a quote should cost, read how much it costs to patch a flat roof in Virginia, and for who to hire, who repairs flat roofs on houses.

Loclis lists DPOR-licensed roofing businesses with their licence number and class. Browse roofing services or read more roofing answers.

To get a flat roof leak looked at, describe the job once and licensed local roofers quote it.