Roof Leak Detection

Roof leak detection is the work of finding the true entry point of water into a roof assembly. It is rarely as simple as looking above the ceiling stain: water can enter at a flashing, run along the top of the deck, a rafter or a sheet of underlayment, and drip out several metres away. A good diagnosis saves money because it targets the repair at the cause rather than at the symptom.

Most leaks on pitched roofs start at interruptions in the covering rather than in the open field of shingles, tiles or panels. Penetrations, walls, chimneys, skylights, valleys and the roof edge are the usual suspects.

Where to look first

• Flashings at chimneys, sidewalls, headwalls and dormers, including step flashing and counterflashing.
• Penetrations such as plumbing vent boots, exhaust caps, satellite or solar mounts and skylight curbs.
• Valleys, where large volumes of water concentrate and debris collects.
• Eaves, where ice dams in cold climates can push water back under the covering.
• Damaged or missing units: lifted or torn shingles, cracked tiles, slipped slates, backed-out screws on metal.
• Low-slope areas: open seams, blisters, splits and ponding near drains.

Working from the inside

In an accessible attic, inspectors look for water stains, dark or wet sheathing, rusty nail tips and mould on the underside of the deck, then trace any trail uphill to its highest wet point. A torch in daylight can reveal pinholes of light through the deck. Stains that appear only in very cold weather may be condensation or frost melt rather than a roof leak, which needs a different fix (see attic condensation).

Water testing

When visual checks fail, a controlled hose test is common: one person runs water on a small, low area of the roof while another watches inside, then the test moves slowly upslope one zone at a time. Testing from the bottom up avoids wetting areas that have not yet been checked. Professional investigators also use moisture meters, infrared cameras and, on low-slope membranes, electronic leak detection.

Safety and limits

Wet roofs are slippery, and steep or high roofs need fall protection. Homeowners can usually do the attic part safely; roof-top testing is often better left to a roofer or a roof consultant. Temporary measures such as a tarp can limit damage until the source is confirmed.