How to Find a Roof Leak: Where the Water Comes In Is Never Where It Shows Up

Roofing

Finding a Roof Leak: The Stain Is Downhill of the Hole, Sometimes by Twenty Feet

Water comes through the roof, lands on the underlayment or the deck, runs down the slope until it meets a nail, a rafter or a seam, drips onto the insulation, soaks sideways, finds a drywall screw, and shows up as a brown ring in the hallway. The hole can be six feet uphill and ten feet sideways from the ring. Roofers who patch the roof above the stain fix nothing, and some do it three times before they look properly. Here is how to look properly.

A method, not a diagnosis. If water is actively coming in, the first section is for right now.

The first hour, if it is leaking now

  1. Bucket under the drip. If the ceiling is bulging, poke a small hole in the middle of the bulge with a screwdriver and let it drain into the bucket; a ceiling that lets go on its own takes the light fixture with it.
  2. Turn off the breaker for any light or fan near the wet area. Water and a ceiling box is the one real danger here.
  3. Move what is under it, lay towels, and photograph everything with the time showing: the stain, the drip, the room, the weather. The photos are the insurance claim and the roofer’s map.
  4. Into the attic with a flashlight while it is still raining if you can get there safely. Running water is easy to find; a dried stain is not. Mark the wet spot on the sheathing with a pencil or tape.
  5. Do not get on a wet roof. A tarp from the ground is a roofer’s job ($300 to $700) and it buys a week. The repair cost page has the rest of the numbers.

The seven usual suspects, in order

On a roof under 20 years old, the shingles are almost never the leak. The things stuck through and against them are.

  1. Plumbing vent boots. The rubber collar around the plumbing stack cracks from sun at 10 to 15 years. Water runs down the pipe into the house. The commonest leak in Ontario, and a $250 to $500 fix. Stain is usually in a bathroom or the hallway outside one.
  2. Chimney flashing. Step flashing pulled away from the brick, counter flashing with the caulk gone, the mortar joint it was let into crumbled, or no cricket behind a wide chimney so water pools against it. Stain near the chimney, often on the uphill side wall. Chimney and wall flashing.
  3. Wall and dormer flashing. Where a roof meets a vertical wall, the step flashing is behind the siding and in front of the shingles, or should be. Caulk on the surface instead means it was done wrong. Stains along the wall line inside.
  4. Valleys. Two roof planes dumping into one channel. Shingles cut too close to the centre, no ice and water underneath, debris holding water, or a metal valley rusted through. Valley leaks.
  5. Nails. A nail that missed the rafter and backed out lifts the shingle above it and makes a hole. A high nail (driven above the seal strip) is exposed to weather. Shows up as a small stain with no obvious cause, usually under a single shingle.
  6. Skylights. Flashing kit installed wrong, or the skylight’s own seal gone at 20 years, or condensation on the glass running down the frame (which is not a leak). Stain at the skylight’s downhill corners.
  7. Ridge vent and roof vents. A ridge vent nailed through the slot without the baffle, or a box vent with a cracked flange, lets wind-driven rain and snow in. Stain high in the attic along the ridge.

Then, far down the list: the shingles themselves, when they are old enough to curl and let wind push rain under them, in which case it is not one leak and the repair vs. replacement page is the one to read.

The attic method

Wait for a bright day and go up with a strong flashlight and your phone. Turn the flashlight off first and look for daylight: a pinpoint at a vent, a line along the chimney, a glow at the eaves. Then flashlight on, and look at the underside of the sheathing for the trail: a dark streak running down the slope, white mineral deposits where water has dried, rust around nail points, a stain that is darker at the top. The top of the trail is the entry point, or within a foot or two of it, and it is uphill of the ceiling stain. Push a nail up through the sheathing there or measure from the ridge and a gable so you can find it from outside. Photograph it. That photo is what you send a roofer, and it is why your quote will be for the right repair.

The hose test

If the attic shows nothing (finished attic, cathedral ceiling, flat roof), the hose finds it. One person inside with a flashlight at the stain, one on the roof or a ladder with a garden hose. Start at the bottom of the suspect area and wet one section at a time for a few minutes each, moving uphill: below the vent, then the vent, then above it; the downhill side of the chimney, then each side, then the uphill side. The moment the inside person sees a drip, the last section wetted is the leak. Do not start at the top, or the whole roof is wet and you learn nothing. Dry day, a roof you can walk safely, or pay the roofer’s service call to do it: $250 to $450, usually credited against the repair.

When it is not a leak

A stain that appears in January, grows through February, and dries up in April, with no rain involved, is condensation: warm house air leaking into a cold attic, freezing on the underside of the sheathing as frost, and melting on the first mild day to rain on the insulation. The fix is in the attic, not on the roof, and a roofer called for this will find nothing to repair. A bathroom fan ducted into the attic does the same thing in one spot. The leak or condensation page tells them apart, and the fan page is the usual culprit.

What to tell the roofer

Where the stain is, when it appears (every rain, only wind from the west, only in a thaw), what you found in the attic with the photo, and what is on the roof above that spot. A roofer who gets that can quote the actual repair from the driveway instead of selling you a roof. The repair prices are on the repair cost page, and if the roofer’s answer is a $15,000 roof for a $400 boot, the $99 review is cheaper than finding out the hard way.

Questions people ask

Why does my ceiling have a water stain but the roof looks fine?

Because the entry point is a boot, a flashing or a nail, not the shingles, and it is uphill of the stain. Look in the attic for the trail on the sheathing; the top of the trail is the hole.

Why does my roof only leak when it rains hard, or only in wind?

Hard rain overwhelms a marginal flashing or a valley; wind from one direction drives rain up under shingles or into a vent on that side. Both point to a specific spot, which is useful: tell the roofer which weather does it.

Why does my roof leak keep coming back after repairs?

The repair was made at the stain, not the entry point, or the real cause (a chimney with no cricket, a valley with no membrane, condensation) was never found. Insist on the attic look and the hose test before the next patch.

Can I find a roof leak from inside the house?

Usually, from the attic: daylight with the flashlight off, then the trail on the sheathing with it on. The top of the trail is the leak, within a foot or two.

Should I call a roofer or a handyman for a leak?

A roofer, for anything involving flashing, valleys or the membrane. A handyman can swap a vent boot. The difference is who warranties it and who will tarp it at 7 p.m. in a storm.

Attic first, roof second

Flashlight, bright day, find the trail, mark the top of it, photograph it. Then the roofer is repairing the hole, not the stain.

What the repair should costLeak or condensation?

Sources and further reading