How to Find a Roof Leak From Inside the House
The stain on the ceiling is not where the leak is. Water comes through the roof at one point, runs downhill along the underside of the decking or a rafter, and drops onto the drywall wherever it meets a nail head, a truss joint, or a light fixture. In a Littleton two-story with a 6/12 pitch, the entry point can be eight feet uphill of the stain.
That's why finding the leak starts in the attic, not on the roof. Here's how we do it, and how you can do the first half yourself with a flashlight before you call anyone.
What you need
A bright flashlight, a tape measure, a phone for photos, and a piece of chalk or blue tape. Go up on a dry day if you're just tracing old stains, or during rain or snowmelt if you want to see it moving. Wear long sleeves; blown-in insulation is itchy and the truss nails through the decking will catch you.
Step only on the truss chords or on boards laid across them. Drywall will not hold you.
Step 1: Find the stain from above
Measure the stain's position in the room below relative to two walls, then find that same spot in the attic. The insulation directly above the stain is usually wet, compressed, or discolored. That's your starting point, not your answer.
Step 2: Follow the trail uphill
Look at the underside of the roof decking above the wet insulation. Water leaves a trail: a darker streak on the OSB, a line of rust down a rafter, a row of rusted nail tips, sometimes a white mineral stain where it dried. Follow that streak upslope with the flashlight. Where it stops — or where it starts, from the roof's point of view — is the entry.
In a South Metro attic, the trail almost always ends at one of these:
A pipe boot. A plumbing vent pipe coming through the deck, usually above a bathroom or the kitchen. Look for staining in a ring around the pipe on the decking. The rubber collar on the outside has cracked after 8–12 years of Colorado sun. This is the single most common leak we find.
A chimney. Staining on the decking on the uphill side of the chimney, or along its sides where the step flashing is. On 1970s–90s brick chimneys in Ridgewood, Southpark and the older parts of Columbine, the flashing was often aluminum and caulk, and freeze-thaw has opened it.
A valley. A long stain running along the line where two roof planes meet. Valleys carry the most water and, in winter, the most sliding snow. If the roof was built with felt under the valley and no membrane, this is where it shows.
A skylight. Staining at the corners of the skylight curb, especially the two uphill corners. Usually the flashing kit or the perimeter seal, not the glass.
A wall or dormer. Staining where a roof plane meets a vertical wall. Step flashing that's short, missing, or buried behind siding.
The eave, on the north side. Staining along the bottom two or three feet of decking, right above the exterior wall, with no penetration nearby. That's ice-dam water backing up under the shingles during a March thaw. See our post on ice dams — the fix is different.
A nail. A single rusted nail tip with a drip stain under it and no penetration nearby. Either a nail that missed the rafter and backed out through the shingle, or a nail from a satellite dish or a solar mount.
Step 3: Daylight check
Turn off the flashlight and let your eyes adjust. Pinpoints of daylight through the deck are holes. Some are harmless — a gable vent, a ridge vent slot — but a point of light in the middle of a slope is a missing shingle or a nail hole.
Step 4: Mark it and measure it
Chalk the spot on the decking. Measure from the nearest gable wall and from the ridge so the spot can be found from on top. Photograph the trail, the stain, and the penetration. That's the information a roofer needs, and it's the difference between a $300 targeted repair and an hour of someone walking your roof looking for it.
Leaks that aren't the roof
About one call in five, the water isn't coming through the roof at all.
- Bathroom exhaust fans ducted into the attic instead of outside. Warm moist air hits the cold decking in January, condenses, and drips. Look for frost on the nail tips in winter and a fan duct that ends in the insulation.
- Attic condensation from a sealed attic with blocked soffit vents. Same pattern — frost, then drips on the first warm day — but everywhere, not in a trail.
- A furnace flue or water heater vent with a failed storm collar. Technically a roof penetration, but the fix is the collar.
- Plumbing. A second-floor bathroom over a first-floor stain, with no wet insulation above. Not our department.
If the insulation above the stain is dry and there's no trail on the decking, stop looking at the roof.
Step 5: The hose test
If the attic tells you the area but not the exact spot, the hose test settles it. One person in the attic with a flashlight, one on the roof with a garden hose running gently on the suspect area, starting low and working uphill in two-foot sections, five minutes each. Water shows up in the attic within a few minutes of hitting the right spot. We do this on almost every leak we can't see from the deck, and it's how we avoid replacing a valley when the problem was a boot three feet above it.
What happens next
Once the entry point is known, most Littleton roof leaks are a $250–$1,500 fix at that one spot. The roof repair page covers what each repair involves and costs. If the trail led to the eave on the north side, or there were three separate trails in three places on an 18-year-old roof, read repair versus replacement before you spend money on shingles.
Need roof work in Littleton?
Call for a free estimate. Mon–Sat 7am–7pm.