How to Find a Roof Leak From Inside the House
The stain on the ceiling is almost never under the leak. Water gets in at one point on the roof, runs down the underside of the decking, follows a rafter, drips onto the insulation, spreads, and finally soaks through the drywall somewhere downhill and sideways from where it started. So the first place to look is not the ceiling. It's the attic.
Here's how we do it, and how you can narrow it down yourself before calling for roof repair.
What you need
A good flashlight, a phone for photos, a tape measure, and something to kneel on. Walk only on the joists or on boards laid across them; drywall won't hold you. If the attic is finished, as on most 1920s isthmus houses, you're looking at knee-wall access panels and the sloped ceilings instead.
Pick your timing. Go up during or right after a rain if you can. In winter, go up on a sunny afternoon after a cold night, for a reason explained below.
Step 1: Start above the stain
Measure from two walls to the stain in the room below, then find the same spot in the attic. Look at the insulation there. Wet, dark or compressed insulation marks where the water lands. Now look up.
Step 2: Follow the trail uphill
On the underside of the roof deck, water leaves a track: a dark streak on plywood, a rust line down a nail, a clean stripe through dusty wood. Follow it up the slope. It ends at the entry point, or at least within a foot or two of it. Photograph the trail and where it ends, then measure its position from the ridge and from the nearest gable wall so you can find the same spot from outside.
Old stains that are dry and grey are history — a leak that's been fixed, or one that only runs in a specific storm. Fresh stains are dark and damp at the edges.
Step 3: Check the usual suspects
Nine times out of ten the trail ends at one of these:
- A plumbing vent. Look for the pipe passing through the deck. The rubber boot around it on the roof side cracks after 10–15 Wisconsin winters. Water shows on the deck right at the pipe.
- The chimney. Stains on the deck along the uphill side of the chimney mean flashing. On old brick chimneys in the Vilas and Regent neighborhoods, this is the leak. Chimney flashing repair Madison homeowners need is usually step flashing plus counter-flashing cut into the mortar, not another tube of caulk.
- A valley. Two roof planes meeting. Look for stains along the line where the decks join. Valleys carry the most water and the most ice.
- A skylight. Stains at the corners of the skylight well. Usually the flashing kit, sometimes the seal on an old unit. Skylight leak repair Madison roofers do is $400–$900 for the flashing.
- A dormer or wall. Where a lower roof meets a wall, water gets behind short step flashing. Common on isthmus houses with a 1950s dormer added.
- The eave. Stains along the bottom three feet of the deck, right above the exterior wall, especially on the north side. That's an ice dam, and it may only leak in February and March. The ice dam page explains what to do about it.
- A nail. A single roofing nail that missed the rafter and backed out, with a rust ring around it. Cheap to fix, hard to find.
Step 4: Rule out condensation
This is the Madison step. In winter, warm moist air from the house rises into the attic through gaps around lights, the hatch and plumbing chases. It hits the cold underside of the deck and freezes there as frost, especially on the nail tips. On the first sunny afternoon it melts and "rains" onto the insulation. Homeowners call this a roof leak. It isn't; it's a ventilation and air-sealing problem, and no amount of shingle work fixes it.
The tells: dripping or staining spread across a whole area of the deck rather than a single trail; frost on nail tips on a cold morning; a bath fan duct that ends in the attic; damp insulation on the coldest days with no rain or thaw. If that's what you see, the roof isn't leaking. Read the section on attic air-sealing on the ice dam page, and get the bath fan ducted outside.
Step 5: The hose test
If the trail is unclear, and the roof is dry and walkable, the hose test settles it. One person in the attic with a flashlight; one on the roof with a garden hose, no nozzle. Start low on the roof, below the suspected spot, and wet a small area for a few minutes. Move up the slope a section at a time. When the attic person sees water, you've found it. Don't spray uphill under the shingles; that makes any roof leak. Don't do this in freezing weather, and don't do it on a steep roof at all — call us.
What it costs to fix
Once found, most of these are a $300–$1,500 repair. Boots are cheapest, chimneys most expensive, and eave damage from ice depends on how many winters it's been going on. The roof repair page has prices by job. If the trail leads to worn-out shingles across the field rather than one spot, the honest answer is on the repair versus replacement post.
When to stop and call
Water near electrical — a light fixture, a junction box — means turn off the breaker and call today. A soft spot underfoot on the joists means the ceiling drywall or the deck is compromised. And if you find fresh stains in three or four unrelated places on a roof over 20 years old, stop hunting for one leak. That's a roof.
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