Repair

How to Find a Roof Leak From Inside the House

By Madison Roof Repairs · September 10, 2026 · 7 min read

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 a roofer does 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

Most of the time 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 a roofer.

Leaks that only show up in certain weather

Some leaks don't appear in every rain, which is why they're hard to pin down.

• Only in wind-driven rain. Water is being pushed sideways under flashing or a lifted shingle on the windward side, usually west or southwest. Check the wall step flashing and the uphill side of the chimney.
• Only in a thaw. That's an ice dam, or snow melting behind a chimney or in a valley. Look at the eave and the valleys, not the open field of shingles.
• Only on sunny winter mornings. Condensation, covered in step 4.
• Only in a long, heavy rain. An overloaded valley, a gutter backing up under the drip edge, or a pipe boot that only lets go when water sheets over it.

Write down the weather each time it drips. After two or three entries the pattern usually points to one spot.

What to record for the roofer

• Photos of the attic trail and where it ends
• Your measurements from the ridge and a gable wall
• Which rooms showed water, and when
• The weather each time
• Anything you've already tried, like caulk on the flashing

That list turns a long visit into a short one, and it lets you check that the estimate deals with the spot you found.

Mistakes to avoid

Cutting into the ceiling to look. The attic usually shows more, without a drywall patch afterward.

Caulking everything you can reach. Sealant smeared over flashing hides the trail and makes the real fix harder.

Assuming it's the shingles. Most leaks are at a penetration or a transition, not in the open field.

Leaving wet insulation in place. Pull it back so the deck and ceiling can dry, and replace it once the source is fixed.

If you can't get into the attic

Some houses don't make it easy: a finished attic, a hatch in a closet ceiling stacked with boxes, or a low-slope addition with no attic at all. In those cases, work from the outside and the ceiling. Note the stain's position, look at the roof above it from the ground with binoculars, and check for the usual suspects on that slope: a vent pipe, a wall, a skylight, a valley. On a 1920s house with knee walls, the access panels behind closets often show the back of the sloped ceiling, which is where the water runs. If none of that works, a roofer can open a small inspection hole where it's easiest to patch.

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.