Repair

When a Roof Leak Is Actually a Condensation Problem

By Des Moines Roof Repair · September 25, 2026 · 5 min read

Every winter we get called to a leak that isn't there. The homeowner has a wet spot on the ceiling, water dripping from the attic hatch, or a dark ring around a bathroom light. It hasn't rained in a week. There's a foot of snow on the roof and the shingles are fine. What they have is condensation, and a roofer who replaces shingles for it is taking their money. Here's how to tell the two apart, and what to do about the real problem.

How a house makes water in its own attic

A family of four puts several gallons of water vapor into the air every day — showers, cooking, breathing, laundry, houseplants. Warm air holds that vapor. When the warm air leaks up into an attic that's 10°F in January, it hits the underside of the roof deck and the nails poking through it, and the vapor turns straight to frost.

Then comes a sunny day. The deck warms, the frost melts, and a hundred nail tips drip at once. It looks exactly like a roof leak. It stops when the deck refreezes. That cycle can run for weeks, and by March the insulation is wet, the deck is stained, and there's mold on the north side of the attic.

Telling condensation from a leak

If it only happens in deep winter without rain, it's almost certainly condensation. If you want to confirm, go up on a cold morning and look for frost on the nails. Finding a leak from inside covers the leak side of that table.

Where the warm air comes from

On Des Moines houses the sources are predictable:

Bath fans that stop in the attic. The most common cause we find, by a wide margin. The fan was installed with a duct that runs a few feet and ends in the insulation, or with no duct at all. Every shower puts a gallon of warm, wet air right onto the roof deck. Common on 1950s–70s ranches in Urbandale, Windsor Heights and the east side, and on a surprising number of 2000s houses where the builder never finished the run.

Recessed lights. Each one is a chimney from the living room to the attic.

The attic hatch and pull-down stairs. An unsealed hatch leaks more air than all the other gaps together.

Plumbing and wiring penetrations, the tops of interior walls, the chimney chase. Small gaps, lots of them.

Dryer vents run into the attic or disconnected at a joint.

Whole-house humidifiers set too high. If the windows are fogging, the attic is too.

Why ventilation alone doesn't fix it

The instinct is to add roof vents. More vents do help, but they're moving the water vapor out after it's already in the attic. The 1920s–40s houses in Beaverdale and Drake have almost no soffit overhang to put intake vents in, so exhaust vents just pull more warm air up from the house. The fix works in this order:

1. Stop the air. Vent the bath fans and dryer outside through the roof or gable with insulated duct. Seal the hatch. Foam the penetrations and the top plates. Box the recessed lights. This is a day's work with a can of foam and a roll of weatherstripping, or $500–$1,500 to have done properly.
2. Insulate to R-49, which is the target for climate zone 5. Insulation slows heat, but only after step one stops the air carrying moisture through it.
3. Then balance the ventilation. Soffit intake with baffles, ridge exhaust. Roof ventilation in Des Moines attics works when there's intake low and exhaust high; a ridge vent with blocked soffits does very little.

The bonus is that the same three steps are exactly what stops ice dams, because the warm air that makes frost also melts the snow at the ridge.

When it's both

Sometimes a house has condensation and a real leak. The tell is a trail on the deck that doesn't match the frost pattern: one dark streak down a rafter from a pipe boot, in an attic that's also frosty everywhere. Fix the leak on the roof and the moisture in the house. A roofer who only looks at the shingles finds one; a roofer who only looks at the attic finds the other.

What we do

On any winter leak call we start in the attic. If it's frost, we say so, show you the fan duct that ends in the insulation, and give you a price for the air-sealing and venting instead of a price for shingles. If it's a leak, the roof repair page covers what happens next. Either way the answer isn't a new roof, and anyone who says it is should be asked to explain the frost.