As featured on KXAN Studio 512
Roof Leak, Condensation, or Your AC? How to Tell
A wet spot on the ceiling sends most people straight to a roofer. In an Austin house with the air conditioner in the attic, the water has three likely sources, and only one of them is the roof. Here is how to tell them apart before you pay anyone to fix the wrong thing.
By Austin Roofing Company • Updated October 2026
Austin's Most Trusted Roofing Company
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The short answer
Match the water to the weather. Water that shows up during or after rain is almost always the roof. Water that shows up on a hot, dry day while the AC is running is almost always the air conditioner: a clogged drain line, an overflowing pan, or sweating ducts. Water that shows up after a cold night, spread out instead of in one spot, is usually condensation in the attic. If the pattern doesn't make it obvious, a free roof inspection will.
Three Sources of Ceiling Water, Side by Side
Each source leaves its own pattern. Note when the water appears, where it appears, and what it looks like, then find the closest match:
When: During or after rain, worst in heavy or wind-driven storms
Where: Near chimneys, vents, skylights, valleys, and walls, though the stain can sit feet away from the hole
What it looks like: A brown ring that grows after each storm; wet or stained wood along one path in the attic
Who fixes it: A roofer
When: Hot days while the AC runs, with or without rain
Where: Under or near the air handler, the return grille, or a supply vent
What it looks like: Fresh, clear water; a full or rusted drain pan; water from a pipe over a window; AC that stops cooling
Who fixes it: An HVAC company
When: Cold mornings after a front, or humid days for sweating ducts
Where: Spread across the ceiling, or near a bathroom fan or duct run
What it looks like: Droplets or frost on nail tips under the roof deck; damp insulation in a wide area rather than one line
Who fixes it: Depends on the cause: HVAC for ducts, a roofer for attic ventilation, either for fan ducts
| Source | When | Where | What it looks like | Who fixes it |
|---|---|---|---|---|
| Roof leak | During or after rain, worst in heavy or wind-driven storms | Near chimneys, vents, skylights, valleys, and walls, though the stain can sit feet away from the hole | A brown ring that grows after each storm; wet or stained wood along one path in the attic | A roofer |
| AC condensate | Hot days while the AC runs, with or without rain | Under or near the air handler, the return grille, or a supply vent | Fresh, clear water; a full or rusted drain pan; water from a pipe over a window; AC that stops cooling | An HVAC company |
| Attic condensation | Cold mornings after a front, or humid days for sweating ducts | Spread across the ceiling, or near a bathroom fan or duct run | Droplets or frost on nail tips under the roof deck; damp insulation in a wide area rather than one line | Depends on the cause: HVAC for ducts, a roofer for attic ventilation, either for fan ducts |
One stain can't always tell you the answer, but a log usually can. Each time you see water, write down the date, whether it rained in the last day, whether the AC was running, and a photo of the spot. Three entries are often enough to see the pattern.
How to Tell If It's Your Air Conditioner
In many Austin homes the air handler sits in the attic, right above a finished ceiling. Every hour it runs in summer, its cooling coil pulls water out of the air. That water drips into a pan and runs out a drain line. When the line clogs or the pan rusts through, the water has nowhere to go but down.
Signs the water is coming from the AC:
- It shows up on dry days. No rain for a week, AC running hard, fresh water on the ceiling.
- It's near the air handler, the return grille, or the closet where the unit lives.
- Water is dripping from a pipe over a window. That's usually the secondary drain. Installers route it somewhere you'll see it, so water coming out means the main drain is blocked.
- The AC stopped cooling. Many systems have a float switch that shuts the unit off when the pan fills. A dead AC and a wet ceiling on the same day point to the drain.
- Supply vents are dripping or ringed with water stains. Cold air running through a duct in a hot attic can make the outside of the duct sweat, especially where insulation is crushed or a vent boot isn't sealed to the ceiling.
A clogged drain doesn't stay small. The Department of Energy warns that blocked condensate drains in attics and closets can go unnoticed long enough to cause severe water damage in ceilings, floors, and walls. If you see these signs, turn the system off at the thermostat and call your HVAC company. A roofer can't fix it, and an honest one will say so.
Sources: U.S. DOE Building America Solution Center: condensate drain pans; sealed and insulated flex ducts
How to Tell If It's Condensation in the Attic
Warm, damp air from inside the house finds its way into the attic through bathroom fans, attic hatches, and gaps around lights. When a cold front drops the roof deck below that air's dew point, the moisture condenses on the underside of the deck and on the tips of the roofing nails. When the sun warms the roof, it drips.
Signs the water is condensation:
- It shows up after the coldest nights, often on a dry, sunny morning.
- Droplets or frost on the nail tips poking through the roof deck, seen with a flashlight from the attic hatch.
- Damp insulation over a wide area, rather than one wet line that runs downhill from a single spot.
- Stains near the bathroom and wet insulation around the fan housing. A bath fan that blows into the attic instead of outside sends shower steam straight onto cold rafters and decking.
The fix depends on the cause. A bathroom fan needs a duct that runs all the way to a roof cap or soffit vent. An attic that can't move air needs better ventilation: intake at the soffits and exhaust at the ridge. That part is roofing work. See how we handle ridge vents and attic ventilation on a new roof.
Source: U.S. DOE Building America Solution Center: exhaust fans venting to outside
How to Tell It's a Roof Leak
A roof leak follows the rain. It's the one source that stays quiet through a dry spell, no matter how hard the AC runs.
- It appears during or within a day of rain, and the stain grows after each storm.
- It's near something that goes through the roof: a chimney, a plumbing vent, a skylight, a valley, or the line where a roof meets a wall.
- From the attic, you can follow a wet or stained path along a rafter or the roof deck, running downhill from one spot.
Water travels before it drips, so the hole is often several feet uphill from the stain. That's why patching the spot above the stain so often fails. Our roof repair team traces the water back to where it enters and fixes the source.
Stay off the roof, especially when it's wet. Looking from the attic hatch with a flashlight is safe. Walking on shingles is not.
What to Do While You Figure It Out
- If water is near a light fixture or outlet, turn that circuit off at the breaker.
- If the water is near the air handler or a vent, turn the AC off at the thermostat. That stops new condensate.
- Catch the water and move your things. Buckets, towels, and furniture out of the way.
- Photograph the spot each time it gets wet, with the date. That's your log, and your record if you file a claim.
If water is coming in during a storm, it's a roof problem and it can't wait. We respond the same day for active leaks, including temporary tarping. Our roof repair page covers what to do while you wait for the crew.
Still Not Sure? Let Us Take a Look
Some cases are mixed: a small roof leak and a sweating duct in the same corner, or a stain that only appears when a storm hits on a hot afternoon with the AC running. Those take someone who can rule the roof in or out.
Our roof inspection and written estimate are free, with no obligation. We photograph what we find, so you see the source for yourself. If the water is coming from your AC, a duct, or a bathroom fan, we tell you so, and you call the right trade instead of paying for a roof repair that won't stop it.
Questions About Ceiling Water, Answered
How do I know if a ceiling stain is a roof leak or condensation?
Can an AC unit in the attic leak through the ceiling?
Why is water dripping from a pipe above my window?
Why are my AC vents dripping water?
Can a bathroom fan cause water stains on the ceiling?
Will a roofer charge me if the water isn't coming from the roof?
Find Out Where the Water Is Coming From
We'll inspect the roof free, photograph what we find, and tell you plainly whether it's the roof. Water coming in right now? Call and we'll come the same day.
Already know it's the roof? See our roof repair page for prices and same-day emergency service.