Ceiling Water Stain: Roof, Plumbing or AC Condensate?
When a ceiling stain grows tells you most of what you need to know. A stain that spreads during or after rain points to the roof. One that grows when a shower, toilet or sink above it is used, or keeps growing on dry days, points to the plumbing. One that shows up in hot weather while the air conditioner runs, under an attic unit or along its drain line, points to the AC's condensate. Then check what's directly above the stain, run a two-hour water-meter test to rule a pressurized pipe in or out, and look outside for water dripping from the AC's overflow pipe. Each answer sends you to a different trade: a roofer, a plumber or an air-conditioning technician.
This page is for houses in the Los Angeles area. If it's the roof, finding exactly where the water gets in is covered in how to find a roof leak; if water is coming through right now in a storm, start with what to do when the roof leaks in a storm.
Start with a pencil and a date
Before anyone opens a ceiling, draw a light pencil line around the edge of the stain, write the date next to it, and photograph it. Then check it after each rain, after the bathroom above has been used, and after a hot day with the AC running. A stain that crosses its line after one of those, and not the others, has told you its source. Note too whether it's damp to the touch or dry, and whether it ever drips.
The clues side by side
Each suspect leaves a different pattern. The table sets them side by side, with the check that settles each one.
| Suspect | When it grows | Where the stain usually is | Quick check | Who to call |
|---|---|---|---|---|
| The roof | During or after rain, sometimes only in wind-driven rain | Top-floor ceilings, near outside walls, below a chimney, skylight, vent or valley | Look in the attic during or right after rain, following the wet trail uphill | A roofer |
| A water supply pipe | Any time, rain or not, and it keeps growing | Near bathrooms, the kitchen, laundry and water-heater lines | The two-hour water-meter test | A plumber |
| A drain line | Only when a fixture above is used | Under a shower, tub, toilet or sink | Run each fixture above it, one at a time, and watch the stain | A plumber |
| The air conditioner's condensate | In hot weather while the AC runs | Under or near an attic air handler, or along its drain line | Look for water dripping from the overflow pipe outside, often over a window or door | An air-conditioning technician |
| Condensation in the attic | In cool weather, with no rain and no water used above | On the underside of the roof deck and on nails, often in a poorly ventilated attic | Look at the deck and the attic's vents | A roofer, for the ventilation |
A stain can have more than one cause, and water held in wet insulation can keep dripping for a day or two after a storm, so a drip the morning after rain isn't proof of plumbing.
If it's the roof
Rain is the giveaway: a roof leak follows the weather. The stain usually appears on the top floor, and often not directly under where the water gets in, because water runs along the underlayment, the deck or a rafter before it drips. The attic, right after rain, is the place to look; how to find a roof leak sets out the steps, and tile roofs have their own patterns in why a tile roof leaks when the tiles look fine.
The first suspect isn't always the right one. On a 2023 job in Granada Hills, the owner blamed a leak on solar panels installed 18 months earlier; it turned out to be an older penetration in the roof.
If it's the plumbing
Plumbing leaks don't care about the weather. Two kinds behave differently:
- Supply pipes carry water under pressure, so a leak runs whether or not anyone is using water. The EPA's WaterSense program gives the test: check your water meter before and after a two-hour period when no water is being used; if the reading changes at all, you probably have a leak. A meter that moves while the stain grows points to a supply line; a running toilet or a leaking irrigation line moves it too, so rule those out first.
- Drain lines only carry water when something above them is used, so the meter test won't catch them. Run the shower, then the tub, then flush the toilet and run the sinks above the stain, one at a time, and check the stain after each. A stain that grows only after the shower has found its source.
A stain under a bathroom, kitchen or laundry room, with no link to rain, is a plumber's call.
If it's the air conditioner
An air conditioner pulls water out of the air as it cools, and that condensate has to drain away. California's mechanical code requires it to be collected and carried to an approved place, such as a plumbing fixture or a drywell, as the City of Long Beach's building department summarizes it.
Where the cooling coil sits in an attic, or anywhere an overflow could damage the house, the code adds a second line of defense in case the main drain clogs. Long Beach describes an extra watertight pan under the coil, or a pan with a separate secondary drain, piped to a place outdoors where it can be readily seen, such as over a door or a window. A handout from the City of Riverside, written for the 2019 edition of the code, lists four accepted methods: a pan under the whole unit with its own drain, a second drain set higher in the unit's pan, a pan with a water-level sensor that shuts the unit off, or a water-level sensor in the drain that does the same.
That gives you two clues to look for:
- Water dripping from a small pipe over a window or door in hot weather. If that pipe is the overflow drain, it's doing its job, and the main condensate drain is blocked. An air-conditioning technician can tell which pipe is which.
- An air conditioner that stops cooling on a hot day, on a system with a water-level sensor. The sensor may have shut it off because the pan is filling.
A stain under an attic air handler, growing on hot days, is a job for an air-conditioning technician, not a roofer.
If it's condensation in the attic
Not every wet attic has a leak. The asphalt roofing industry's manual warns that stains in the attic can come from condensation when the attic isn't ventilated well enough, and IKO, a shingle manufacturer, calls these false roof leaks. Moisture from the house rises into the attic and condenses on the cool underside of the roof deck and on the nails. It shows up without rain, and it points to the attic's ventilation, which is roofing work, rather than a hole in the roof.
Don't let it sit
Whatever the source, a stain that keeps growing is water inside the house. The EPA's mold guidance is to dry water-damaged areas within 24 to 48 hours to prevent mold. The California Department of Insurance notes that a leak that has gone on for a while may already have grown mold, and that cleaning it up yourself can spread the spores; it also tells homeowners to report water damage to their insurer immediately. Drying out after a leak is covered in what to do when the roof leaks in a storm.
Frequently asked
Can a roof leak show up when it isn't raining?
How do I check for a plumbing leak above a ceiling?
Why is water dripping from a pipe above my window?
Can my air conditioner leak through the ceiling?
Is a water stain on the ceiling always a leak?
Who should I call first?
Related reading
Sources & methodology
Code sections, rules and figures on this page come from the primary sources below. Codes, local requirements and product terms change; each was verified October 5, 2026.
- U.S. EPA WaterSense, Fix a Leak Week (finding leaks with the water meter)
- City of Long Beach, Building and Safety: Mechanical, Electrical and Plumbing Code Requirements (air conditioning drain lines)
- City of Riverside, Building and Safety: 2019 California Mechanical Code Residential Condensate Disposal Requirements
- Asphalt Roofing Manufacturers Association, Residential Asphalt Roofing Manual, 2014 edition (Chapter 17, Roof Care and Maintenance)
- IKO, How to Find a Roof Leak (updated June 2, 2026)
- U.S. EPA, A Brief Guide to Mold, Moisture and Your Home
- California Department of Insurance, Residential Property Claims Guide (Form 405, February 2025)
We'll tell you whether it's the roof
Call (323) 844-7777. We look in the attic and on the roof. If it's the roof, we find where the water gets in and repair that section; if the clues point to the plumbing or the air conditioner, we say so plainly. Have ready when the stain grows, what's above it, your photos and the dates you marked on it.
Prepared by Cali Energy, October 5, 2026. This article is for general educational purposes only and is not legal, tax, financial, engineering, or utility advice. Rates, incentives, codes, permit requirements, equipment specifications, prices, and program terms may change; figures and timelines are estimates, not guarantees. Confirm current requirements with the applicable utility, AHJ, program administrator, manufacturer, or a licensed professional. See our Content Disclaimer. Cali Energy · 19201 Parthenia St, Unit E, Northridge, CA 91324 · CSLB #1032379 (B, C-10, C-39)