The stain on your ceiling is not where the water got in. It's just where the water gave up. In hundreds of jobs, I've rarely seen the entry point sitting directly above the damage. Water runs downhill, wicks sideways, and rides wiring chases until it finds a low spot — and that low spot is what you see. Seal where the stain is, and you just sealed a perfectly good piece of ceiling and left the leak wide open.
Start with the clues
Before you touch anything, interrogate the damage. A soft floor means water got past the surface long ago. A brown ring on a ceiling panel is a leak that dried; a dark, spreading blotch is one that's still active. Musty smell with no visible damage? Start with the compartments and under-sink cabinets — mold starts in the dark spots you don't look at. And pop the lens off a clearance light: water sitting in a light cover is a guaranteed leak. It got in around that fixture and had nowhere else to go.
Rule out plumbing first
Half the "roof leaks" I get called to are plumbing. Two-minute test: turn on the water pump (or hook up city water), close every faucet and toilet valve, and listen. If the pump cycles on its own, water is going somewhere it shouldn't. Then put your hands on every fitting you can reach: under sinks, behind the toilet, around the water heater, at the city inlet. Wet only after it rains? Roof or walls. Wet all the time? Plumbing — and you don't need a ladder for it.
Work from the top down
Rain leaks start high, so start high. Check every roof seam and penetration — vents, skylights, antenna bases, the front and rear caps. Read the sealant like a skeptic: press it with a finger. Good sealant flexes; dead sealant cracks and crumbles. Check the marker lights too — their screws go straight through the front cap, and the sealant behind them bakes out in the sun.
Then walk the walls: window frames, compartment doors, every exterior screw head. Compartment doors are leak factories — the seals compress and the frames shift just enough to let water in. A roof leak can show up at the floor line three feet away, because water follows the framing down. Always suspect uphill of the stain.
Scan it with a moisture meter
When your eyes can't find it, cheat with a tool — a pinless moisture meter. Hold it flat against the surface and it reads moisture below it without putting a hole in anything. I sweep the ceiling, walls, and every roof penetration after a rain, and tape-mark anything reading clearly higher than the dry areas around it. It's the one tool that finds a leak before it becomes visible. I carry a Klein ET140 pinless moisture meter — it reads up to three-quarters of an inch deep, through the panel and into the real story.
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The hose test, done right
Still can't find it? Bring the water to it. Get a helper inside with a flashlight, then work the hose from the bottom up — one section at a time, five minutes each. Low to high: start at the roof and everything below gets wet, and you'll never know which level let it in. One suspect area at a time — a window, a compartment door, a roof seam. The second the helper sees a drip, stop. Mark it, dry everything, and keep going anyway, because there's often a second one.
When to call in the pressure test
Some leaks refuse to show up under a hose. For those, shops use a SealTech machine: a calibrated fan that pressurizes the inside of your RV while the tech sprays soapy water over every seam and penetration outside. Where air pushes out, bubbles form — there's your leak. It runs roughly a hundred to a hundred-fifty bucks, and it's worth it when you've been chasing a mystery leak for weeks. Expect to seal, retest, and repeat once or twice — closing one path changes the pressure and reveals the next one. Don't DIY it with a shop vac cranked to full blast: too much pressure blows out window and door seals, and then you're fixing two problems.
Mistakes that waste a weekend
- Sealing the stain instead of tracing the water back to the entry.
- Assuming the roof is the only suspect. Clearance lights, compartment doors, and plumbing fittings all leak too.
- Running the hose over the whole rig at once, so every section is wet and none of them tells you anything.
- Ignoring a musty smell because there's no visible damage yet. By the time you see it, it's old.
- Finding one leak and stopping. There's often a second one right behind it.
A leak you find this week is a tube of sealant. A leak you find next year is a wall. Work the system top-down, verify with water, and test instead of guessing.
Learn it once. Fix it right.
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