RV Roof Leak Around an AC Unit

Here’s the thing almost nobody tells RV owners: your roof air conditioner isn’t sealed with sealant. Unlike every vent and skylight up there, the AC sits on a thick foam compression gasket around the standard 14-by-14-inch opening, clamped down by bolts from inside the ceiling. No caulk, no lap sealant — by design. Which is why the most common “fix” we see, a bead of caulk run around the AC shroud, doesn’t work and usually makes the diagnosis harder.

First question: rain water or machine water?

Water at the ceiling grille has two very different sources, and timing separates them.

• Drips during or after rain, AC off: the gasket, or the roof membrane near the opening — a genuine leak
• Drips on hot, humid days while the AC is running hard: almost certainly condensate — the unit is making water faster than its drain pan sheds it, often because the pan’s drain holes are clogged with debris or the RV is parked off-level
• Both: it happens; work through the condensate side first because it’s the quicker check

Condensate problems are maintenance, not roof repair — clear the pan, check your level, and the “leak” often vanishes.

Why gaskets fail

The gasket is foam doing a compression job for years, through thousands of kilometres of road vibration and every freeze-thaw swing Ontario offers. It takes a compression set — squashed flat, no spring left — and stops sealing. Installation errors accelerate it from both directions: overtightened bolts crush it flat on day one, undertightened bolts let the unit rock and work loose. If your AC was ever removed and reinstalled, the gasket and its torque are the first suspects.

The proper fix

From inside, with the shroud and grille off, the mounting bolts are accessible. Sometimes the honest fix is as simple as bringing them back to spec — snug and even, compressing the gasket to roughly half its free height, not wrenched down to the stops. If the gasket has taken a set, the unit gets lifted, the flange cleaned, a new gasket fitted, and the bolts torqued evenly in a cross pattern. While the unit is up, the membrane around the opening gets inspected too — every so often the gasket takes the blame for a membrane split hiding under the unit where no inspection can normally see it.

Why the caulk trick backfires

Sealing the shroud to the roof traps water inside the shroud (the shroud is ventilated on purpose), hides where water is actually entering, and smears product onto membrane that the eventual proper repair has to deal with. If there’s water at your ceiling grille, resist the caulking gun. This one is a diagnosis job first — and it’s a quick one for anybody with a torque wrench, a moisture meter, and a reason to look.