Sealing RV Roof AC Gaskets Before They Leak

Every rooftop AC I pull for a ceiling leak has the same story underneath it: a foam gasket that did its job for years, slowly flattened out, and finally quit. No drama — just a tired seal over a big hole.

Here's the part most owners miss: that gasket is the only seal between the AC and your roof. There's no bead of caulk doing backup duty, and there shouldn't be. The AC sits on a picture-frame gasket around a standard 14-by-14-inch opening — some units use 14-by-16 — and four bolts sandwich it to the roof from inside. When the foam loses its spring, rain walks in and shows up as a stain around the ceiling assembly.

How the gasket fails

Foam takes a compression set. Years of the unit's weight, Florida heat, and road vibration leave it thinner and harder than it started, and the bolts loosen as the foam gives up thickness. Eventually a hard rain finds the gap — and by the time you see drips from the air return, the ceiling and decking around that opening have usually been wet more than once.

There is no fixed replacement interval — inspect it every year and replace it when it's flattened, cracked, crumbling, or leaking. That's a better rule than any calendar.

First check: are the bolts just loose?

Before you order anything, drop the interior ceiling assembly and look at the four mounting bolts, one in each corner. Snug them evenly, a little at a time, in an X pattern. Dometic and Coleman manuals I work from spec roughly 40 to 50 inch-pounds — inch-pounds, not foot-pounds. That's barely more than snug, and it's enough to compress a healthy new gasket to about half its roughly 1-inch thickness. Some kits even have indicator tabs that show when you've got there.

I use a small 1/4-inch drive inch-pound wrench like the TEKTON 24320 torque wrench (20-200 in.-lb.). Your foot-pound wrench can't read this low, and guessing is how gaskets get crushed. If a re-torque stops the drip and the foam still has spring, recheck it after a few trips — travel settles a gasket. If it leaks again, or the foam is flat and hard, stop tightening. More torque on dead foam distorts it and can warp the pan.

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Do not caulk around the AC

This is the mistake I peel off other people's rigs. Smearing sealant around the base does not help the gasket — it dams up the condensate that should drain out of the pan, across the roof outside the gasket, and off the rig. Trap it and it comes inside instead. Sealant like Dicor 501LSW-1 Self-Leveling Lap Sealant belongs on vent flanges and screw heads — not as a moat around your AC.

While the shroud is off, keep the drain holes in the pan clear. A clogged drain makes a good gasket look guilty: rain-only leaks point at the gasket, but drips while the AC runs on a clear day point at drainage first.

Replacing the gasket

• Kill the power — shore cord unplugged and 12-volt off — then pull the interior assembly and disconnect the wiring so the unit is free.
• Back the four bolts out and lift the unit off the opening, or tip it onto blocks beside it. It's heavy and awkward; get a second set of hands and don't drag it across the membrane.
• Scrape the old gasket off the bottom of the unit and clean both mating surfaces. Old foam and dirt left behind become the new leak path.
• Stick the new self-adhesive gasket to the bottom of the AC, centered on the opening, and set the leveling pads where the kit calls for them — they carry the outer edges so the gasket doesn't take all the weight.
• Set the unit square over the hole, start all four bolts by hand, then bring them down in an X pattern, a little at a time, to 40-50 inch-pounds — about half the gasket's thickness, or to the indicator tabs.
• Put the interior back together, restore power, run the AC, and hose-test the roof around the unit. Recheck bolt snugness after a week and after your first trip.

Match the kit to your unit before you climb up — confirm the opening, and whether your unit uses drain cups or a ducted plenum needing a specific gasket. A universal kit is cheap insurance; the ceiling and decking you replace when you skip it are not.

Mistakes that bring the leak back

• Cranking the bolts with a foot-pound wrench or a driver — crushed foam seals worse, not better.
• Tightening one corner all the way down, then the next. Uneven compression leaves a low corner that leaks.
• Caulking the perimeter of the unit and trapping condensate on the roof.
• Reusing a flattened gasket because it "looked okay" while the unit was off. If you lifted it, replace it.
• Forgetting the drains — then chasing a "gasket leak" that was a clogged pan all along.

This is a half-day job with basic tools, and it's one of the cheapest leak preventions on the whole rig. Check the bolts every year, respect the inch-pounds, keep the drains open, and swap the foam when it's tired. Fix it right the first time, before the ceiling tells you about it.