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What the roof gasket does

Your rooftop air conditioner sits over a roughly 14 x 14 inch opening in the roof. A thick foam gasket between the unit's base pan and the roof seals that opening, and four mounting bolts from inside the RV pull the unit down to compress it. Over time the foam flattens, dries out or cracks, the seal fails, and rain finds its way in.

Signs your AC gasket is leaking

• Water dripping from the ceiling assembly or vents after rain, when the AC hasn't been running.
• Stains, soft spots or sagging in the ceiling around the AC.
• Water marks on the ceiling assembly or the filter.
• A unit that rocks, vibrates or rattles on the roof.
• Gaps between the base pan and the roof, or foam that looks crushed flat or crumbly.

Rule out condensation first. Water dripping while the AC is running in humid weather is usually condensate, not rain. A clogged drain or a frozen coil can also cause drips (see our post on frozen coils). If it drips after rain with the AC off, suspect the gasket.

Can you just tighten the bolts?

Sometimes. If the gasket is still in good shape and the bolts have loosened, bringing them back to spec can stop a leak. But don't crank them down. Over-tightening crushes the gasket and can crack the base pan or the ceiling assembly, and a gasket that has been flattened for years won't spring back. Follow your manufacturer's spec:

• Dometic: its installation instructions for current 15,000 BTU rooftop units say to tighten all four bolts evenly to 40 to 50 inch-pounds (inch-pounds, not foot-pounds). That's far lighter than most people expect, so use a torque wrench or driver that reads in inch-pounds.
• Coleman-Mach: the 47000 series instructions use the gasket itself as the gauge: each bolt is tight enough when the gasket's indicating tab has been pulled down even with the roof. On spring-type mounts, Coleman-Mach's Mach 8 manual says to tighten until the spring coils are compressed and just touching.

Check your owner's manual for your exact model.

How to replace an RV AC roof gasket

Before you start: rooftop units are heavy, so plan on at least two people (or a lift) to raise the unit. Only work on a dry, walkable roof.

1. Turn off the AC breaker, disconnect shore power and the generator, and turn off the inverter.
2. Inside, remove the ceiling assembly cover and then the ceiling assembly, unplugging its wiring connectors. Photograph the wiring first.
3. Remove the four mounting bolts.
4. On the roof, lift the unit off the opening and set it aside on a protected surface. Watch the power cable and the wiring connectors as you lift.
5. Peel off the old gasket and clean the roof and base pan. Coleman-Mach's instructions say to remove all old caulking so the roof surface is clean. Use a cleaner that's safe for your roof material.
6. Check the opening. Coleman-Mach's 47000 series instructions call for a 14 to 15 inch square opening; Dometic's call for 14 x 14 inches.
7. Install the new gasket on the base pan (or over the opening, depending on the kit), centered.
8. Set the unit back over the opening, lined up front to back.
9. From inside, reinstall the bolts and tighten them evenly in a crisscross pattern to your manufacturer's spec.
10. Reinstall the ceiling assembly, reconnect the wiring, restore power and test the AC. After the next rain, check for drips.

Universal 14 x 14 gasket kits: Dicor Seal-Tite ACGR-1 (left) and Camco 25071 (right).

Gasket kits by brand

• Coleman-Mach: gasket installation kit 8332-3301, 2-pack 8332A3302 (14 x 14 opening), gasket installation kit 47233-3301 and Mach 3 Plus roof gasket package 48203-3301.
• Coleman-Mach, low-friction roofs: on aluminum, steel or gelcoat fiberglass roofs, Coleman-Mach's instructions recommend the 8333-3871 kit to help hold bolt tension. For units that see heavy side loads, they recommend the Roughneck gasket and bolt package 48207-3301.
• Dometic: Blizzard NXT roof gasket kit 3316315.013, FreshJet 5 roof gasket installation kit 4450037541, and more in Dometic installation kits.
• Furrion: gasket installation kit C-FACR15SA-010.
• Universal 14 x 14: Dicor Seal-Tite ACGR-1, Camco 25071, VentMate 62901 and VentMate 65526. For a 14 x 16 opening, see VentMate 65527.
• Uneven or sloped roofs: Valterra leveling pad kit A10-1414.

See the full range of RV AC roof gaskets.

While the unit is off the roof

It's the easiest time to check the rest of the unit. Look for a cracked shroud (see our shroud replacement guide), clean the coils and filter, and inspect the roof around the opening for soft spots. If the roof deck is soft or rotted, repair it before reinstalling the AC; a new gasket won't seal on a damaged roof.