Water dripping out of the ceiling around the air conditioner is a very common RV leak complaint, and it gets an equally common wrong fix: a ring of sealant around the base of the unit on the roof. It looks like the obvious move. It almost never addresses the actual cause.
How a rooftop AC is sealed
Most rooftop units sit over a large square opening in the roof, typically about fourteen inches on a side, and they do not seal to the roof with caulk at all. They seal with a thick foam gasket between the base of the unit and the roof. Inside the coach, the ceiling assembly connects to the unit with long bolts, and tightening those bolts pulls the unit down and compresses the gasket.
Dometic's guidance for many of its units is to tighten the four mounting bolts evenly to roughly 40 to 50 inch-pounds, which compresses the gasket to about half an inch. That is not much torque. It is a snug, deliberate setting, not a hard pull with a ratchet.
Why it starts leaking years later
Foam takes a set. A gasket that has been compressed for years in summer heat and winter cold loses some of its rebound, and the seal that was tight on delivery day becomes a gap at one corner. Road vibration plays a part, and so does the weight of a heavy unit on a roof that flexes. Rain then runs under the edge of the unit, through the opening, and into the ceiling.
There is an equal and opposite mistake too. Overtightening the bolts crushes the gasket past the point where it seals evenly, and it can distort the ceiling assembly. Owners who discover the bolts and crank them down hard sometimes trade one leak for another.
The air conditioner does not seal to the roof with caulk. It seals with a gasket you cannot see from the roof at all.
Why caulk around the shroud does not help
The gasket sits under the base of the unit, inside the footprint. The edge you can reach on the roof is the outside of the unit's base pan and the roof around it. Sealant there is not in the water path that matters, and if it bridges onto the unit itself, it can interfere with removing the AC later.
It can also hide the evidence. A ring of fresh caulk around an AC is often a sign that someone was chasing a gasket leak from the wrong side, and it is worth knowing that before assuming the problem has been dealt with.
Gasket leak or condensation?
Not every drip near the air conditioner is rain. A rooftop AC pulls moisture out of the air while it runs and drains that water onto the roof. If the rig is out of level, the drains are blocked, or the unit is working hard in humid conditions, condensate can end up where it should not.
- Drips that appear only while the AC is running, especially on muggy days, point toward condensate.
- Drips that appear after rain, with the AC off, point toward the gasket or the roof around the opening.
- Water that shows up after driving in rain can be wind pushing water under a tired gasket from the front of the unit.
- Staining on the ceiling panel around the grille, spreading outward, usually means the leak has been going on across several storms.
Sacramento summers are dry, so the condensate scenario is less common here than on the coast or in the Southeast. When an AC drips in the Valley, the gasket is the first suspect.
What the fix involves
Sometimes the first step is simply checking the bolt torque. If the bolts have backed off or were never set evenly, bringing them back into spec can restore the seal on a gasket that still has life in it. That is a quick check from inside the coach.
If the gasket has taken a permanent set, it needs replacing. That means removing the ceiling assembly, lifting the unit off the roof, scraping away the old gasket, cleaning the roof surface and the opening, and inspecting the decking around it for moisture. Then a new gasket goes down, the unit is reset square, and the bolts are brought down evenly to spec. Dicor sells a universal replacement gasket kit with leveling blocks for exactly this job, which is a fair indication of how common it is.
Lifting a rooftop AC is a two-person job on a roof that can take the weight, and the unit is heavier and more awkward than it looks. It is also the one moment you can see the roof underneath it, which is valuable if water has been getting in for a while.
While the unit is off
Check the edges of the opening for soft or darkened decking. A gasket that leaked for a season or two has often let enough water in to start the wood around the opening going, and it is much easier to deal with that while the unit is already off than to discover it later from below. Look at the electrical connection and the condition of the membrane where it wraps into the opening, too.
If your AC drips after rain, start with the bolts, and if that does not settle it, plan on a gasket. What you do not need is another ring of sealant on the roof around the unit.
The Nor-Cal Crew
Mobile detailing, ceramic coating & paint correction · Sacramento, California