Leak Points

Symptom: A stain on the ceiling panel near — but rarely directly at — a vent. Water runs along the rafter before it drops, so the stain is usually a foot or two off to one side.

Confirm it: Hose the vent alone for three minutes with someone inside watching the ceiling seam. Look at the flange corners with a flashlight first; cracked lap sealant at the corners is visible from the roof.

Why it fails: The flange is screwed through the deck and sealed with a bead. Highway vibration works the screws, and UV hardens the bead. It is a wear item, not a defect.

Spec: Remove all lap sealant to bare flange. 7001 primer on the flange plus 6 in. of deck. Alasta Seal built to 80 mil across the transition, feathered into the 40–60 mil field.

Symptom: Water dripping from the AC ceiling vents or running down the interior shroud during rain. Distinctive because it does not stain a panel — it comes straight out of the duct.

Confirm it: This one is nearly self-diagnosing. If the water is coming out of the air distribution box, the gasket is the source and nothing else is.

Why it fails: The foam gasket between the rooftop unit and the deck is compressed by four through-bolts. It takes a permanent set over a few seasons, the bolts back off with vibration, and the compression seal opens.

Spec: Pull the unit, replace the gasket, torque to spec. Then seal the perimeter of the mounting opening and the surrounding deck with 7001 and Alasta Seal at 80 mil before the unit goes back down — so the next gasket failure has a waterproof deck under it.

Symptom: Staining in the overhead bunk, at the front ceiling corner, or in the rear closet. Often accompanied by a musty smell before any stain is visible.

Confirm it: Water-test the cap seam alone, and give it a full ten minutes before you rule it out — the path from the cap joint to a visible stain can be long.

Why it fails: A molded fiberglass cap meets an EPDM or aluminum roof at a compound curve, covered by a trim strip and a run of sealant. The two materials expand at different rates, the joint moves every time the unit is towed, and it faces directly into rain at highway speed.

Spec: Remove the trim and all sealant. 7001 on both substrates. Alasta Seal at 80 mil across the joint, feathered 6–8 in. onto both the cap and the deck. This is the detail most worth over-building.

Symptom: Water on the slide floor or along the slide ceiling line, usually only when the slide is out or only when it has just been retracted wet.

Confirm it: Test with the slide in one position, then the other. A leak that only appears in one state points at the wipe seals or the rail sealant, not the roof.

Why it fails: Nothing else on the unit moves as much. The rail is through-bolted into the sidewall, the topper is tensioned across it, and the sealant at both ends is worked constantly.

Spec: Alasta Seal at 60 mil over the rail perimeter and header, with 7001 on the aluminum and the sidewall skin. High-elongation membrane is specifically why polyurea rather than a rigid coating belongs here.