Trailer Roof Leaks: How to Find Them and Seal Them Before They Spread

The roof on an enclosed cargo trailer is a sheet of aluminum over roof bows, sealed at the front cap, the rear header, the side rails and every hole cut for a vent or an A/C unit. It does not leak in the middle. It leaks at those seams and holes, and by the time you notice water inside it has usually been getting in for a while. Catching it early is a ten-minute job. Catching it late means replacing a ceiling panel and possibly the plywood under it.

Where to look

• The front seam where the roof meets the V-nose cap. This takes the full force of rain at highway speed.
• Around every roof vent and A/C unit. The screws and the flange gasket are the usual culprits.
• The side rails where the roof sheet turns down and meets the wall trim.
• The rear header above the ramp or barn doors.
• Any screw in the roof, including ones holding a rack or a solar panel.

Inside first

Start by looking up inside the trailer with a good flashlight on a dry day. Water leaves marks: a stain on the ceiling liner, a swollen or discolored spot on the plywood, a drip line down a wall post, rust on a roof bow. Where the mark is tells you which seam is upstream of it, keeping in mind that water runs downhill and along bows, so the stain can be a couple of feet from the entry. If the trailer has a ceiling liner, the leak often shows as a soft spot before it shows as a stain.

Then the roof

From a ladder, walk the seams with your eyes and your hand. You are looking for sealant that has cracked, pulled away from the aluminum, or gone chalky and thin. Look at each vent flange for lifted edges and each screw head for a missing dab of sealant. If you cannot find it, a helper inside with a light while you run a hose slowly along one seam at a time, starting at the bottom and working up, will find it in a few minutes. Start low so you do not soak everything at once and lose the trail.

Sealing it

Use a sealant made for aluminum roofs, not silicone from the kitchen aisle. Self-leveling lap sealant is the right product for horizontal seams and around vent flanges; a non-sag sealant is right for vertical joints at the cap and rails. Clean the old sealant off to bare, dry aluminum with a plastic scraper and a solvent that does not attack the paint, then apply a bead that overlaps the seam on both sides. Do it on a warm, dry morning so it cures before the afternoon storm. Resist the urge to lay new sealant over cracked old sealant; it looks fixed and leaks again in a month.

Vents and A/C units

A vent that leaks is usually a gasket that has flattened or a flange whose screws have backed out. Retighten the screws, reseal the screw heads and the flange edge. If a rooftop A/C is leaking, check the gasket between the unit and the roof, which compresses over time; the fix is a new gasket and the unit torqued back down evenly. Our post on roof vents, A/C and airflow covers how those are installed, and a leak around one is often an installation problem rather than a roof problem.

How often

In Georgia, twice a year: once in spring before the storm season, once in fall before hunting season and the cold. Add a look after any hail, after a tree limb comes down on it, and after any trip where the trailer sat under a sprinkler at a campground or a car wash pushed water at the seams. Fold it into the routine in our maintenance checklist, and if the trailer sits outside all summer, our summer heat post explains why the sealant ages faster than you expect.

If a leak has already gotten into the plywood, the fix is more than sealant, and it is worth pricing against what a new unit from our inventory costs before spending on a ceiling and floor. Newer builds from our lot use a one-piece aluminum roof, which takes most of the mid-roof seams out of the picture; ask about it when you spec a build.