CargoandHorseTrailerRoofsLeakforDifferentReasonsThanRVsDo

RV roof advice is mostly written for rubber and TPO membranes. That leaves out a lot of trailers. Enclosed cargo trailers, car haulers, concession trailers, and many horse trailers have metal roofs, and they fail in their own way. The fixes that work on a membrane roof are not always right for them, and some of the most common problems are not about the sealant at all.

How a typical metal trailer roof is built

On most enclosed trailers the roof is a sheet of aluminum, or on some models coated steel, laid over roof bows and fastened at the perimeter. Many are one continuous sheet with no seams across the top. The edges are where all the joints are: the roof sheet meets the side walls under a rail or trim strip, screws go through it at intervals, and sealant is what keeps water out of every one of those holes and joints.

That design is simple and it is good at shedding rain across the middle. Nearly all the risk sits along the edges and at anything that penetrates the roof, such as vents, lights, and air conditioners on finished-out trailers.

The physics working against the sealant

Aluminum expands and contracts with temperature at roughly twice the rate of steel. A trailer roof in the Sacramento Valley can go from a cool morning to a surface temperature well above the air temperature in afternoon sun, every day, all summer. The aluminum sheet grows and shrinks along its full length while the steel frame under it moves far less.

All of that movement concentrates at the fastened edges. Sealant around each screw and along each seam is stretched and compressed thousands of times, and the sealants that last are the ones flexible enough to follow it. A hard caulk, or one that has aged and lost its flexibility, cracks at exactly the points the movement is concentrated.

On a metal roof, the middle sheds water and the edges do all the work. Look at the edges.

Where these roofs leak

• Screw penetrations along the roof edge, where sealant has cracked or shrunk back from the screw head.
• The front and rear roof caps, where the roof sheet transitions over the curve of the nose or meets the rear header. Wind-driven rain on the highway hits the front hard.
• Seams in the trim or rail where two pieces meet end to end.
• Roof vents, especially the plastic vent lids that go brittle in sun, and the flanges under them.
• Marker lights mounted on the roof edge or front cap. Each is a hole through the skin.
• Any accessory added after the fact: a roof rack, a light bar, an antenna, a solar panel. Every new screw is a new place for water to get in.

Sealant versus tape on metal roofs

Two approaches show up on metal trailer roofs, and they are often used together. Self-leveling lap sealant flows into the small gaps and channels at the roof edge and around screw heads. Roof tape, the best-known being EternaBond, is a thick, flexible tape with an aggressive adhesive that bridges an entire seam in one continuous strip.

EternaBond lists aluminum and galvanized steel among the surfaces its tape bonds to, and it states that a proper installation can last twenty years or more. Its own guidance also says the old cracked, loose, and dirty material should be removed first for a durable seal, and it gives a minimum application temperature of 40 degrees. On a metal roof that usually means taking failed sealant all the way back to clean metal before anything new goes on. Tape laid over crumbling caulk is only as good as the caulk.

Tape is particularly good along long perimeter seams on trailers that flex a lot on the road. Sealant is better in tight, irregular spots like around screws and fixtures. A combination, sealant into the gaps and tape capping the seam, is a common and sound approach.

Water inside that is not a leak

Horse trailers and uninsulated cargo trailers have a problem RVs mostly do not: condensation on the underside of a bare metal roof. Warm, moist air inside a closed trailer meets a roof that has cooled overnight or under a cold rain, and water forms on the inside of the metal and drips. With horses aboard, who breathe out a lot of moisture, it can be considerable.

Owners chase this as a leak and reseal a roof that was not leaking. The clue is the pattern. Condensation drips from across the ceiling, often in many places at once, and shows up on cold mornings or after hauling animals even when it has not rained. A leak tends to show up at the edges or near a fixture, and it tracks with rain.

Condensation is managed with ventilation and, on trailers built for it, insulation or a roof liner. Sealant does nothing for it. It is worth working out which problem you have before paying to fix the wrong one.

Riveted aluminum trailers are their own category

Vintage and riveted aluminum travel trailers, the polished shells that turn heads at campgrounds, are sealed differently again. Every rivet and every overlapping panel seam is a potential leak path, and restorers use seam sealers made for that construction. Silicone is a poor choice on these for the same reason it is a poor choice on a membrane roof: very little bonds to it afterward, so whoever does the next repair has to strip it out first.

Keeping a metal roof tight

• Look along the full length of both roof edges twice a year, and after any long trip on rough roads.
• Press on the sealant at a few screw heads. Healthy sealant has give. Hard, cracked sealant has stopped moving with the roof.
• Check every light and vent on the front cap. Highway rain is hardest on the front.
• Before resealing, remove the failed material and clean the metal. New sealant over old failure is not a repair.
• If the trailer is finished out with a wood subfloor or wall panels, check the corners inside for darkening or softness. Water that gets in along the roof edge runs down the walls.
• Keep silicone off the roof entirely.

If you are not sure whether you are dealing with a leak or condensation, a photo of where the water appears inside, plus when it shows up, is usually enough to tell. If it is a leak, the roof edges are where to start.