File 03: Two Saturated Decking Bays and Two Entry Points

A 28 foot single axle trailer, thirteen years old, EPDM membrane over lauan decking. The interior ceiling panel above the bed carried a brown stain about the size of a dinner plate with a defined edge, and the lap sealant along the front cap to roof seam was cracked and lifted in two places. The roof read springy on a two foot section just behind the seam.

We ran a seam by seam hose test from the top down with an observer inside. Ten minutes on the front cap seam produced a wet line along the interior ceiling edge within four minutes, which confirmed the obvious entry. Every other seam on the front half tested dry that afternoon, and those negatives were logged, which turned out to matter later.

Two decking bays were saturated to the point of separating into plies, running from the cap seam back about four feet. The rafter between them had rotted at one end, where water had pooled against the wall junction rather than draining. The front wall top plate was wet for roughly thirty inches, soft enough to take a screwdriver without resistance.

None of that was a surprise given the symptom. What was mildly surprising was how far the moisture had travelled laterally along the top plate compared with how little had appeared on the ceiling. Water in an RV runs along framing until something stops it, and a top plate is an excellent channel. The stain marked where it finally found a path down, not where it came in.

With the decking, the rafter end and the top plate section replaced and the cap seam properly re bedded, we ran the hose test again before installing new membrane. Standard practice, and this is why. The interior wetted again, in a different place, about five feet back along the same wall. The cap seam was sealed and dry. Something else was letting water in.

It was the awning rail. The rail is through fastened into the sidewall, and several of the screws had backed out enough to break their sealant. Wind driven water was tracking down the screw shanks and into the wall cavity, then along the same top plate. That entry had existed independently of the cap seam and would have been sealed permanently under new membrane had nobody looked.

Once both entries were closed, the remaining structure had to be proven dry. Saturated plies were already gone, so what was left was framing and the underside of adjacent decking. Air movement ran for several days, with pin meter readings taken on a grid and compared against a dry reference point on the same materials at the rear of the trailer.

The first reading was close but still elevated at the wall junction. The second, two days later, matched the reference. That is the point at which membrane and interior panels can go on. It is also four days that produce nothing an owner can see, which is why this stage gets skipped by people in a hurry and why we will not skip it.

Seventy four body hours at $210 per hour, twenty two shop days. Roughly nine of those hours were the second entry: retesting, pulling the awning rail, cleaning fastener holes, re bedding and refitting. The owner funded the job, because thirteen years of seepage is deterioration rather than a covered event, and we said so at intake rather than letting them find out from an adjuster.

The lesson transfers to any water job. Finding a leak is not finding the leak. On a wall with an awning rail, a window, marker lights and a cap seam, there are four independent candidates and nothing stops two of them failing at once. Test, close, then test again. It costs an hour and it is the difference between a repair and a repeat.