Water Enters High And Leaves Low

An RV roof is not flat. It is built with a crown, a shallow arch from one sidewall to the other, so rain sheds sideways instead of standing in the middle. That crown is the first thing to understand about a leak, because it means the highest line on the roof is the centre line, and anything that gets in under the membrane begins by moving away from it. A penetration near that centre line, an air conditioner opening or a bathroom vent, feeds water toward both sidewalls at once, which is how one entry point produces evidence on two sides of the coach.

The second slope is the one nobody thinks to mention, which is the attitude of the coach where it sits. A trailer parked nose down on a driveway is a roof tipped forward by a degree or two, and that is enough to carry water from a rear vent flange over half the length of the coach before it finds anywhere to leave. Owners who always park on the same slope get the same leak path every time. That is genuinely useful at estimate, and it comes up perhaps once in twenty conversations because it does not feel like information.

Roof framing on almost every coach runs transversely, side to side, with decking laid over it. The space between the decking and the ceiling panel is therefore not one open void but a row of channels, each one bounded fore and aft by a rafter and open across the full width of the vehicle. Water that gets past the membrane lands in one of those channels and stays in it. It cannot cross forward or back without help, and it is completely free to travel left and right.

The practical consequence is worth writing on a notepad. Measure from the stain forward and back, not side to side. An owner with a ring in the ceiling eighteen feet behind the front cap needs to look at the penetrations that sit eighteen feet back, and can ignore the vent that happens to be nearest. The exception is a coach with ducted air conditioning, where the ducting has already cut through the bay walls to run air, and water uses those openings exactly the way the air does. Ducted coaches are the difficult ones, and it is fair to say so up front.

Once water reaches the roof edge it turns vertical, and the space it enters is a narrow cavity between an outer skin and an interior panel with framing, wiring looms, window headers and cabinet cleats crossing it. Every one of those is a shelf. Water falls until it meets one, spreads along it, and only continues down when it has enough volume to get past. That is why a wall can be soaked at three separate heights with perfectly dry material between them, and why a single reading proves nothing about the height above or below it.

It is also why walls hide water longer than ceilings do. An interior wall panel is a vinyl faced board, and vinyl is a vapour barrier facing the room. Moisture behind it has no route out into the living space, so there is no stain to see. The first visible evidence is dimensional: a cabinet edge that has swollen, a valance lifting at one end, a window surround that no longer sits flat. By the time a wall shows you a shape change, it has been holding water through at least one wet season.

Every water job that arrives here presents as one of a short list of symptoms, and the symptom is a position on a route rather than a diagnosis. What it tells you reliably is how far the water has already travelled, which is the same thing as how long it has been running.

• A ring on the ceiling panel, meaning water reached a low point above the ceiling and stopped there rather than continuing
• A dark line at a slide sill, meaning water arrived along the floor and not through the ceiling at all
• A swollen cabinet edge or a lifting valance, meaning a wall cavity has been holding water at that height for a season
• A soft floor at the base of a wall, which is the end of the route and the most expensive place to find it
• A musty smell with nothing visible anywhere, which means the water is sitting in a cavity that has no interior surface to stain

The method an owner can run without tools is short. Fix the fore and aft position of the evidence with a tape measure and write the number down. List every penetration that sits within a couple of feet of that band, both sides of the roof, including the ones nobody counts such as a solar mounting foot or a ladder bracket. Then ask which of them the coach's parking slope would carry water toward. That usually leaves two or three candidates out of a dozen, and it takes twenty minutes.

What that does not do is settle it, and this is where we decline to be helpful in the way people want. We do not write a water estimate from photographs, and we say so when a set arrives by email. A photograph of a ceiling ring proves water got in. It does not carry the path, and the path is the estimate. Insurance walk-ins are welcome and we write the estimate here, with the coach in front of us and the interior evidence mapped before anybody climbs a ladder.

Resealing the nearest flange when the real entry point is two bays forward is not a neutral mistake. Roof vent and fan work runs $250 to $1,200 and skylight work $350 to $2,500, so the wasted money is modest. The expensive part is the calendar. A leak that keeps running through another wet season does not pause, and in this climate it never gets a chance to dry between wettings, which is how a defined decking repair over a few square feet turns into a strip. Flooring and subfloor work runs $500 to $8,500 and water damage restoration $750 to $15,000.

We have guessed wrong ourselves, and the correction is the reason the sequence here starts inside. A stain in a bedroom corner sent us to the vent above it, which was resealed properly and did nothing, because the entry point was an antenna base on the opposite side of the roof and two bays forward. Diagnostic time posts at $285 per hour with a one hour minimum, credited against an authorised repair, and that hour spent mapping the interior first is the cheapest hour on a water job.