Finding and fixing rainwater leaks

In response to an earlier blog about finding plumbing leaks, I got this comment:

"I was wondering if you have any suggestions on where to look for the cause of a leak in our left outside compartment. It only occurs when it rains severely.

We have exhausted our options. We have taken it to an Airstream dealership twice, and basically they couldn’t figure it out."

I've heard versions of this one before. It can be extremely frustrating to keep going back to the service center for different "fixes" that don't solve a leak.

Part of the problem is that service technicians sometimes just guess at the likely cause of the leak but don't do real diagnosis, because diagnosis takes time and effort. So they throw some sealant at the problem, and (in this case) tighten the compartment door latch, and hope for the best.

As with plumbing leaks, solving a rainwater leak is mostly a matter of being methodical. The first step is to make sure it's really rain that's leaking, and not part of the plumbing system.

You know it's a rainwater leak when...

• it only occurs after rain, snowmelt, very heavy dew, or washing
• the plumbing is completely de-pressurized when the leak occurs (pump is off, no hose is connected, and the faucets have been left open to relieve any pressure in the pipes)
• water or water stains appear above countertop level, such as on walls, emanating from wall seams, on curtains or ceiling

It may seem basic, but you should check the obvious things first: if this is the first time you've spotted this leak were all the roof vents, fans, windows and door fully closed? What about the outside shower compartment (if equipped) and all the exterior access doors?

If the cause isn't obvious and it's still raining, just do what you can to collect the water and mop it up until the rain stops. You should make every effort to prevent water from settling in cabinetry or on the floor, as it will eventually cause permanent stains or delamination if left to sit.

Ask yourself, when does it leak?

In this case, the owner said "It only leaks when it rains severely," and indicated that the leak has been ongoing even after having a service center look at it.

This is both good and bad news. Good news because it's a hint that the leak is small. It might take longer for the leak to do severe damage. But it's kind of like the difference between being shot with a .22 versus a .38 caliber gun. Either way, you want to get to the hospital.

It's also bad news because small leaks are harder to find. A gap in sealant the size of the tip of a pen can let a steady drip-drip of water that really adds up over time.

Noticing that "It only leaks when it rains severely" is a strong indicator that this isn't a plumbing problem. But I have to point out that despite appearances, the leak almost certainly always occurs when it rains, not only in severe rain. It's just that the water is not always visible, because in a short or light rain the water doesn't always have a chance to migrate to somewhere that you can detect it. Instead, it stays inside the walls or ceiling.

That's a pretty good indication that this isn't a plumbing problem, so we can move onto the next step.

Understand how your Airstream is built

Airstream trailers are semi-monocoque, double-wall construction. Let's break that down:

• "double wall" means that the body of the Airstream is made of two sheets of aluminum separated by a gap. Ribs made of aluminum separate the two sheets, and electrical wires and insulation are installed in-between. In the photo above you can see the exterior body aluminum sheet, the ribs, and the wires (the interior body sheet is not yet installed).
• "semi-monocoque" means that the body is curved like the frame of an airplane but the floor is flat.  

Now imagine a stream of water running from a leak in the roof somewhere. Where would it go? 

Naturally the water would run downward, but it would soon run into obstacles. For example, in the photo above you can see how water running from a leak from a clearance light above the front window would quickly hit the frame of the window (the rectangular area). The water would either drip into the window area or it might be forced to go left or right and then continue downward to the floor before it became visible.

Airstream puts Knauf Ecobatt insulation inside the walls of current production travel trailers. (They previously used fiberglass insulation, which is similar.) The insulation doesn't absorb water but it can hold water like a sponge, and this is why a small leak sometimes doesn't show itself. Until there's a lot of water, it might just stay in the wall, soaking the insulation.

Electrical wires and other objects in the walls can also slow down or alter the path of water. For all these reasons, you have to accept that a rainwater leak may not take the straightest path downward. Water tends to meander inside the ceiling or walls and emanate from odd places, depending on the tilt of the trailer and obstacles in the walls.

Start at the top and know the likely causes

Now you understand why exterior leaks are particularly hard to trace. Working backward to trace a leak can be fruitless. In many cases, you may not be able to find the leak source until you can get a good look at the roof.

In my experience, rainwater leaks are 90% caused by failed sealant around something on the roof, like a roof vent or antenna. A few rainwater leaks are from failed sealant over the entry door or windows. Both of these are easy to fix, and so you should almost always start at the top and work down.

Keep in mind that exterior leaks are rarely caused by missing or broken rivets, bad gaskets, or hatches on the sidewalls, although people often jump to one of those conclusions.

The owner who wrote to me also mentioned this:

"[the dealership service center] caulked the rim and severely tightened the latch on the door to the compartment. It leaks a bit less, but still the compartment floor carpeting is damp or wet after a good rain. I can’t find any signs of a leak down the inside walls or the roof of the compartment."

Can you understand why the dealership's repair didn't solve the problem? It's likely that the leak wasn't in the sealant at the edge of the compartment. From what we discussed above, you can guess that the leak is actually coming from somewhere much higher.

The owner can't find signs of a leak inside because the water is probably running down between the two aluminum sheets, soaking the insulation and eventually ending up at the compartment floor.

So my advice would be to start looking at spots above the compartment to see if there's any obvious failed sealant, cracked plastic, or broken gaskets. This includes checking all roof penetrations (vents, antenna, lights, etc) within 10 feet or so of the compartment. I'd also take a good look at the screws that attach the awning in that area, to see if the screw holes have been enlarged.

The owner gave us one final clue:

"This is a 2006 30’ Classic with slide out."

While not a smoking gun, slide-out trailers are notorious for weird leaks. If the compartment is anywhere near the slide-out, I'd be extra careful about checking the area. 

Get a pressurized leak test if the cause is not obvious

In tough cases, exterior water leaks may require a "pressurized leak test" where a service center puts a blower inside the RV. With soapy water they can find places where air is leaking out, and use that information to find leak points.

Keep in mind that a pressurized leak test will find every point that air can get out, but not all of those places will actually let rainwater in. The photo above shows an example. While bubbles showed that air was able to leak out around the water fill, this wasn't a place where rainwater was getting into the trailer.

A pressurized leak test can be done at a service center.

Leaks are often very simple to repair—once you know what to fix. Usually the solution is as easy as replacing some sealant. Don't go overboard and start applying sealant to every bubble revealed by the pressurized leak test. All RVs naturally "breathe." Like most houses, they're not hermetically sealed. Your attention should be focused on leaks at the roof or along the tops of doors and windows, where water is likely to get inside the walls.