The Textbook Treatment – RV Subfloors and Leaks

This article is not an instructable on how to replace your RV floor.

That’s because there are a hundred of those instructables online and in popular forums like FiberglassRV.com and iRV2.com.

Also on YouTube. May I recommend this one from Brian at RVwithTito.com?

But if you’re struggling with a damaged RV subfloor, then I recommend you START here. You won’t finish here (you’ll wind up on one of those 42-point DIY instructables), but this article will get you aboard the right flight.

Because here’s a dirty secret:

RV manufacturers generally consider a damaged subfloor a totaled camper.

As a rule, RV subfloors are not designed to be replaced.

So if you want to fix yours, you’ll have to get creative.

Where Are RV Floor Leak Locations?

Yes, this article is about floors, but let’s start by talking about leaks.

Because a leak is what caused your rotten floor. And if you don’t fix it, you’re just wasting your time.

Where are common leak origins?

Slide-Out Rooms

Slide-outs are basically magnets for every RV problem. A faulty seal can easily channel water into or atop a subfloor.

Door Thresholds

Door thresholds are classic hotspots for water leaks. Wind-driven rain soaks the edges of the subfloor where it butts against the aluminum door threshold.

Wheel Wells

Water and grime thrown up by spinning tires can find its way around plastic wheel wells, leaching into the untreated edges of the routed subfloor.

DWV Pipe Penetrations

Common culprits for water leaks are water pipes, especially 1-½” gray water and 3” black water drain pipes. These leaks can be particularly insidious, since the water may contain effluent and other bacteria.

Body Corners

All corners of the RV are fair game for leaks. Check the two front corners behind the fiberglass cap, and check the two back corners of the rear wall.

Why Is My RV Floor Soft or Spongy?

Uh oh! Is that … a puddle?

… why is my RV floor WET?!

Looking for something to blame?

Here are three mug shots:

Criminal 1: Condensation

When warm, humid air is cooled, water condenses on surfaces. After all, that’s why dew forms on morning grass.

And when water condenses atop a subfloor, the subfloor may absorb that moisture. And when the air warms back up, the moisture evaporates.

This repeated wetting and drying cycle can actually be worse than total immersion in water. It causes severe rot and disintegration.

Condensation is a problem with RVs that are poorly insulated or stored outside with the windows and vents closed.

It may also be a problem seen with single-pane windows. Condensation forms inside the windows on cold mornings, drips down the wall, and seeps into the subfloor.

Note that condensation is NOT caused by a water leak! It has two causes: the movement of moisture air in and out of a camper, and the differences in daily temperatures.

Condensation can be eliminated or reduced with dual-pane windows, window covers, indoor storage, and properly venting your RV during storage.

Criminal 2: Roof Leak

Ah, the dreaded RV roof leak! Difficult to properly seal — and even more difficult to track down. RV roof leaks can wreak havoc with your camper.

Due to the design of most RV roofs, where water exits into the camper may not be within 10 feet of the entry! So frustrating!

If you find a roof leak, do three things:

1. Try to visually identify the source by looking for watermarks. Pour a bucket of water on the roof and watch where it goes, looking for any low-spot puddles or dirty drips.

2. No luck? Recaulk your exterior trim and components if the caulking is more than 90 days old.

3. Still leaking? You might need to remove and reseal all your corner trim. I highly recommend a corner seal tape like EternaBond!

4. Still no luck? Then you might need a permanent fix: A seamless roofing waterproofing barrier like RV Armor.

Criminal 3: Your Pets

Sorry to be the bearer of bad news, but no pet is ever 100% potty-trained.

Many RV owners have wondered why their floor smelled of ammonia, only to rip up a corner of the flooring and discovered a urine-soaked subfloor stained red and yellow.

Check the corners of your RV periodically for secret pee corners. And don’t lock up your pets for hours on end.

Criminal 4: Your Pipes

This might be the worst of the four. Water leaks from busted water pipes and leaky drain pipes can soak a camper from the inside out in a matter of minutes.

As you probably know, frozen water can cause pipes – whether PVC, flexible vinyl, PEX or copper – to bust. A busted pipe can spew 3-5 gallons of water a minute into your camper.

Occasionally, high water pressure can cause leaky pipes, but since RV systems are tested to 80-100 psi, this should be rare.

Bad quality or cross-threaded fittings can cause leaky pipes. Since most RV manufacturers use plastic fittings rather than brass, an over- or under-torqued fitting can allow water leaks. I’ve even seen fittings arrive from wholesale distributors missing the required O-rings!

Common locations for bad fittings include the water pump connections, the city fill connection, and the outdoor shower box.

Faulty ABS or PVC solvent-weld fittings can cause gray- or blackwater leaks. These leaks tend to be small and slow. They are difficult to diagnose. Sometimes, they aren’t detected until the surrounding subfloor is soaked in sewer water. So gross.

How to Repair a Rotted RV Subfloor?

I’ll be honest with you: A soft spot in an RV floor can be a death knell. Short of surgery and a good bit of luck, your floor might be doomed.

By the time a wooden subfloor has become noticeable soft and spongy, it’s in bad shape. This happens because of repeated wetting and drying cycles. Plywood will have suffered delamination, and OSB will have suffered disintegration.

Here’s the bad news: There’s no way to restore a disintegrated wooden RV floor. It’s permanently damaged.

The good news is that there are workarounds.

SUPPORT

The easiest workaround is to support the damaged subfloor. For instance, maybe you can screw a ¾” treated plywood backer to the underside of the subfloor. This helps isolate the affected area.

Unfortunately, this hack only works for solid subfloors. And you must have access to the underside, which might mean removing the underbelly covering and anything else in the way.

REPLACE

It some locations, it is possible to remove and replace sections of the subfloor. Depending on your camper design, you may have access from the underside or top (once you remove the flooring materials). If you’re lucky, you can use a jigsaw or router to cut away the rotted area and replace it with a healthy subfloor.

This solution works best for framed floors, although with some luck, it can be done with solid subfloors. It is cheap, but time-consuming. You’ll run into a thousand little things that must be done, like matching trimwork, to ensure a seamless final installation.

PENETRATING EPOXY

This solution is almost too good to be true. Turns out that if you thin epoxy resin with enough solvent, it can soak into rotted plywood or OSB. Once it sets, the epoxy becomes strong and sturdy, just like the original OSB!

These products are called penetrating epoxies. Popular products include:

    • Minwax Wood Hardener
    • Bondo Rotted Wood Restorer
    • TotalBoat Penetrating Epoxy Sealer

These products are rather incredible. They’re like magic!

Unfortunately, they’re also quite expensive. And if you’ve never worked with epoxy before, you have a learning curve ahead of you.

And don’t spill it on your floor, or it will never, NEVER come off. EVER.

What About Floor Leveling Compounds?

Many DIYers have used floor leveling and patching compounds, usually latex-based, to “waterproof” sections of rotted subflooring.

I don’t really recommend this. It can work well in some applications, but penetrating epoxies are SO. MUCH. BETTER. In every way.

If you’re going to all the work of repairing rotted wood flooring in your RV, do it the right way, the first time.

Types of RV Floors

Alright, here’s the good stuff!

I certainly don’t know of every subfloor on the market. But I’ve used, seen or researched quite a few. I feel confident saying that 98% of the RVs in the country use one of the following types of subfloors.

SOLID FLOORS

Once Upon a Time: 4x8 Plywood Sheets

In ye olden times, RV floors were made from 4×8 sheets of plywood. This size of sheet – four feet wide, eight feet long – is the standard size for a construction sheet good in the U.S. construction industry.

These sheets were commonly laid side by side – not end to end – along the length of an RV. I’m not an expert in vintage RV construction, so I’m not sure how they were seamed – butted, scarfed, glued or screwed, etc.

Very few manufacturers still use 4×8 sheets to build RV floors. It’s too time-consuming compared to a single jumbo-sized piece of flooring. Plus, all those seams are leaks waiting to happen.

Construction-grade plywood can be a challenging material to work with.

  • It bows and warps after fabrication.
  • Its size changes with humidity.
  • The layers can separate in a failure called delamination.
  • Voids inside the plywood can trap moisture, calling the plywood to rot from the inside out.

Premium-grade plywood has very few of these issues, but it is prohibitively expensive (easily over $100 per 4×8 sheet!).

Ol’ Faithful: The Rise of OSB Subfloors

Oriented-strand board (OSB) was born in the 1970s. OSB is a type of manufactured wooden sheet made from layers of wood chips bonded with glue. Unlike plywood, it was truly flat. And it was cheap. Really cheap.

Unfortunately, early versions of OSB were prone to rotting and disintegrating. In fact, your Grandpa still probably doesn’t trust OSB.

But the truth is that OSB manufacturing has improved 10x, while the quality of lumber has decreased by the same. OSB is now the manufactured wooden sheet good of choice, and it shows no signs of slowing down.

What Makes OSB the Most Popular Subfloor Choice?

Whereas plywood suffers from warping and voids, OSB is consistent. Engineers can specify the strength axis, the exact thickness, the density, etc. Because OSB lays flat, it is easier to route and machine.

And here’s another reason: The size and thickness of OSB floors are only limited by the size of the machines. With plywood, sheet size is limited by the size of the tree!

Today, OSB sheets are easily available in sheets as large as 8×40-ft! This means most RV manufacturers can use a single “Jumbo” OSB sheet as a seamless subfloor in their RVs.

For all these reasons, a solid “jumbo-sized” OSB subfloor remains a popular subfloor in many RVs.

Not all OSB Is Created Equal

Don’t judge all OSB by the flaky stuff peeling apart in your grandparent’s basement.

Cheap commodity OSB used for basic sheathing doesn’t even begin to compare to advanced OSB products.

Most OSB subfloor products have sealed edges – edges coated with waterproofing resin – and a waterproofed underside. Huber Woods Performax 500, for instance, has a thermally-fused waterproofing coating bonded to the bottom.

Leading products include:

  • Huber Woods AdvanTech/Performax 500
  • Tolko T-Strand PRO
  • Georgia-Pacific DryMax
What Kills OSB?

What kills OSB is repeated wetting and drying. Even treated OSB, which can survive immersion in water, cannot survive repeated wetting and drying cycles. The chips will swell and then slowly disintegrate. In extreme cases, the RV owner might not notice the issue until they fall right through the floor!

FRAMED FLOORS

Insulated Framed Floors

Plywood and OSB floors are both examples of solid subfloors. There is another type of floor, a farmed floor, which isn’t solid.

Imagine a wall in your house. It’s framed with stick lumber – studs, plates, cripples and so forth – and sheathed on both sides.

Now lay that wall on the floor. That’s basically an RV framed floor. Only unlike a wall, which is framed with 2×4 lumber, RV floors are usually framed with 2×2 lumber to save weight and reduce overall height. The thickness of the floor decking will vary as well, from ⅜” to ¾” depending on the joist spacing, desired quality, etc.

This construction style creates hollow joist bays, which are typically insulated with fiberglass insulation (not my pick, but that’s status quo).

It’s also common to route wiring and sometimes plumbing or piping through framed floors.

Check out the Grand Design Transcend floor.

Now, this sort of floor has a more robust style of construction. It’s also easier to build and easier to repair.

The big downside to this floor is the choice of insulation. 9 times out of 10, the RV manufacturer choose loose-fill fiberglass, which IMHO does not belong anywhere in an RV. Fiberglass easily molds, and it loses all its R-value when wet.

COMPOSITE FLOORS

I.
Luv <3
Composite floors.

They are the coolest.

But the term “composite” has been loosely stuck onto many different types of floors. So it’s kinda confusing.

So I have broken down composite floors into three categories:

  • Laminated Composites
  • Fused Composites
  • Solid Composites

The first two categories – laminated and fused floors – are types of sandwich construction.

It’s a really important concept to wrap your head around, so I’ll say it again: sandwich construction.

All sandwich floors have (at least) a core inside of two skins, like this:

So all sandwich floors have at least three layers: Side A skin, the core, and Side B skin. More advanced composites may have even more layers!

  • In laminated floors, the layers are glued together. This is usually accomplished by rolling or spraying the adhesive onto the materials and then feeding them through a pinch roller or vacuum press to apply pressure to the bondline.
  • In fused floors, the layers are melted together. This can be accomplished through thermal fusion (i.e., heat) or chemical fusion (i.e., solvent-welded).

I’m not going to get into the mechanics and physics of sandwich construction. They can get rather complicated. But here are the three main takeaways:

  • Sandwich floors boast an incredible strength-to-weight ratio. They are often half the weight of plywood but twice as strong!
  • The skins do most of the work.
  • Sandwich panels excel at distributed loads. Where they usually fail is point loads: face screws, high impact, etc.

Composite subfloors are being adopted by premium RV manufacturers. It’s only a matter of time until they become an industry standard. RV consumers are just too sick and tired of dealing with rotten subfloors! It’s high time for a change.

LAMINATED COMPOSITE FLOORS

The RV industry originally attempted composite floors by simply gluing cheap 1/4″ plywood skins to Styrofoam cores.

Those …. Bombed. Badly. They came with all the problems of a wooden floor (rotting, bowing) plus the problems of a laminated floor (delamination)! And the solution is usually … replace the whole thing.

A lot of manufacturers fell hard on their own swords: Keystone, Jayco, Forest River, etc. Lots of reports of soft, squishy floors and catastrophic delamination. These floors were commonly used on travel trailers marked as “lightweight!’

The industry attempted some workarounds, such as inserting floor joists every 16, 24 or 32 inches into the foam core, and then screwing the face skins to those joists.

But those workarounds merely prolonged the inevitable. It was all too easy for water to wick between the lauan and the foam core and slowly rot the skins from the inside out.

A more modern version of a laminated composite floor is the Keystone HyperDeck floor, which the company debuted in 2020.

The HyperDeck substitutes fiberglass-reinforced polypropylene skins for lauan plywood (thank goodness!). No more mildew. No more rot. Far less likely chance of delamination.

Some laminated floors use sheets for the skins; others use roll materials. Roll materials are preferable. The resulting skin is seamless. Sheets lead to seams, which can

FUSED COMPOSITE FLOORS

TransCore

In 2020, Airstream announced it was moving away from 5/8-in OSB flooring in favor of TransCore composite flooring. According to RV News:

“The bonding method in creating this composite material means you don’t have the squeaking that can come with plywood pieces rubbing against each other, and it improves the longevity of the flooring,”

Airstream VP of Sales Justin Humphreys said:

“In addition to being slightly lighter than wood, it could potentially extend the life of your vinyl flooring – and the value of your trailer over time. With the addition of TransCore, the iconic Airstream shell is now completely free of any wood product.”

Here’s what TransCore looks like:

You can see typical sandwich construction: a lightweight foam core with structural fiberglass skins. The claim to fame here is fused fabrication. The skins and core are fed concurrently into a giant heated machine that heats the materials and presses them together. There’s no glue; the plastics fuse together.

Transonite

Transonite, made by Creative Pultrusions, is a unique offering in the composite subfloor space. Whereas most composite panels have cores made from high-density foam or honeycomb, Transonite pultrudes reinforced resin into a vertical matrix.

According to RV Pro magazine:

“We actually bring our glass and aluminum and resin into a die and the die is then heated,”

said Ted Harris, market development engineer for Creative Pultrusions.

“Polymerization takes place that creates a solid structural panel. It’s a 3-D composite panel. We actually stitch fibers and those fibers connect the top and bottom skins. They become saturated with resins and solidify and actually attach to the top and bottom skins to create what is essentially mini I-beams. We call those fiber inserts. We can vary the density of those I-beams from three up to 19 per square inch. Those fiber inserts create significant compressive strength within the panel.”

Yeah … it’s easier seen than explained. Here’s a picture.

The four big benefits here are:

  • The pultrusion process creates a fused product; no bondlines to fail!
  • Seamless core AND seamless skins! No leak points; no weak joints.
  • Wide variety of skin thicknesses and strengths, from 0.55 to 0.250 inches thick.
  • Wide variety of core thicknesses and strengths, from less than half an inch to four inches thick!

SOLID COMPOSITE FLOORS

When I say “solid,” what I really mean is homogeneous: the same material or matrix runs all the way through. In other words, the panel is not built with distinct layers.

This is a rare breed. Solid composite floors are eye-wateringly expensive. I don’t know of any mainstream RV manufacturer that uses solid composite panels for – well, just about anything. But you do see solid composite subfloors in turnkey construction, especially for off-road and amphibious RVs.

Most solid composite panels come from the marine industry. And most are made of Reinforced Polyurethane Foam (PRF), which is an extremely dense closed-cell foam reinforced with a fiberglass matrix. You’ll also hear it called structural foam.

A popular pick is the Coosa Bluewater Series, from Coosa Composites. Coosa board is

“polyurethane foam filled throughout with layers of continuous strand fiberglass and woven roving fiberglass.”

There’s not much that can kill a structural foam board. They can be machined, adhered, and mechanically fastened. If you’re lucky enough to have structural foam inside your RV, count your lucky stars!