Resealing an RV underbelly

We’ve seen more of the underside of our RV in the past couple of months than in the six years we’ve owned the fifth wheel. Since removing the covering over our RV underbelly, we learned where our slideout motors are positioned, where our wastewater and freshwater tanks are located, and where rust accumulated on the I-beam spanning the length of the coach. 

That exposure also gave Bob the opportunity to attach a dangling leveling sensor to the frame, add an aftermarket sensor to our kitchen gray wastewater tank, clean away rust, apply rust protectant, and add a fresh coat of paint on the frame for increased protection.

After he completed all that, it was time to reseal the RV underbelly. Here’s what the experience was like.

Purchasing a replacement

Our rig originally had corrugated plastic underneath. On top of that sat a loose layer of foil bubble-wrap-type material as a radiant barrier to help with insulation. 

Bob’s research for an underlayment replacement revealed a newer material from RecPro with the foil layer attached to it for simpler installment. We purchased a 25-foot roll that measured 77 inches across and were surprised to find that the material felt more fabriclike than the original plastic. Despite that, it’s still water- and temperature-resistant and considered “extreme duty.” 

Preparing for installation

Bob conducted more research to determine the best way to install the new underbelly. Because the original sat in very narrow grooves that spanned wider than the beams stretching from the front to the rear of the RV, we knew that getting the new material in without cutting it (our goal) would be a challenge.

Someone had installed the same type of underlayment solo by using a 2-by-4 on top of jack stands to help hold it up as he forced the material into the grooves. Although we’d be working together on the project, we decided to follow that example. 

We secured a 2-by-8 (which doubles as a board for under our tires to help us get level at uneven sites), heavy-duty jack stands, a chalk line, a winding tape measure, scissors, self-tapping screws, and washers. 

Measuring twice, cutting once

Once we designated Bob as the leader of this project, the first thing we had to do was unroll the new material. Bob’s brother Tom, a fellow full-time RVer, took on this task, leaving the expanded roll in the sun to help flatten it out.

Next, we laid out the old pieces of the underbelly that Bob had cut and removed to help us determine the width and length we’d need. We had to cut off about 5 inches of width the entire length of the new material. That’s where the chalk line came in. 

We marked our desired width measurement at the two ends and in the middle of the new material, using blue tape that we could write on to see our nicks. Then we measured again and pulled out the chalk line, stretching it the length of the new material and carefully aligning it over our markings. We each snapped it, leaving a nice orange line.

I took the heavy-duty scissors and cut the material. They were the right tool for the job. 

Marking points of interest

Tom ran to the store to secure a yellow marker that would help us see important points on the black material. 

While he was away, Bob and I positioned the pieces of old corrugated plastic on top of the new material to prepare to mark the areas we’d need to be mindful of for installation, such as the spare tire holder, water pipes, slide motors, and things like that. 

Tom returned and helped us make our marks, including yellow dots to indicate existing screw holes.

Installing the new material

We talked through our plan for installation: pulling the material in from the back to the front. Once we were all on the same page, Bob and I crawled under our rig and lay on our backs on crushed gravel to try to drive the material into place. 

Remember how I said the grooves were wider than the beams spanning the length of the coach? Yeah, that caused a bit of a problem. We weren’t able to get the material into the slots at the start, so we decided to maneuver it under the hitch at the back of the rig and up over the axles and the board across jack stands. We had to force the corners and sides of the stiff material up toward the trailer for this to work. 

I grabbed the axles and support beams above me and scooted on my back toward the front of the rig, pulling the material as best as I could as I went. I felt like an airman from WWII dashing through the tunnel of a B-25 to the navigator/bombardier seat while on a mission.

Pulling the material the length of the coach proved cumbersome, but little by little, keeping our communication open, we were able to complete the task. 

Attaching the underlayment to the rig

Getting the material in position the length of the rig was only part of the process. Next came forcing the material into the grooves on either side of the frame and securing it. 

Tom joined Bob under the trailer toward the front while I moved around to work the material into place. Once we thought we had enough in the grooves and had it pretty straight, we all held the piece up and in place while Bob attempted to attach a screw with a thick washer. The metal frame didn’t want to let the screw in. Finally, the screw made a hole. 

We repeated this process. But as Tom tried to help attach another screw, he accidentally kicked the 2-by-8 above his legs, knocking it onto his shin. I crawled out from under the trailer to move the board off and out of the way, finding a rock in my back pocket from my scooting adventure. 
Tom got a surface wound on his leg but wrote it off as part of the experience. “Every scar is a sign of adventure,” he told me.

Back under the trailer, I continued to push the material into the slots, holding it up while the guys secured what they could and cut holes as needed. 

Once we got to the axles, Tom left Bob and me to complete the project. It took a bit longer than we’d hoped. The hardest part was cutting off excess material so that we had just enough to fit into the end cap at the back of the rig.

Bob secured the underbelly, and we celebrated a job well done with a dip in a hot tub to rest our sore muscles and bruised backs, behinds, heads, knees, and elbows. We finished the project just in time to insulate the bottom of our rig from cold weather.

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