Foam/plywood over "hump" in metal floor?

[dbsoundman]

Hi all, my wife and I were discussing the steps leading to finally putting some foam and plywood down on our metal floor last night, and she mentioned she had concerns about one piece of the metal flooring between the two rear wheel wells which has a "hump" to it. To be clear, this is a Thomas bus, and the metal floor is made up of about 6" wide "planks" that go across the bus from side to side. One of these planks has a 1" or less hump to it, which is definitely noticeable but I didn't think it would be problematic. However, since we're planning to use PL Premium to glue down 3/4" XPS, then put 1/2" or 3/4" plywood (likely tongue and groove) over the metal, I wanted to make sure we wouldn't end up with a major bump in the floor there. My hope is that the foam would compress a bit to compensate for the hump.


Any thoughts? I can't be the only person that has an uneven metal floor...

 

[jazty]

If the hump extends the full width of the bus I don't think you'll be able to compress the foam enough for it to not be noticeable in the finished floor.


Luckily, foam can easily be formed. A knotted wire wheel will remove foam material quickly, though it will make a huge mess. Perhaps you could make it fit this way? To make it easier you could even cut the foam into more manageable squares for forming. Maybe make them 2'x2' squares and form them one at a time along the hump.

If you are using thick enough plywood (at least 5/8" thick) you could even bring the foam XPS sheets up to the hump on either side; spray foam the hump; cut if flat; and position the plywood so that it is centered over top of the "weak" spray foam spot. That might do the trick, too. 6" isn't such a wide gap as long as the plywood sits over it properly.


Just some ideas...

 

[dbsoundman]

If the hump extends the full width of the bus I don't think you'll be able to compress the foam enough for it to not be noticeable in the finished floor.


Luckily, foam can easily be formed. A knotted wire wheel will remove foam material quickly, though it will make a huge mess. Perhaps you could make it fit this way? To make it easier you could even cut the foam into more manageable squares for forming. Maybe make them 2'x2' squares and form them one at a time along the hump.

If you are using thick enough plywood (at least 5/8" thick) you could even bring the foam XPS sheets up to the hump on either side; spray foam the hump; cut if flat; and position the plywood so that it is centered over top of the "weak" spray foam spot. That might do the trick, too. 6" isn't such a wide gap as long as the plywood sits over it properly.


Just some ideas...

My wife actually suggested not putting foam over the hump, I think using spray foam is an excellent compromise! Good call.

FYI I’m basically using your assembly techniques, leaving a gap at the edges and filling with spray foam. There’s a metal lip that’s about 1-1/2” at the edges anyway so there was going to be a gap no matter what!

 

[jazty]

Neat! Glad my previous experiences can be of service!

A question just crossed my mind: why is there a hump in the first place? There are a couple lateral beams below the floor sections of Thomas bodies. If you are seeing bulging of the floor, I bet that the beam is rusted and has expanded. You might want to investigate the beam from the underside before foaming over top of it.


If the beam has rotted out, you can decide whether or not to fix it. Playing with the bus for a handful of years? Meh, it'll probably be fine. Plan to hang onto it until the end of time? Now is a good time to fix the issue.


I had to replace one of my lateral beams. I cut out a floor section from the top; put in a new beam; welded it in place; then welded a new floor section into place. It was a good bit of effort, though I generally had fun doing it

 

[dbsoundman]

Neat! Glad my previous experiences can be of service!

A question just crossed my mind: why is there a hump in the first place? There are a couple lateral beams below the floor sections of Thomas bodies. If you are seeing bulging of the floor, I bet that the beam is rusted and has expanded. You might want to investigate the beam from the underside before foaming over top of it.


If the beam has rotted out, you can decide whether or not to fix it. Playing with the bus for a handful of years? Meh, it'll probably be fine. Plan to hang onto it until the end of time? Now is a good time to fix the issue.


I had to replace one of my lateral beams. I cut out a floor section from the top; put in a new beam; welded it in place; then welded a new floor section into place. It was a good bit of effort, though I generally had fun doing it

Man, don’t tell me these things!

I think it goes right over the rear axle so it’s not easy to reach. I’ll take a peek underneath but I’m not inclined to over engineer this build so I’ll likely just let it go. These buses are so overbuilt I don’t think one, two, or even three rusted supports would really be noticeable!

 

[DeMac]

Firebuild had a similar floor problem between her rear axles.

https://www.skoolie.net/forums/f27/whats-up-with-this-floor-hump-34840.html

In the above thread, Musigenisis mentioned the same with his and a possible cause.

 

[dbsoundman]

Firebuild had a similar floor problem between her rear axles.

https://www.skoolie.net/forums/f27/whats-up-with-this-floor-hump-34840.html

In the above thread, Musigenisis mentioned the same with his and a possible cause.

Good info. I'll take a closer look tonight hopefully and see if the hump is easily fixed, but if not I'll probably just go with spray foam and see what happens.

 

[rossvtaylor]

I'd take Jazty's thoughts seriously...it sounds like "rust jacking" which is the condition where an underlying steel member rusts enough to begin expanding as the layers of corrosion stack up. If that happens with a thinner metal against a thicker piece, it will displace the thinner one...sometimes considerably. That may not be the case here, but it might be. And, if it is, the problem won't stop and it will continue to expand until the rust is remediated (which, when it's that bad, means cutting it out and replacing it). Fingers crossed that's not the issue here, but it's something you ought to check before you put in the over floor materials.

I have some good examples of rust jacking, if you want to see them!

 

[musigenesis]

My bus had some severe humping going on in the back (this was in the past, not the present :biggrin. The source of this could be seen underneath the bus pretty clearly. The cross-members underneath the floor are formed from the edges of adjacent panels bent down into a c-section, then two panels are spot-welded together along the edges to form a virtual I-beam at the seam. This I-beam then rests on top of the chassis rails, with small squares of rubber (about 1/2" or so thick) between the beams and the rails.

In some spots (on the rear of my bus in particular), these small rubber pads were misaligned (either from the factory or they had moved over time) so that rather than being directly under the center of the I-beams, they were off to one side and only partially supporting the beam on one side. In a brand-new unrusted floor this isn't a big deal and the floor as a whole is supported flat, but once the I-beams become rusted and somewhat weakened, the misalignment of the supporting pads allows the beam to twist in place, causing the side of the floor with the pad under it to bend up slightly. This twisting and bending causes the hump to form (no more than maybe 1/2" at most in my case).

For my subfloor, I welded upright wood screws to the floor, then placed my 2" XPS foam board down over them, then screwed 2" pieces of oak dowel through matching holes in the XPS onto the wood screws, then screwed my 3/4" plywood onto the dowels from above. This resulted in a completely flat subfloor - bulges be damned - in this case because the plywood straightens out the bulge in the steel floor.

In your case, using just glue, you will not have this solid mechanical connection between plywood and sheet metal to force the bulges down to any significant degree. What I would do in your case, if the source of your bulges turns out to be these misaligned pads, is get some additional pads to force under the unsupported sides of the cross-members, which should be possible to do with the help of a big prybar (moving the existing pads is probably not going to be possible because of the weight above them - I wasn't able to budge any of mine). Then from the inside, use a big sledgehammer (with serious ear protection) to hammer down the bulges. You should end up with something reasonably flat (although not perfect).