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Old 02-12-2019, 02:06 AM   #1
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Question Sub-floor fastening

I just pulled all the wood flooring out of my bus, it is down to the bare steel. No rust at all thank goodness! welded closed all the bolt holes and put two coats of rustoleum down. Now getting ready to put in a sub floor of 1x2's 16 inches on center so I can insulate under the new wood floor. The question is what would be the best way of fastening the 1x2's to the steel floor?
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Old 04-07-2019, 09:14 PM   #2
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This was a good question did you ever get it answered?
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Old 04-08-2019, 03:50 PM   #3
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I think it's a good question too. But obviously it's not a popular question. I've found numerous posts on this forum about floating floors, but I'm not comfortable with that concept.

I want my floor as well-secured as possible. Just sitting there, held in place only by gravity does not fit into my definition of well-secured.
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Old 04-08-2019, 04:18 PM   #4
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I think it's a good question too. But obviously it's not a popular question. I've found numerous posts on this forum about floating floors, but I'm not comfortable with that concept.

I want my floor as well-secured as possible. Just sitting there, held in place only by gravity does not fit into my definition of well-secured.
I'm planning on gluing my foam board to the floor, and gluing tongue and groove plywood flooring to the foam board- the floor won't be moving anywhere and it will do a better job of insulating the floor without the framing
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Old 04-08-2019, 04:31 PM   #5
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There are screws designed for this. They are made to deive through wood, then self tap into steel. Get the countersunk head version and it goes real fast. They can even drive right into the ribs below the floor. Just make sure you look at what is underneath first.
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Old 04-08-2019, 05:06 PM   #6
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There are screws designed for this. They are made to deive through wood, then self tap into steel. Get the countersunk head version and it goes real fast. They can even drive right into the ribs below the floor. Just make sure you look at what is underneath first.
Do you have any additional details on these? Maybe a web link?
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Old 04-08-2019, 06:11 PM   #7
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Without furring strips every couple of feet or so I would be worried the foam would collapse. Perhaps not too quickly, but eventually I think you would end up with a low spot in the middle. I think 2X2s every 18-24 inches screwed through the steel floor into the support structure, would prevent the floor from sagging. Then the plywood could be screwed to the 2X2s with plenty of screws that are too short to penetrate through the 2X2. So there would be no direct thermal bridging, just some places where wood was the only insulation, which I could live with..

I realize that there is foam that is denser and more resistant to compression than the standard foam, but I would still want the plywood to be supported by 2X2s at least every 2 feet. Overkill perhaps, but I have several extremely large friends and I am not small myself. I would really hate to have a floor that sagged in the middle.
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Old 04-08-2019, 06:29 PM   #8
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Without furring strips every couple of feet or so I would be worried the foam would collapse. Perhaps not too quickly, but eventually I think you would end up with a low spot in the middle. I think 2X2s every 18-24 inches screwed through the steel floor into the support structure, would prevent the floor from sagging. Then the plywood could be screwed to the 2X2s with plenty of screws that are too short to penetrate through the 2X2. So there would be no direct thermal bridging, just some places where wood was the only insulation, which I could live with..

I realize that there is foam that is denser and more resistant to compression than the standard foam, but I would still want the plywood to be supported by 2X2s at least every 2 feet. Overkill perhaps, but I have several extremely large friends and I am not small myself. I would really hate to have a floor that sagged in the middle.
there was someone that posted on this subject before - they had the numbers calculated of how much pressure would be exerted on the foam through 5/8ths plywood - I don't remember the exact figures, but they were huge - the plywood spreads the pressure out very well - using rough figures, a 300# person standing on one foot would exert about 7.5 lbs per square inch - standing on 2 square feet of 5/8 plywood would reduce that down to about 1.5 pounds per square inch or less
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Old 04-08-2019, 06:45 PM   #9
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Without furring strips every couple of feet or so I would be worried the foam would collapse. Perhaps not too quickly, but eventually I think you would end up with a low spot in the middle. I think 2X2s every 18-24 inches screwed through the steel floor into the support structure, would prevent the floor from sagging. Then the plywood could be screwed to the 2X2s with plenty of screws that are too short to penetrate through the 2X2. So there would be no direct thermal bridging, just some places where wood was the only insulation, which I could live with..

I realize that there is foam that is denser and more resistant to compression than the standard foam, but I would still want the plywood to be supported by 2X2s at least every 2 feet. Overkill perhaps, but I have several extremely large friends and I am not small myself. I would really hate to have a floor that sagged in the middle.
I'm using standard Owens Corning Formular 250 rigid pink foam in 1" thickness. It has a compression strength of 25psi. That's 3600psf. Plywood has a compression strength of between 4k and 6k/psi. So after plywood is laid you have a compression strength of 4025-6025psi (approx. 750000psf). What is it you will have on that floor that would make you think you will compress the foam enough to sag over a 2' square?
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Old 04-08-2019, 09:26 PM   #10
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Do you have any additional details on these? Maybe a web link?
I got these self drilling and tapping screws at the local Fastenal store to attach new deck boards to my 10k lbs flatbed trailer. Going to use the same type screws for the floor of my box van conversion.
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Old 04-08-2019, 09:42 PM   #11
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I'm using standard Owens Corning Formular 250 rigid pink foam in 1" thickness. It has a compression strength of 25psi. That's 3600psf. Plywood has a compression strength of between 4k and 6k/psi. So after plywood is laid you have a compression strength of 4025-6025psi (approx. 750000psf). What is it you will have on that floor that would make you think you will compress the foam enough to sag over a 2' square?
The PSI values of the two layers do not just add up like that. The plywood itself is effectively incompressible, but the foam has a PSI of 25 regardless of whether anything is on top of it. If a 100-pound ballerina stands on pointe directly on the foam board and the tip of her shoe occupies one square inch, she will dent the foam in that spot (since she's producing a pressure 4X the rated value).

The benefit of the plywood on top of the foam is that because it is relatively stiff, it distributes the force/weight of the objects on top it over a much wider area - which reduces the pressure on the foam board underneath it to much less than 25 PSI.

If the PSI values of layers were additive in the way that you suggest, then two layers of foam board should be able to resist 50 PSI - which is not the case.
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Old 04-08-2019, 10:39 PM   #12
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The PSI values of the two layers do not just add up like that. The plywood itself is effectively incompressible, but the foam has a PSI of 25 regardless of whether anything is on top of it. If a 100-pound ballerina stands on pointe directly on the foam board and the tip of her shoe occupies one square inch, she will dent the foam in that spot (since she's producing a pressure 4X the rated value).

The benefit of the plywood on top of the foam is that because it is relatively stiff, it distributes the force/weight of the objects on top it over a much wider area - which reduces the pressure on the foam board underneath it to much less than 25 PSI.

If the PSI values of layers were additive in the way that you suggest, then two layers of foam board should be able to resist 50 PSI - which is not the case.
Point is is it is much ado about nothing.
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Old 04-09-2019, 01:26 AM   #13
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There are screws designed for this. They are made to deive through wood, then self tap into steel. Get the countersunk head version and it goes real fast. They can even drive right into the ribs below the floor. Just make sure you look at what is underneath first.

Good post, Miltruckman!


Here are the screws I used:


Teks Screws for Flooring.jpg


I used them for attaching the furring strips to the metal floor. Since the floor metal is thicker than the skin I did end up drilling pilot holes through the metal floor. It was easier for me.
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Old 04-09-2019, 01:38 AM   #14
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I don't completely trust the plywood, any plywood, because I know that all plywood is not created equally. I spent somewhere around 8 or 9 years all together working in 4 different plywood mills spread around Oregon and British Columbia. So I am very aware of the shortcuts people take and what they get away with.

There can be big holes in core or in centers, and the people laying up the plywood just break a chunk off a piece of scrap, either with glue or without glue depending on what's missing and put it in the hole as quickly as possible while paying basically no attention to how well it fills the hole. There just isn't time for that.

If the holes in one ply are even a little too big there will be an area nearly the size of the hole where the other plys will not be stuck together really firmly, and they will often come apart or delaminate while the plywood is cooling in a stack before it gets cut into finished size and graded.

Delaminations can also result from either too much glue or too little glue. And there are a number of other things that can weaken plywood All those small defects are pretty much undetectable when the panel is new. A few small holes in inside plys don't matter much unless you're building a boat, but enough small weak spots can add up to seriously impact the strength of the plywood.

So I want my plywood screwed to something that is screwed to the bus. And I don't want the plywood spanning too much, especially where people will walk, because even flexing the plywood a little can be expected to enlarge any small delaminations in the area where people walk.

When I build something I want it to be as strong and durable as I think I need to make it, and I think understanding the limitations of one's materials is crucial to doing that. And my bus will be spending a lot of time on rough backroads in National Forests and such, so I want it to be tough.
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Old 04-09-2019, 06:16 AM   #15
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Gs1949, of course it's your bus and you can do what feels right, but there's overkill and there's way overkill.

Foam sandwiched between steel and 5/8"+ plywood and adhered with a good glue will withstand all the abuse that any of us have subjected our buses to.
Avoiding thermal bridging from firring strips is plenty worth it, in my opinion.

Maybe there are stinker batches of plywood out there. As a consumer I have yet to experience such shoddy workmanship. I'm sure it happens, but even if it a ply was missing it would have little to no effect on the foam. The plywood might squeak, but the weight is still plenty distributed. That's why I recommend at least 5/8" plywood. 1/2" would likely do if it were uniform, but 5/8" gives you additional buffer.

Again, it's you bus, your build. Do as you will.
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Old 04-09-2019, 06:51 AM   #16
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I'd also like to add that on a personal note I use to be way skeptical of this method. It took lots of conversations with building professionals and merchants and reading data sheets before I was comfortable. Even then I brought some XPS home to test with. It seems ridiculous, but the stuff is indeed structurally sound... as long as it's installed properly.


You made mention of the "hammering" action of uneven plywood against the foam. It is true that if there is a gap between the foam and other building material and the foam is repeatedly impacted that it can crush. Just like a hammer hitting a nail, when there is room for the material to "swing" it can gather additional force before impacting the foam. This is mitigated by using adhesive or screwing the plywood down. I recommend adhesive. Use some pails of water or cinder blocks to weigh the plywood down while the adhesive cures.


Foot traffic on XPS no longer concerned me when I started seeing things like this:


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Old 04-09-2019, 10:00 AM   #17
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I've glued my foam to the treated and painted floor, and then glued my foam to my ply and used no furring strips. After that I used sealant along the edges at the wall and then wedged plywood from the bottom of the chair rail to the top of the subloor, this will then again be sealed to prevent any internal water leaks from finding a home below the subfloor. As well as force the floor down along the entire edge surface of the bus. This coupled with the leading edge tack strip that will separate the driving compartment from the living compartment I'm satisfied that there is enough retention in the subfloor itself. As then the hardwood will go atop that subfloor and all of the furniture framing will attach to the walls, and floors at a min.
I'm not saying don't secure your floor, I'm also not saying don't bother with furring strips. I'm saying you have to do what provides you peace of mind, but to that end just think of all the work you're going to do to your bus and does each step build on the previous step and if so do all the steps together provide you that peace of mind? One match stick is easy to break, but a bundle of match sticks.....right.
In my case the adhesive, plus the edge pressure, plus the furniture fastening plan, plus the static weight atop the floor... I have the peace of mind that my floor will take whatever I put it through with out the added need of addition fasteners/holes in my recently converted/treated floor and without the load bearing and fastening points of furring strips/subfloor framing.

I'll take some picks of the edge wedging plywood soon as my new phone arrives..........old one had an accident.....
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Old 04-09-2019, 10:13 AM   #18
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I've glued my foam to the treated and painted floor, and then glued my foam to my ply and used no furring strips. After that I used sealant along the edges at the wall and then wedged plywood from the bottom of the chair rail to the top of the subloor, this will then again be sealed to prevent any internal water leaks from finding a home below the subfloor. As well as force the floor down along the entire edge surface of the bus. This coupled with the leading edge tack strip that will separate the driving compartment from the living compartment I'm satisfied that there is enough retention in the subfloor itself. As then the hardwood will go atop that subfloor and all of the furniture framing will attach to the walls, and floors at a min.
I'm not saying don't secure your floor, I'm also not saying don't bother with furring strips. I'm saying you have to do what provides you peace of mind, but to that end just think of all the work you're going to do to your bus and does each step build on the previous step and if so do all the steps together provide you that peace of mind? One match stick is easy to break, but a bundle of match sticks.....right.
In my case the adhesive, plus the edge pressure, plus the furniture fastening plan, plus the static weight atop the floor... I have the peace of mind that my floor will take whatever I put it through with out the added need of addition fasteners/holes in my recently converted/treated floor and without the load bearing and fastening points of furring strips/subfloor framing.

I'll take some picks of the edge wedging plywood soon as my new phone arrives..........old one had an accident.....
a good description of what I have in mind
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Old 04-09-2019, 11:52 AM   #19
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I don't completely trust the plywood, any plywood, because I know that all plywood is not created equally. I spent somewhere around 8 or 9 years all together working in 4 different plywood mills spread around Oregon and British Columbia. So I am very aware of the shortcuts people take and what they get away with.

There can be big holes in core or in centers, and the people laying up the plywood just break a chunk off a piece of scrap, either with glue or without glue depending on what's missing and put it in the hole as quickly as possible while paying basically no attention to how well it fills the hole. There just isn't time for that.

If the holes in one ply are even a little too big there will be an area nearly the size of the hole where the other plys will not be stuck together really firmly, and they will often come apart or delaminate while the plywood is cooling in a stack before it gets cut into finished size and graded.

Delaminations can also result from either too much glue or too little glue. And there are a number of other things that can weaken plywood All those small defects are pretty much undetectable when the panel is new. A few small holes in inside plys don't matter much unless you're building a boat, but enough small weak spots can add up to seriously impact the strength of the plywood.

So I want my plywood screwed to something that is screwed to the bus. And I don't want the plywood spanning too much, especially where people will walk, because even flexing the plywood a little can be expected to enlarge any small delaminations in the area where people walk.

When I build something I want it to be as strong and durable as I think I need to make it, and I think understanding the limitations of one's materials is crucial to doing that. And my bus will be spending a lot of time on rough backroads in National Forests and such, so I want it to be tough.
I'm 65, was in the residential construction business for 15 years. Never have I run into an issue like you describe. It must be a one in a million occurrence. But you shouldn't be using "plywood" but a grade specific for flooring which is quite different than cheap plywood. I won't even go into depth on OSB, none will get near my bus. I plan on installing the rigid foam pressure fit between the sides, laying my snap together flooring right on top and it will semi-float Everything I build on top will be secured to walls.
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Old 04-09-2019, 12:11 PM   #20
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I'm 65, was in the residential construction business for 15 years. Never have I run into an issue like you describe. It must be a one in a million occurrence. But you shouldn't be using "plywood" but a grade specific for flooring which is quite different than cheap plywood. I won't even go into depth on OSB, none will get near my bus. I plan on installing the rigid foam pressure fit between the sides, laying my snap together flooring right on top and it will semi-float Everything I build on top will be secured to walls.
for sure, use tongue and groove plywood sub floor, other wise the plywood will flex at every joint
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