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Attaching the floor should be necessary only to hold it in place, not to keep it flat. If it does not lie flat on its own, you need better plywood. Depending on your location, you may need to visit a real lumberyard. I haven’t seen anything decent in a box store in several years. It doesn’t have to be fancy—my 1/2” exterior ply was flat as a flitter.
 

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I used HD’s lowest grade 1/2” plywood - it was all pretty warped but all they had available with everyone working on building projects now. I installed it to see if it would work, but doesn’t seem like it will...although I haven’t tried gluing the 1/2” polyiso or the plywood. I should have waited for better quality/heavier plywood, but am now just hoping to salvage this since it’s already cut and bolted in.

What did you use for your subfloor? There is a vast difference in quality and suitability for subflooring. I’m assuming you haven’t put anything on it yet - is that correct? I used ¾" poplar plywood and it’s quite flat and heavy. I’ve only held it down at the existing tie down locations. I used wood biscuits at the joints along with wood glue. I don’t particularly like the idea of screwing or bolting they the metal floor for many reasons and it should not be necessary other than perhaps in one or two places at the most. You must use solid core flat, good quality plywood to begin with or it will never lie flat. If it goes in warped it will most likely never really be truly flat. The factory floors are glued down but if you plan to insulate under the subfloor, as you should, it becomes pretty much impossible to glue it down. Some people swear that ¼" plyscore is good enough but I personally feel ½’ quality plywood is the minimum one should use and thicker is far better and doesn’t really cost much more in the long run.
 

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Is the polyiso going below the ply? It is dead flat and stiff enough it might hold your ply flat if glued tightly enough. Make the polyiso span the ply joints.
 

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Pull that ply, go to a lumber yard not a box store. Ask for “hardwood plywood” and have them show you the product. Pay more but you will appreciate it later. Mine was birch but poplar, maple, osk, and some more are available. Poplar is the cheapest and paints well.
 

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Biscuit joining is for sure the way to go to connect multiple floor pieces. Cheap, strong and very easy to use. Most folks probably not familiar with it unless they are into woodworking a bit. You can do it before hand, or as the pieces are installed into the van. The joint with them is extremely strong.
 

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I used biscuits on my ¾" poplar plywood floor. It’s the simplest, easiest way (if you have the tool).
 

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They do make biscuit cutter bits for routers tho. A lot better and easier than making a lap joint (IMHO)🤔
 
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Biscuit tool is very inexpensive and would make a good reinforcer for the cabinet joints.
 

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I would agree that if you are doing any edge joining, Ls, or solid wood capping it is a must have tool and super easy to use.
 

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Stumbled across this thread for my planning. The lapping of the joints is an excellent idea. The question is regarding the material. Seems that plywood of superior grade is the standard. The question is regarding Coosa board? Anyone tried it? I know it is pricey, but it is lightweight and will never rot. It got its fame in the boating industry (where I have some interest). Thoughts? You can actually hose out your van and never worry about rot.
 

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I don’t think you would ever want to hose out your van - it’s NOT a boat!
 
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I don’t think you would ever want to hose out your van - it’s NOT a boat!
Yeah, I know :), it seems that I am taking alot of boating building techniques with me. I figure if it can salt water.....people talked about lapping plywood joints....a couple of layers of vinyl ester and 1708 and you're good to go!:)
 

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Lapping joints is fine but too much work. A simple biscuit or 3 in the joint and some yellow glue is more than enough.
 
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