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I've got thin foam strips on the floor in the gaps between the raised sheetmetal floor ribs. Above this I've got 3/4" plywood (then will be laying a closed cel foam subfloor and a vinyl floor.)

The 3 Plywood pieces I used maybe had a little bend or bow to them to begin with - and after a week of laying in the van floor (with minimal weight pressing them down) there is still a noticeable spring and squeak to them in a few places.

Each of the 3 pieces is bolted down in 2 spots (using existing tie-down threaded holes).

So:

A - Should I try and glue down the plywood to the floor rib tops, focusing on the springy places? (which glue?)

B- Should I use some self-tapping sheet metal screws (countersink the heads, drill pilot holes through the van floor)

C - Apply more heavy/weighted items to the floor and wait a few weeks to see if things settle?
 

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Only the third is nondestructive and I doubt it will work. I bought hardwood plywood which tends to be a bit higher quality and my floor stays flat except for a small bow in the center back. I have a cross divider under my bed and so I added a trim piece to hold it down. Any chance of flipping the bowed sheet(s). 2nd choice self taping screws.
 

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A - Should I try and glue down the plywood to the floor rib tops, focusing on the springy places? (which glue?)
Glue it down with a polyurethane sealant / adhesive. That's how the factory optional floor is held down. I used Loctite in a tube for a calling gun from HD
 

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If you do pre-drill then don't use a huge bit, keep it as undersized as possible, the self-tapping or self-drilling screws push enough metal aside without cutting it away that fastener holding power is much improved by the thickened sheet metal surrounding those bored holes... make it just a pilot hole for alignment, handy to drill from beneath to ensure clearances & then daub of butyl rubber caulk or similar to weather proof the fastener and protect the sheetmetal from corrosion...
 

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I'm installing my subfloor right now. I'm using the threaded holes where the tie down loops were bolted in to bolt the floor down. If I need more, I'll use T-nuts counter sunk about 1/16" (enough to be flush), screw the bolt up from the bottom, then cut it off flush.
 

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... I'll use T-nuts counter sunk about 1/16" (enough to be flush), screw the bolt up from the bottom, then cut it off flush.
I like this idea for better holding power. I am concerned that bouncing on roads might cause just self tapping screw threads alone in sheet metal to work loose. Hoping to start my build soon...
 

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I'm installing my subfloor right now. I'm using the threaded holes where the tie down loops were bolted in to bolt the floor down. If I need more, I'll use T-nuts counter sunk about 1/16" (enough to be flush), screw the bolt up from the bottom, then cut it off flush.
You shouldn’t need more if you fit the plywood, joint the edges, and use the tie downs. Mine covers only the exposed floor as the cabinets go down through it and it somewhat floats (held at the perimeter) w/o any movement or squeak.
 

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a couple of T-nuts where your problem areas are, I used stainless bolts short enough to avoid cutting, thread lock, and caulked the exposed hex nut and washer on the bottom.
 
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