Attach floor 1/2 plywood to 2x4, screw direction

[DesertApprentice]

I'm working on the floor.

I have 2x4 floor frame sandwiched by 1/2 plywood on the top and bottom. I wonder what's the better direction to screw the bottom 1/2 plywood to the 2x4 frame:

1) I see people screw from the 2x4 frame down towards the bottom 1/2 plywood. I guess they use 2" screws.
2) I'm thinking to use 1.5" screw, start from the plywood on the bottom and go up into the 2x4.

I think 2) has better connection strength. Is there any drawback to screw from the bottom up?

Thank you!

[dmdc411]

I would use carriage bolts through the entire floor. Heads up. Lag bolts pull out. Not sure of your build plans, 2x4 sandwiched between two 1/2" sheets of plywood is very heavy. Same results could be abtained with 1x2, with blocking at attach points, and 3/8" plywood top, and 5mm on the bottom. All cavities filled with high density foam. Everything glued as you go. The assembly would be about half the weight.

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[MickinOz]

Here's how I reckon it's done:
You make the frame, then glue and screw the plywood to it.
So, you screw up into the floor frame through the bottom plywood.
You should seal your timber anyway, so your screws should seal up at the same time.
Then the whole lot is through bolted to the chassis.

Conventional floor assembly.JPG (40.69 KiB) Viewed 696 times

2 x 4's are pretty darn big though. I'm assuming it's under that size slightly?
e.g. we get a timber size which is 90mm x 45mm.
That equates to about 3 1/2 inches by 1 3/4 inches.
Basically off the saw it'd be 4 x2, but it is then dressed all round (DAR) to 3.5 x 1.75.
Going by what I've read in the generic Benroy plans, I'm talking about "dimensional lumber".

That would be a big timber for that job. Way oversize for a conventional 8 x4 or 8x5 teardrop in my semi-informed, but worth no more than anyone else's, opinion.
Especially with 1/2 inch plywood top and bottom. Unless this thing is a monster off-road ready beast, I think it would be much much stronger than necessary.

The next size timber down here in Oz is 70 x 35. That's roughly 2 3/4 x 1 1/2 inches.
It would be a 3 x2 off saw, the DAR to 2.75 x 1.5

That is what I used for the floor frame for #2, wide face vertical, with a single top layer of 1/2 inch hardwood marine ply. It's not going anywhere, and it does not sag when I get my 6 feet tall son to sit in the doorway area. If I were double skinning the floor, I'd probably try for even smaller timber and at least thinner ply on the bottom.

Floor #2 build.JPG (27.21 KiB) Viewed 696 times

Of course, the only real reason to say your floor timbers are on the big side is the weight consideration. If the chassis is strong and the outfit has brakes, that's no biggie really.

[tony.latham]

Not sure of your build plans, 2x4 sandwiched between two 1/2" sheets of plywood is very heavy.

That jumped at me too... weight builds up fast during a teardrop build.

My floor system has evolved but here's where I'm at and where it will stay:



The plywood chunks are removed after the backside get's its 1/4" layer of ply. Then it's flipped and another 1/4" is added after the 3/4" foam board is installed in the voids.



All sheathing is glued and screwed (or crown stapled) from the sheathing side.



And by the way, it's stiff enough to walk on.

Tony

[DesertApprentice]

dmdc411, MickinOz and Tony: thank you all for your replies!

I have the 2x4 (actual size is 1.5 x 3.5) frame with 1/2 plywoods. It is indeed very bulky and heavy. It has been a learning experience for me every step of the way.

Floor will be through-bolted to the chassis. I plan to screw the bottom ply to the 2x4 from the bottom up, screw the top ply from the top down, like shown in MickinOz' picture. They will be glued as well.

camper-floor-2.jpg (141.31 KiB) Viewed 662 times