Light Weight Floor

[Postal_Dave]

I was just answering another post about light weight campers and I realized that floors on foamies are usually made really solid and very heavy.

Using this Northern Tool 5 x 8 trailer as reference, https://www.northerntool.com/shop/tools/product_200612541_200612541 , with each cross member placed at a 2 foot spacing.

How would you build the floor to make it sturdy but light weight?

[Ottsville]

In a teardrop the floor will take the most abuse during building. Once you're done and there's a mattress in there it's just supporting (mostly body)weight with the area near the door taking the most abuse. I think if you are primarily concerned about weight you could go with something to protect/seal the bottom and then a thick piece of foam insulation, as long as that insulation wasn't supporting the walls. Not ideal, but pretty light.

[tomhawk]

My floor is a 5' x 9' composite of 3/4" foam covered with top and bottom 1/4" plywood.
It seems quite sturdy and weighs around 110 lb.

I would be interested how much other people's floors weigh.

Thanks,
Tom

[eLink]

My floor is a 5' x 9' composite of 3/4" foam covered with top and bottom 1/4" plywood.

If I were to build another one, I would definitely go with this method.
With an NT trailer locate 1x (3/4" actual) framing at side rails, cross-members and at anticipated bulkhead locations. Make sure to add shim strips over the cross-members.

[John61CT]

Planning stage only, not yet built. Design also suitable for a bed platform, maybe raising or folding in a standy.

Greenhouse plastic bench grids, sufficiently load bearing but super lightweight and pretty cheap, easily cut to fit like wood.

Come in sizes designed for load bearing support "beams" on 18" or 24" centers both sideways and longitudinally.

I would use aluminum profiles for that, could be 8020 style but pricey.

I'd do the outermost edge in angle, likely already decent support there in the trailer chassis / frame design, so maybe can do without.

Main load-bearing cross members spaced (exactly, tight fit with bench grids snapped in) at 18 or 24", in rectangular tubing or maybe I-beam

Then lighter /smaller secondary "mid support" pieces longitudinally, T-profile in the middle, angle at the outermost edges if needed.

Can use spray or pourable closed-cell PU foam to infill the grid for insulation, bring the surface up to a smooth level plane higher than any of the framing to prevent a thermal bridge

optionally add a slab of rigid foam for more insulation as desired, above or below.

Skin the top with luan, vinyl, gym mat foam or whatever you like.

[tony.latham]

...and I realized that floors...

Dave:

Keep in mind, I'm a woody, not a foamy. But I believe in building light (and strong).

This is how I made my last floor.

It has an internal 3/4" framework:



The gussets are temporary. They hold it together until it's flipped over and skinned. With this floor, I skinned it with 1/4" subflooring plywood on both sides. You could use 1/8" ply on the bottom to shave weight. The voids are filled with foam board.

It's remarkably stiff and strong.

The skinny cross members are doubled 3/4" plywood with the grain up, not flat to aid in stiffness. The plywood is there to support the vertical upright pieces in the galley along with the batter, 7-gallon water jug, and 54-quart cooler. The lateral 2x6"s are there to support the walls and provide "meat" for the attachment points to the frame.

The 1 x 4" cross-member gives support to the headboard that will be above it.



I epoxy the bottom and give the top three coats of an oil-based poly.



Tony

[ghcoe]

My floors are just a 4'x8' sheet of 7x16" OSB. 53lbs. is the heaviest part of the build, but you have to have a strong foundation.

[pchast]

I've got ZIP wall outside down. with 1.5" blue board foam and 4 mm underlayment on top.


The ZIP Wall company responded... 45 to 48 lbs per sheet from the factory."
So the total floor weight is likely 79 lbs from the data sheets.

[linuxmanxxx]

I did all 3 of mine with 1x2 framing and 3/4"foam in the gaps and Luan on both sides. Use the mix on bottom and match cross members in framing for bolting and it's way lighter than anyone else has posted and it's strong as long as you glue completely the Luan to the foam top and bottom. You can stand and walk on it and under 50 pounds. It's a torsion box which has massive strength due to the glue surfaces.
Steve

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[John61CT]

Yes the extra tensile strength of rigid material well-adhered to a "filler" with just compressive strength

in effect, a DIY composite

can be major.

I did not emphasize that in my "greenhouse bench" description,

a sheet material epoxied directly to the underside if the plastic grid would drastically increase its strength,

most likely to the point of eliminating many of the supporting frame members.

But with them, it is completely unnecessary, would just be overkill.

[aggie79]

Like others have done, I used 1x2 framing with 1/4" plywood (actually 6mm Baltic/Russian Birch plywood) top and bottom with the voids insulated:



With framing on in the floor, you can eliminate almost all of your transverse metal framing in your trailer to save weight.

To further save weight, you could reduce the size of the 1/2 framing to say 3/4" thickness x 1" width and, if you don't go rock-crawling and protect the floor during the build, I feel you could use 3mm Baltic/Russian birch plywood instead of the 1/4"/6mm.

[tony.latham]

I used 1x2 framing with 1/4" plywood

Aggie:

I think you were the one that tipped the scales for my last floor construction.

Thanks,

Tony

[GPW]

The other day I was checking out a friends supply of #1 Cedar boards … straight grained , clear of knots ,not quite 3/4” and Very Light ( bug resistant too ) … Our homemade trailers floors evolved now to a “board floor” which we like and is very durable …

My friend also had a piece of foam ( 1” ) that he’d fiberglass on each side with cloth and epoxy , he placed it across a couple blocks two feet apart and then stood on it in the middle … A Big Boy (~270#) , it flexed slightly but made no noises … Another option for those that like fiberglass , and being a floor , there’s no worry with Epoxy resin and UV light …

As exemplified in the above posts , it may be better to use a Hybrid floor with specific materials used in places of need/stress … The material should fit the job .

Nice to have so many options besides a HEAVY sheet of wood chips .

[Longn46]

...and I realized that floors...

Dave:

Keep in mind, I'm a woody, not a foamy. But I believe in building light (and strong).

This is how I made my last floor.

It has an internal 3/4" framework:



The gussets are temporary. They hold it together until it's flipped over and skinned. With this floor, I skinned it with 1/4" subflooring plywood on both sides. You could use 1/8" ply on the bottom to shave weight. The voids are filled with foam board.

It's remarkably stiff and strong.

The skinny cross members are doubled 3/4" plywood with the grain up, not flat to aid in stiffness. The plywood is there to support the vertical upright pieces in the galley along with the batter, 7-gallon water jug, and 54-quart cooler. The lateral 2x6"s are there to support the walls and provide "meat" for the attachment points to the frame.

The 1 x 4" cross-member gives support to the headboard that will be above it.



I epoxy the bottom and give the top three coats of an oil-based poly.

Tony

Hello!

Can you help me understand this part?

1. What does cross members grain up, not flat mean? Does it mean instead of = we cut it so grain is like this |||||

2. What’s the width of these cross members (cross bar 3-5)?

I’ve been reading your book and it says two layers of 3/4” plywood set on edge. Do you mind explaining it to me? I tried googling and YouTube with no success.

4. Do you need to join the floor frame together (the 1x6 and 1x4)? Or is it being held in place only by the crown staples and glue from the 1/4” plywood?

Thank you!


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[QueticoBill]

It would be my opinion you don't need more than a perimeter frame and frame or ply and thinner foam at ply skin seams. The foam needs to be fully adhered to the ply skins but ultimately the glue and skins will hold the frame together, and the fasteners will have outlived there usefulness when the glue cures.