foamie flooring alternatives?

[Tonto]

After a year of delay, I am back to designing a foamie lift-up hybrid which will be mounted on a boat trailer. It will be larger than average (14' to 15' in length and 7' wide) so weight is a major issue. Using plywood as flooring adds significantly to weight. Has anyone used a composite / SIP floor (thin plywood on both sides with XPS in the middle, potentially using PMF also)? Other thoughts? If so, what thickness should be adequate?

Wooden joists 24" on center will run cross-ways on the trailer, and we will have a joist running down the center length-wise. The trailer itself is only 5' wide, but the wheels extend outward 12" on each side, so I planned to extend the width to where the wheels are, thus making the trailer 7' wide. Thus maximum dimensions of each individual floor panel dimensions would be 24" x 30", while the panels outside of the main Frame would be 12" x 24".

Any thoughts?

[QueticoBill]

some drawings would help, but yes, I know that a ply with foam between is very strong. I think thin ply - 1/4" or maybe 3/16" if you can find it - with w" foam would span the 5' easily and cantilever 1' both sides. Skip the 2x4s.

Build a 4' x 7' test panel and try it. The walks being foam and pmf don't provide much support. If they were foam with 1/8" ply skins instead of canvas they'd help.

[Tonto]

Wonderful! I will give it a try.

QB: Just one question - I think you first suggested a plywood/ foam sandwich with the top and bottom being either 1/4" or 3/8" plywood. In your second paragraph you mentioned 1/8" plywood, but I think referred to the walls. Would walls (foam and 1/8" ply on both sides) attached on top of the floor add rigidity / reduce deflection for the floor?

[QueticoBill]

It would depend on 1/8" plywood and it's specifications. Baltic Birch should be fine, but not sure about the quality of some thin panels.

My plan is 1/4 on both sides of floor and (side) walls (though I suspect 1/8 would be fine for bottom of floor) and 1/8" for both faces of roof - from floor at front through hatch at rear.

3/16 would be a good compromise throughout. I know that is what is used for a lot of portable platforms, like those used for concerts in arenas - 4x8s supported just at corners and tested for 150 psf. You can drive cars on them, but 2" thick.

[QueticoBill]

Since you seem interested in stressed skin panels, I'll just throw out there the possibility of using cork instead of foam. Natural and I think basic wood glue would work great. Not sure where it stands price wise with foam these days.

[Tonto]

One 4' x 8' by 1/2" cork panel on Amazon is $229 (US). Don't know if that is representative of the market, but at anything near the amount I was planning, 4x8 at 2" thick XPS is a far better deal! I may be able to get this tried over the next 10 days or so, if I can get some work and home duties finished. Many thanks,

Also, I was planning to use PVC panels as an outer skin on the roof and sides (4' x 8' x 1/8") with PMF underneath. Have you ever heard of that? I am still trying to figure out the best way to adhere the PVC panels to the PMF Foam, however.

T

[Pmullen503]

If you are using panels on the outside, you really don't need the canvas underneath.

[Tonto]

My reason for canvas is that I am not sure the 1/8" PVC and 2" thick foam will bond securely. Without a strong bond similar to PMF, the structural strength of the side and top is reduced. (Note: I am not an engineer, however.) I was even planning to frame the 4' x 8' foam panels in 1" x 2" wood to add to rigidity and create attachment points for the PVC to the wood (with screws?). If anyone knows of adhesive or glue that will create a strong bond between foam and PVC without damaging either, I am all ears.

[twisted lines]

If anyone knows of adhesive or glue that will create a strong bond between foam and PVC without damaging either, I am all ears.

Gorilla construction Ultimate is good with the foam and plastic.

[Hubert]

My reason for canvas is that I am not sure the 1/8" PVC and 2" thick foam will bond securely. Without a strong bond similar to PMF, the structural strength of the side and top is reduced. (Note: I am not an engineer, however.) I was even planning to frame the 4' x 8' foam panels in 1" x 2" wood to add to rigidity and create attachment points for the PVC to the wood (with screws?). If anyone knows of adhesive or glue that will create a strong bond between foam and PVC without damaging either, I am all ears.

If you are not sure that you can achieve a good bond between the PVC and XPS, make some test panels. The strength in a composite panel depends on the glue joint between the skins and the core, otherwise you are just flexing two thin membranes and a weak core independently. Adding canvas won't do much if the PVC is not bonded to the core. Furthermore, if you have issues bonding PVC to XPS, you will likely have similar issues bonding PVC to canvas.

I have used HH-66 cement to glue vinyl to vinyl with great success, but have no clue whether it would work with foam. Aside from bonding issues with PVC it has a poor strength-to-weight ratio. I think it is about 2x heavier than hardwood plywood for the same thickness with a 2-3 times lower modulus in the in-plane directions. Neither can compete with fiber glass for strength-to-weight ratio.

Noseoil built his floor out of 3mm Baltic birch with wood framing and foam (https://www.tnttt.com/viewtopic.php?f=50&t=60248&start=15#p1093220. I've read through other build journals where 3mm BB was used for the bottom, and thicker plywood on top.

[KCStudly]

5mm (3/16) marine grade okume ply over foam is okay for crawling on, especially once a mattress is put down, but will flex a little bit under direct knee pressure (I'm north of 200 lbs and that is with 1x and 2x2 cedar framework on fairly close centers. The idea of reducing the thickness of the ply on the bottom has merit for weight reduction and there is less need for puncture resistance there (theoretically), but in bending (i.e. sagging) it is the bottom skin that is in tension, so does a lot of the work, too. For 2ft on center framing I wouldn't go below 3/16, or 2 plies of 6oz glass. For a standy I would go a little thicker on top.

[QueticoBill]

Keep in mind that foam is between 15 and 25 psi and some higher. That's 2160 to 3600 pounds per square foot. It's not going to crush from humans with even 1/8" ply.

[Pmullen503]

I'm with KC on this one. 1/4" ply over OC 250 XPS with 24" x 24" wood framing is what I have on my trailer and it will flex noticeably when you kneel in the center of a bay. Flex, per se is not that big of a deal, especially in a TD you don't walk in. But in a standy, you might notice it. Flex does not mean structurally unsound, just something you should expect.

[Tonto]

I will make a test run in the next few weeks after I finish a long list of honey-dos.

My primary concern about rigidity is that I am building on top of a boat trailer which - for the back half - has the steel frame 5 1/2’ from side to side, with the wheels sticking out another 12” on each side, for a total width of 7 1/2’. Therefore my floor will be cantilevered, approximately 12 inches past the steel frame on each side, and increasingly cantilevered towards the front since this is a boat trailer and it narrows to a single point. So I don’t want the floor to bend/ break where it is cantilevered.

This will be a pop-up, with interior height slightly over 6’.

The floor will be SIP panel constructed of XPS foam sandwiched between plywood on top and bottom with the four edges consisting of 1“ x 2“ cedar. Cedar is light and naturally water-resistant.

Any opinions before I test about whether I should reinforce the interior of the 4’ x 7.5’ foam panel with a 1” x 2” x 7.5’ center support down the panel?

Unfortunately, I did not inherit any engineering genes, so I have not entirely figured out how to stabilize and support the portion of the floor where the steel frame narrows from 5’ wide at the trailer’s rear to the front tip. Perhaps I should create a 2x4 frame for the front portion of the trailer and attach the floor on top of that. It would weigh less than welding angle iron to the existing trailer.

Several of you were concerned about the 4 x 8 PVC panels. They will not be on the floor, but the sides and roof. My question about canvas PMF covered with the 1/8 PVC panel was because I thought that the combination of 2” xps, canvas with TightBond II and then the panels on the outside would create greater rigidity than just XPS and PVC by itself, and that the pvc outer layer would offer increased protection and water proofing.

[Pmullen503]

I'm going to suggest that you do more of a torsion box with foam infill rather than a straight SIP. Not that the sip wouldn't be strong enough if you have the right glue and good clamping.

I would run wood 1x2's every 2 feet glued and screwed to your plywood, then infill the cavities with foam. When you put the top on, glue and screw the ply.

7 1/2' x 14' is huge and unless I did something like vacuum bag the whole thing, I don't know how I would clamp it well enough to trust a pure SIP.