Foamie floor

[gregkn73]

Hi all,

I would like to make a roof top tent, mounted onto my trailer and if family like it, proceed then , making a tear drop trailer. I would like to make it as light as possible, so I am thinking to make the bottom, floor panel, from foam & fiberglass, instead of plywood . I have red few threads here, and I can see that mostly, you use a plywood floor for your foamies. The bottom panel of my RTT , will be 215cm*125cm. If I use 3cm foam, covered with 6oz fiberglass and epoxy, won't be strong enough, to support two of us? It will have for extra support two longitudinal aluminium rails, for mounting on the racks of the trailer. Thanks in advance for any info and suggestion.

Greg

[RJ Howell]

I'm a build for my truck. The Loft (over cab section) is 1.5" foam, fiberglassed over (outside only) 1.5 sq tube .125 steel at 16" center (long way, so 16 x 74 foam span) and I'm still going to put at least 1/2" plywood over. Chances are 5/8" will be my final choice. Just as it is, and I lean on it, I can hear things I'd rather not hear..

Ya, they build boats from this stuff, but it's a bit more than just 6oz cloth (so I read). With your RTT, you'll need plywood in several places to reinforce for hinges, clamps, mounting screen panels.. Ya, as I did, sandwich the foam with 1/4" inside and out for the sides and roof sides. But... I'd plywood the deck. MHO..

Nice project! A slant roof will be fairly straight forward! You want to go for gusto, try a full lift!


Then comes the sewing.. Where I'm at right now..

[Pmullen503]

The question is can you stand in the middle of the least supported section. Fiberglass over foam is good under tension but bad under compression (exactly what you'd subject the top surface to by standing in the middle of a poorly supported section.) For a wall or even a roof, probably OK because the per square foot loads are lower and more uniform.

I used 1/4" ply skins over 1.25" (30mm) foam with perimeter solid wood and it will flex a little when I kneel in the middle of an unsupported section. I don't think 6oz glass alone would have held up.

[gregkn73]

The question is can you stand in the middle of the least supported section. Fiberglass over foam is good under tension but bad under compression (exactly what you'd subject the top surface to by standing in the middle of a poorly supported section.) .

So if I use at the entrance area
, where we will be kneeling, an extra layer of 6oz glass, do you think, I will increase enough the compression resistance?

[tony.latham]

So if I use at the entrance area...

Are you glassing on both sides?

T

[Tyrtill]

As long as the foam is semi supported (16" centers) you could probably use it as a floor for a roof tent since you are keeping your weight fairly distributed. If the foam is supported by the vehicle roof completely you would probably just have to protect it from sharp objects gouging it which fiberglass, PMF, or a heavy blanket would do.

I was experimenting with foam because I was thinking about making my benches out of it. Here is the video from that experiment.

I believe it was 16.5" space between the supports and the foam was supported at the one end completely by wood but the other end was completely open.
https://photos.app.goo.gl/MjAN2hbgB4pTJr3r9
(sorry about the weirdness of the dialogue in the video it was only meant for me)

I would think that if you put a 1/8" skin of wood on the bottom or wrapped the whole piece of foam in fiberglass or pmf you could probably get to 24" without support especially since you will be laying down for the most part.

[gregkn73]

So if I use at the entrance area...

Are you glassing on both sides?

T

Yes

[gregkn73]

As long as the foam is semi supported (16" centers) you could probably use it as a floor for a roof tent since you are keeping your weight fairly distributed. If the foam is supported by the vehicle roof completely you would probably just have to protect it from sharp objects gouging it which fiberglass, PMF, or a heavy blanket would do.

I was experimenting with foam because I was thinking about making my benches out of it. Here is the video from that experiment.

I believe it was 16.5" space between the supports and the foam was supported at the one end completely by wood but the other end was completely open.
https://photos.app.goo.gl/MjAN2hbgB4pTJr3r9
(sorry about the weirdness of the dialogue in the video it was only meant for me)

I would think that if you put a 1/8" skin of wood on the bottom or wrapped the whole piece of foam in fiberglass or pmf you could probably get to 24" without support especially since you will be laying down for the most part.

How thick is the foam ,in your video? I have made the same with 5cm foam , with similar results :-).... That was the reason I thought, that if wrapped with 6oz fiberglass and epoxy, will be pore than sufficient, for using it as floor for my RTT. But then, i red many threads here, that all used plywood for floor and hesitate!

The floor will be 215cm*125cm, 3cm thick foam, wrapped with 6oz fiberglass and Epoxy, will have two longitudinal aluminium rail isupports 40cm from each longitudinal end, and at each longitudinal end will add 1cm*3cm wood slats, to attach ,hardware. If needed ,only at the entrance area, I can add an extra layer of 6oz glass. The rtt will be attached more often on cross sectional racks onto my trailer .

I hope it so more clear , the description of my construction plan.

[tony.latham]

So if I use at the entrance area...

Are you glassing on both sides?

T

Yes

That will help solve the tension strength issue.

I've done a fair amount of fiberglass work and can't answer your question about adding a second layer of glass for kneeling strength. You're probably considering doing something that no one on this forum can give you a definitive answer on. I would suggest that you laminate a small piece of foam --4 square feet or so-- and give it a test.

It would make for a good practice run if you haven't used epoxy/glass before too.

Tony

[OP827]

I did one and two layers of 6oz cloth over 300 Foamular XPS in my build and for walls and ceiling it works, but for the floor that could be a problem with foam compressing under a knee etc. XPS comes in various density. 300 means it will compress 10 percent under 30 lbs/in2 load , so it might not be suitable for floor if only covered by a single or double layer of 6 oz fiberglass, something thicker to distribute concentrated load may be required. I build a table with 3mm Baltic birch on both sides and foam in the middle and can stand on it no issues, no epoxy and cloth was added on the table. Just sharing my experience. You could try GRP or FRP sandwich with foam, looks like more research and testing.

[gregkn73]

So if I use at the entrance area...

Are you glassing on both sides?

T

Yes

That will help solve the tension strength issue.

I've done a fair amount of fiberglass work and can't answer your question about adding a second layer of glass for kneeling strength. You're probably considering doing something that no one on this forum can give you a definitive answer on. I would suggest that you laminate a small piece of foam --4 square feet or so-- and give it a test.

It would make for a good practice run if you haven't used epoxy/glass before too.

Tony

Thanks Tony and all others, for helping me.

Unfortunately where I leave, I have access only to foam. Fiberglass and epoxy should be ordered, with pretty expensive shipping cost, so I can't order small quantities to just experiment, as I wanted to do :-(

So if 3cm foam wrapped with 6oz fiberglass and epoxy, won't have any tension issues and only compression is a possible issue, I wonder how all these DIY surfer's boards are made. If compression is the problem and not the tension , my RTT floor where I will kneeling have the same or less compression forces as the surfer's boards, where they are standing on feet . Also most of the boards are wider than 40cm, which is the width of unsupported foam parts at my RTT floor plan.

[Pmullen503]

A surfboard is supported by water which spreads the forces out in the most advantageous way. Put that same surfboard between between two sawhorses and try standing on it. No one is saying it won't work but no one has tried it. Try it, if it works, great! Worst case you glue on another layer of foam and glass that too.

[gregkn73]

A surfboard is supported by water which spreads the forces out in the most advantageous way. Put that same surfboard between between two sawhorses and try standing on it. No one is saying it won't work but no one has tried it. Try it, if it works, great! Worst case you glue on another layer of foam and glass that too.

That is exactly the reason, I make the question here and not at surfer's forum :-) But you first, among others, wrote me, that fiberglass is great under tension and weak under compression! As I understand it, if it is great under tension, it doesn't matter if it is supported by air or water underneath. And since compression is its weakness, I thought that the upper surface where we will kneeling on it , need a greater strengthen. Surfer's use mostly 6oz+4oz fiberglass on top of their boards , and only 4 or 6oz underneath, as I just red.

Again I know that the better way, to find answers ,is experimenting, but as I wrote earlier, fiberglass + epoxy should be shipped, with great shipping cost, so I don't want to order materials.just to experiment.

[John61CT]

Actually foam is much better against compression forces, they just need spreading over more square inches.

Similar to puncture resistance, which would be critical for a floor

both are the same even if the foam were supported by sitting on top of solid 1"steel plate.

Now, the **tensile** strength, ability to resist deflection by a point load when only the edges are supported

is **very** weak for plain foam slabs of any sort. This is what is addressed by PMF, glue holding even weak fabric, once all well "laminated" so long as the adhesive is strong, the
fabric prevents the surfaces from bending, at least to some extent.

But not for a floor without support, say 18-24" centers.

Now, sheets of even thin plyboard will be much stronger. As would many plastic sheeting, especially if all one piece covered the whole surface.

Then your cross beam support could go to maybe 30-40" apart.

I would not do any lightweight sheeting on its own, even 3/4 marine ply only supported around the edges. Maybe fine for the first ten years but. . .

So, pick a rigid sheet material (not stretchy or flexible) you like for the flooring surface, Baltic would come up nice with protective sealing treatment.

Maybe a rougher thicker version for underneath, or even a strong canvas PMF, you just need to protect the foam from water, punctures and abrasion breakdown there.

Personally a thin layer of metal sheeting would be better.

And really glue well, laminating in effect a DIY composite panel.

Seal fully, protect the edges well too.

[John61CT]

Another approach I think stronger and likely just as light.

Build a framework with rectangular aluminum profile, precisely designed to enclose and support plastic grid bench / shelving sold for commercial greenhouses.

Not expensive stuff, and very strong, supported at 18-24" centers like 300lbs psf.

Fill those grid holes with pourable floatation / insulation PU foam, maybe more on the underside, bring the floor surface up right to the top of your aluminum "joists"

and then skin for aesthetics and abrasion/puncture resistance only, could use a cork, vinyl rubber even carpet.

That skin does not need to be "strong" in the tensile sense, not even glued, could literally be a rug.

The floor itself is insulated and waterproof, nothing that will ever rot.