It's A Sandwich

November 29, 2020

The Floor Sandwich

The flooring system I'm installing (not quite done yet) is a sandwich of materials. The bottom or outside layer is 0.025" 5052H32 sheet aluminum. This sits directly on the steel trailer frame which is coated with POR15 which I'm counting on to moderate any galvanic corrosion between the steel and aluminum.

The 5052H32 is fastened to the frame with 1/8" stainless steel closed-end pop rivets spaced around the outside edge about every 12". The closed-end rivet will prevent water from entering through the typical mandrel hole in the pop rivet. There are three 48" wide aluminum sheets laid "east-west" (north being forward) and they overlap by about 3". Where they overlap I used a double bead of Sikaflex 221 sealant-adhesive, then I fastened the sheets together with 1/8" closed-end pop rivets.

Above each frame, on top of the aluminum, I laid down 1 1/8"tall x 1.5" wide kiln dried fir (ripped from 2x4 lumber). The lumber is crossed using half-lap joints. The lumber is attached to the frame using #14 2" stainless steel flat head screws driven into 7/32" holes (my Dewalt driver did this with ease). This is similar to how the original floor was attached to the frame except I used stainless steel screws with Philips head vs a straight screw slot which is a major pain to remove. Also the original lumber spacers were not half-lapped, they simply laid them down.

A few other deviations from the original:

I added a couple cross members were there was no frame below them. I'll screw these to the plywood flooring and they'll act like battens on a wide wood surface (like a table or bench top). They are strong by themselves but when used in conjuction with the floor they'll add increased stiffness.
I cantilevered the ends at the front and back by about 2 inches each where the trailer shell bends outward. This provides an additional 4" of floor space. There will be minimal load on these outside edges but every inch counts in a small space like this.
I made sure to drive a fastener through each lapped joint. Many of them had fasteners to the frame but those that didn't got a #10 stainless steel screw similar to what I'll be attaching the plywood with.

You may notice some coloration on the lumber next to the aluminum sheet. This is a coat of penetrating epoxy to help seal the wood from water damage. The #1 cause of trailer damage is water. A wise engineer once told me that you can either hermetically seal something or plan for what happens when water gets in, because it WILL get in. So my plan goes like this:

Provide 1/4" weep holes in strategic locations in the aluminum sheet to allow water to drain out.
Cover the weep holes with 1mm square stainless steel mesh on the outside where it can be replaced when it inevitably gets clogged or damaged. Some bugs will be able to get through the mesh, but there are other, larger openings.
Used strips of corrugated plastic ("coreplast") to create an airgap between the aluminum sheet and the rigid foam insulation.
Dado 1/8" channels in the bottom of the wood cross-members so water and moisture can migrate (hopefully out through the weep holes).
Seal the wood cross member bottoms with penetrating epoxy where they would com in contact with water.
Use Extruded Polystyrene (EPS) foam as insulation. It's the closest option to waterproof. The other options, polyisocyanurate (polyiso) and expanded polystyrene (XPS, i.e. Styrofoam), aren't nearly as water resistant. And stuff like fiberglass or rockwool may technically be waterproof, but water will get trapped in the batting. The trade off is EPS has about 20% lower insulation value per thickness that polyiso (but better than XPS). And the cost of EPS is much higher than XPS.
Use "marine grade" plywood and seal it with penetrating epoxy. Marine grade plywood does not have voids so water can't get trapped internally. And it's constructed with waterproof glue. Unlike exterior grade plywood it is not treated with chemicals that might damage the fastener hardwear. Marine grade plywood is more expensive, particularly if you live in the desert where it's a special order item.

Strips of Coreplast used to create void between 5052 AL and the EPS insulation

So I made good headway the past few days, but I didn't get quite done because the penetrating epoxy hasn't cured on the plywood. I cut the plywood out and dry fitted it, but need the epoxy to cure before I can screw the floor to the wood cross members. It's cold so I ordered the cold weather epoxy variant and the cure rate is much slower, it could be days. So it won't get done this weekend.