Tom&Shelly wrote:My experience with underlayment is that some brands aren't particularly waterproof. I bought a sheet from Home Depot and tested a chunk in water, and it started to become unglued....
On the other hand, Tony Latham, on this forum, got some good waterproof underlayment from his local lumber yard..
* I used thin underlayment on my floor, and due to good prepping, it has been flawless...as has my cabin structure.
* I started my trailer build using a 1/2" sheet of birch plywood that had been sitting unused in one of our sheds for twenty years. It was good quality, and hadn't warped or delaminated in any way, so it became the bottom of the floor (undercoated with automotive spray undercoat). I decided that I wanted a 3/4" floor, so I bought a sheet of Luan underlayment, which was 1/4" thick, and used Titebond2 and 38 Tek screws around the perimeter (plus eight 3/8" carriage bolts thru to the crossmembers) to securely attach it all.
* I made the floor before I learned of the polyurethane+paint thinner "mix" for waterproofing, but since I undercoated and filled all underside seams with PL adhesive, I figured that my floor would be effectively waterproofed after I put 2-3 ? full coats of polyurethane on the topside of the Luan. I guess it was sufficient since it once had two inches of standing water on it, overnight plus a full day, after I left a door ajar in a monsoon. No warpage or delamination, six years later.
MickinOz wrote:...Here in Oz, the type of glue used is generally reported in the specifications on the retailers website.
For example, Bunnings tell you that their Australian structural ply is constructed with type A waterproof phenolic resin.
I bought plywood from various suppliers. With two exceptions, It has always had the type of bonding printed on the back.
I look for A-Bond, or WBP (water boil proof). South American Aaruco ply is all made with WBP, even their structural grades. Love it....
* My trailer is built from 3/4" Arauco plywood (no insulation), with phenolic resin between plies, except for the floor; I got a great deal on six sheets for $100 ??, with one side pre-sanded already. Once I studied the poly "mix" threads, I totally waterproofed the plywood before assembly, and again used PL adhesive inside and out (and in-between) all seams, for more waterproofing. 10-12 tubes. And afterwards did the same with poly "mix" basecoats, and multiple topcoats of acrylic farm equipment enamels. Ten years after the trailer was constructed/poly'd/painted, and after many modifications, there's been zero instances of wood rot or delamination. I love that plywood, too.