plywood floor decisions...to fiberglass, epoxy or not to do either?
Reasons for making a decision to put fiberglass cloth and resin on both sides of plywood flooring. You would do this if your make of trailer is known to have a problem with sagging floors. The fiberglass cloth and resign will considerably stiffen the plywood. If your trailer make is not know for this problem there is no real reason to go to that added expense, labor and additional weight
Area under a bathroom, I would add a layer of fiberglass and resin to the upper plywood surface and be sure sure to use epoxy on the cut edges of any holes cut through the plywood for drains from sink, toilet or shower. Then also be sure that when you install the plywood you have a lot of the underneath flooring adhesive in at least an inch wide solid band right around those holes so water from drain leaks
can't run back underneath the plywood.
ALWAYS coat the edges of the plywood to seal them. You can do it with epoxy or by rubbing a no voids coating of a good quality lifetime caulk along the edges. The cut edges have a lot of end grain wood in them and that end grain is how trees transfer moisture within their structure. So it makes logical sense that those natural fibers on end grain will still readily take up water that is next to them and transfer it deeper into the plywood layers. This is why the edges of plywood will quickly rot and delaminate. The drilled through the plywood bolt holes are also to be considered a cut edge that should be sealed before installing a bolt. So ALWAYS seal the cut edges before you install plywood flooring. As a remedial measure on existing plywood flooring put some caulking along any exposed cut edges creating a nice radius fillet that goes right onto the fiberglass shell surface as well.
Putting in new plywood flooring. There will be voids and channels in your mastic used to lay down your new floor. When you have a leak from windows
doors, plumbing or even just spills water can move to the edges of the plywood flooring and then run under the floor. There is no way for it to evaporate which means it will wick up into the bottom surface of the plywood. So do run a bead of polyurethane caulking along all the edges of the plywood after installation. Polyurethane will stick nicely to the fiberglass shell. Tool the caulking with a fillet radius to direct the water away from the plywood edge. Use a lifetime rated caulking, one labeled for marine purposes is going to be known to stick well to fiberglass.
I just purchased some Locktite Marine Polyurethane adhesive caulking at Home Depot for a reasonable price versus some of the other choices typically marketed to the marine supply stores. Stay away from silicone caulking.
I do coat the upper surface of the plywood with epoxy in any areas that are known to have a likely potential standing water issue such as in a compartment with an outside door or in a compartment with a lot of water hose connections such as where fresh water tanks and pumps are located or where a water heater is located. Or if you have an electrical
cord in one of the push through a hole types of hatches then I would coat the floor in that area. Or in the area under a sink cabinet. Better to have the water easy to wipe up and/or run off to a more visible place than have it sit there hidden and soaking into the bare plywood for a very long time.
Be the water and think "where can I go and what will happen when I get there?". Then find an easy to implement plywood rot preventative measures which pretty much amounts to caulk and or surface coatings along with a few tiny weep hole drain escapes to the outside. Gravity can work for you too
Of course no water leaks
equals no rot.