And the waterproof plywood does a wonderful job of providing an unbroken vapor barrier (stagger those seams), to keep all the wood in the one RH level of the room you are laying the floor in. It’s also a good idea to put down 15 pound roofing felt, or rosin paper. But as this paper will get punched with a thousand holes, it’s not an effective moisture barrier, but gives the customer the feeling like you are doing the job according to NOFMA standards. But really you are doing much better than these minimum guidelines from the trade associations.
If you just cannot add even this much underlay you certainly will need to apply at least one coat of polyurethane floor finish on the underside of all the prefinished boards. This alone is a time consuming and messy project, and you really ought to let this finish cure for a week, so it doesn’t add an odor to the floor. Shellac or lacquer would be a quicker choice for this undercoating, but these finishes are a bit less of a barrier. Don’t use a water based finish for this. This all seems to defeat the purpose of prefinished, maybe some of the prefinished plank manufacturer’s could think ahead and do this undercoating at the factory. Nah, that would be too easy.
Rack out about 6-8 rows, and really try to keep the joints staggered well, with no H’s. Squiggle a bit of urethane adhesive on the ends, and on the middle of the long boards (4 feet and up). Because these boards have stress relief grooves in the bottom, (it would have been better to have flat milled bottoms for gluing) you will be able to only squiggle several 4″ by 1″ spots of glue on the ends. Don’t apply too much glue, or the planks themselves may get glued together. This could tear the boards in half when they get stressed by excessive shrinking. I know I’ve seen it happen. All the sides and ends of the planks need to be independent of each other. I hope this is clear. Stay with me on this.
The glue is only meant to prevent cupping and crowning, and most urethane adhesives have some degree of flex to them to allow for some seasonal expansion. Yellow carpenter’s glue does not, and PVA glues in general are not used in the wood floor business anymore because of this. I would prefer you to use DriTac 7500 urethane adhesive, but Lepage’s makes calking tubes of some pretty fair urethane adhesive (PL Premium), and I had good luck with it so far. Also Bostik’s Best urethane adhesive is quite well known in the trade, and Bostik has urethane adhesives in calking tubes for your convenience. Be sure to pick up some urethane adhesive remover and some latex gloves, or you’ll be going home with black hands every day. You’ll soon see what I mean.
We use urethane adhesives for flooring these days, because this glue contains no water or solvents. It uses the moisture in the air to cure itself, and has a bond much stronger than maple wood. If the EMC of the subfloor and the hardwood is below 8% in the winter you should mist a little water on the subfloor area and this will speed the curing. Especially since these are prefinished boards, the trapped glue may never fully cure otherwise. It always says this on the tube label, but I thought I would mention it here. Oh, and don’t get any of this glue on the finished face of the hardwood. Keep the urethane cleaner handy, and get it right off, or you will be replacing that board when the glue dries permanently on the surface, and the customer complains.