Preventing Warping From Indoor Relative Humidity On Hardwood Plank Floors


Thank you Joseph for your last email. Yes I would very much like to hear what you have to say about this upcoming job of mine. I had very little to do with the pricing of this job, it was handled by the salesman. It appears that they have not treated the job properly. I do know that the customer wants the work done properly, and she would insist that store take the proper steps.

As a subcontractor I have been able to build a solid reputation here for doing good work, with out complaints/callbacks. I do not want this job to go bad. I am schedule to start the prep on Monday.

Also, what are the special techniques to install a refinished plank floor?



Dear Todd

First off, I would have preferred if the customer had been sold a laminated (engineered) plank floor, which would have looked and sounded the same, only it would have minimal or no warping during wide indoor relative humidity (RH) swings. The salesman’s job is to find out what “look” the customer wants and then lead them to the best suited product. Anyway, what’s done is done.

It’s going to be ABSOLUTELY CRITICAL that you measure the EMC (equilibrium moisture content) of the hardwood plank, any underlay and the subfloor. Let this floor acclimatize (for weeks if needed) until all these components come to the same EMC. Keep photographic and written records of this, or everybody will blame YOU when this floor fails. Find out what the EMC for indoor wood is in your area, any wood floor installer or cabinet shop will know.

I’m going to assume this is a solid wood 3/4″ by 5″ tongue and groove plank floor.

So the first consideration is too beef up the subfloor. In your case the 3/4″ plywood is not quite thick enough for a plank floor. I would like to see you staple 1/4″ waterproof underlay plywood with 7/8″ narrow crown staples (every 6″ square). What you want to accomplish is a vapor barrier under the floor. You understand that the 2″ flooring cleats you will be using will penetrate 7/8″ into this subfloor. Without this extra thickness they will go right through the subfloor and expose the underside of the hardwood to possible dampness or at least a different humidity level from below.

What causes cupping and crowning in wide hardwood floors is the different EMC levels between the top face of the plank and the bottom face. 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 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.

If you just cannot add even this much underlay you will need to apply at least one coat of polyurethane floor finish on the underside of all the 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.

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) you will be able to only squiggle several 4″ by 1″ spot 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. All the sides and ends of the planks need to be independent of each other. I hope this is clear. Stay with me Todd.

The glue is only meant to prevent cupping and crowning, and most urethane adhesives have some degree of flex to them. 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 I doubt you could find some before Monday. Lepage’s makes calking tubes of some pretty fair urethane adhesive, 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% this 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.

Next, once this is done use a regular nailing schedule (I’m so glad you are not using staples) but 6″ would be better and make sure that each board has at least one nail about 4″ from each end, but no closer. Keep the pressure on your nailer so that the nails just nicely tuck into the little groove on top of the tongue, check often. Take your time so that you don’t get glue everywhere. The glue sets up in about 30 minutes, so if you have some stubborn boards, get them in fast, because once the adhesive has set, it’s almost impossible to remove.

Oh, and that’s another reason for the 1/4″ underlay. If this floor fails you will be able to remove the plank from the underlay (destroying the underlay, not the subfloor).

For the first few and last three rows that your nailer cannot reach, use a nail spinner available at: 1,180,42334

Use the nail spinner with spiral (ardox) finishing nails instead of the lousy thin pneumatic finishing nails, that I’ll bet you use. Don’t try shortcuts here.

After the job is done tell the customer that this type of installation, only works when they keep their indoor humidity levels, within a narrow 20% range. Try to find the MEAN indoor RH that this house experiences, and have them keep the house + and – 10% of this figure. And strongly suggest that they buy a wet/dry bulb hygrometer to accurately measure this. The cheap metal ones at the hardware stores are not accurate. A cheap but good one is available though:

This assures only minimal seasonal movement of the planks. Less gaping, crowning and cupping. I hope this wood has a satin finish on it. With a high gloss or semi gloss finish, the home owners will see the crowning effects right away once the spring hits, as the wood gets slightly damp from below. Especially if there is only a crawl space under the floor. I would prefer not to install such floor over an unheated crawl space, this is just asking for trouble.

Again Todd, I do not advocate this type of installation. Basically the wood becomes trapped in it’s own glue. And when the home owners subject the wood to extremes in RH changes (believe me they will) they will blame the store, or you, or the manufacture of the wood when this floor starts bucking in humid weather or gaping in the winter.

And then I will finally hear about it when they (or someone like them) write to me complaining of shoddy workmanship and material. What they need to understand is the hygroscopic nature of wood, but I doubt they will. I never tackle jobs like this.

Any more questions you may have on this subject or clarifications of your original question feel free to write again at no cost. I hope you have enjoyed this personal service, real human responses are the best.

P.S. If you are interested, I have a long (12 screen page) article on how to get your own better paying customers. It’s at the bottom of the Quick Links.

As always your Most humble servant, Joseph, the Wood Floor Doctor.