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Plank Floor: What Special Considerations This Type of Floor Raises

My definition of a plank floor is anything 4" and wider. One of the biggest mistakes I see in the wood floor industry is the failure to treat plank floor like the special item it is. And the very worst kind of plank floor is the prefinished wide boards. If you are thinking about installing a wide plank floor read this article carefully, and consider if this product is worth the trouble for you. In the North East here we have hot and humid summers, and then,heat our houses to desert dryness during the winter. So be sure to install the 4" and wider boards in the manor I describe below. In some very moisture stable climates, 4" boards may be still OK to install like strip floor. But don't try to get away with a simple nailing when you go wider that 4", you'll regret it no matter what climate you experience. In any climate you must try to keep the relative indoor humidity within 40-60% if you want minimal movements of these wide boards.

With a sharp increase in humidity (30% or more) the 4" board will expand twice as much as the 2" one. These wide boards will exert tremendous pressure against each other. They can actually crush each other slightly and push out along the sides of the floor into the expansion spaces (hopefully provided). The floor will remain tight until the winter heating season sets in. A lot of homes are heated very well but no humidity is added to the air. A simple attachment to the furnace duct will provide this with a forced air system. Or you can get various portable humidifiers and put them in several locations. The prefinished floors are more subject to moisture changes because they are not sealed in any way on the many seams. The best thing one could do with a prefinished plank floor would be to recoat the surface several times after it has been installed. It sort of defeats the purpose or prefinished, and you may also find that these durable conversion finishes are difficult to get any kind of finish to stick to them.


Central heating has been a great improvement to our home comfort but in most houses not enough humidity is added to the air. A simple attachment to the furnace duct will provide this with a forced air system. Or you can get various portable humidifiers and put them in several locations. Prefinished plank floors are more subject to moisture changes because they are not sealed in any way on the many seams. The best thing one could do with a prefinished plank floor would be to recoat the surface several times after it has been installed. It sort of defeats the purpose of prefinished, and you may also find that these durable conversion finishes are difficult to get any kind of finish to stick to them.

All the subfloor specs I will mention here are assuming your floor joists are on 16” centers. If they are 20” - 24” apart you must add another layer of plywood to give enough stiffness for the hardwood plank floor. This is best done by using underlay plywood of at least the 1/2” thickness. And it would behoove you to squiggle some urethane construction adhesive between these layers of plywood to prevent future floor squeaks.

For all plank floor I recommend at least having at least 7/8" to 1" thick plywood or softwood board subfloor. This insures the 2" flooring nails full engagement. A thick diagonal (to the joist) softwood board will make a great nailing surface for the new plank floor. But never lay a new floor in the same direction as the old subfloor boards. Read about the pitfalls of this (it’s free) in this web site. If you find you have a softwood board subfloor, laid at right angles to the joist, you will have to install the 1/2” plywood underlay as a buffer between the two floors. Plank flooring should be always laid at right angles to the joist. This is so the long boards can be screwed right through to the joist, for extra stability.

Never, never, never nail a new hardwood plank floor to a old hardwood strip floor. The flooring nails will crack the old hardwood and you will have a double squeaky floor. Remove the old hardwood and use the softwood subfloor under it. It makes a much better nailing surface. Always use flooring nails every 6-8 inches. Never use floor staples.

A wood plank floor 4" or wider will only get half the flooring nails of a 2" board. So to compensate for this you will need to pre bore, screw and peg all the ends down. A few of the longer boards (over 5 feet) will need screws on their mid length. If you are considering installing really wide 6" and greater boards, it's best to install longer screws into the joist themselves. You won't want too many pegs showing so this added screwing should be confined to only the longest boards and should not repeat itself for at least a half a dozen rows. Install the floor by nailing it just like a strip floor. And then go back and mark where off the pegs should go to make sure that the overall effect isn't too busy.

If you find that the new floor will look a lot better laid parallel to the joists (for instance down a hall instead of cross ways), use thicker plywood subfloor, about 1 1/4" or simply two layers of 5/8", as a subfloor. In this case you can still screw the occasional long plank to a makeshift joist. Counter bore and drill the holes in the hardwood where you want them. Now install a 2 by 6 bridge between the joist from down below, just where the screws start to come out. Then you can screw the plank into that solidly nailed 2 by 6, acting just like a joist.

It's best to use a counter bore and pilot hole drill combination bit. Use a drill guide to keep your pre boring perpendicular to the wood surface. Better yet simply go to, and order their drilling kit. It contains all the drill bits you could possibly need for this task, and most importantly the kit also contains two plug cutters. Instead of looking around for the same species of wood for your plugs, cut your own on a drill press or a drill guide. Or you can also use a contrasting wood species like walnut. You will want to use at least 2" screws on all the ends, and 3" screws only on really wide plank floor where they intersect the joist. The size of screws you use depends on the bore hole. Two modest 3/8” to 1/2" plugged holes would look fine on 4-6" Planks. But you should use a 3/4" bore hole on 8-12" Planks. Once the plank goes over 8" you will be using 3 screws in all the ends and long boards. Generally the screw sizes will range from #10-#14, they just have to match your counter bore unit. Oh, I almost forgot, don't do the ends where they meet the wall, this will look silly, unless they are completely cover by the molding.

I have found that the best screws are the German made Spax screws, available at Lee Valley tools. They have a thread design that allows for the very smallest pre drill hole to be made. The design of the screw itself then cuts it own threads and holds almost like a machine screw. Very effective, and worth the extra cost. Don’t use drywall screws here, they are brittle and will snap off in time.

The pegs will get glued into the bore holes by a good PVA carpenters glue. If you have used the Snug Plugs provided by the Lee Valley drill kit your plugs will have seated in without a visible glue line. Only when the plugs have dried overnight should you cut them flush with the floor, with a special flush cutting saw. You can see why this method won’t work well with a prefinished or presanded plank floor. The surface of the wood will be so marred with all this screwing and pegging that the floor logically will have to be sanded and finished on site. But there is a way to end fasten these types of floor, without using screws at all.

You could try a short cut to this end fastening, and use nails instead. But no ordinary nails will do. Use the old fashioned cut nails, and yes they are still in manufacture. You can get them from the Tremont Nail co. at You can use their 2" flooring nails for face nailing between the joists, or go up to the 3" floor nail when hitting the joist. You should pre drill for these also. These flooring nails have small head so that they can be set below the surface (then filled), and would be suitable for hardwood plank. But for pre-sanded (sanded at the factory with beveled edges, but no factory finish applied) softwood plank floors you might be better off with Tremont Nail’s rose headed nails pounded just to the surface and not filled. You'll have to install these after any final sanding, and several, but not all coats of finish are on the floor. Just screen buff the floor before you pound in the nails, and this way it will save having to buff around the rose heads. Apply the final coat of finish over and around the nails. This makes things a little more forgiving.

Speaking of floor finish, this is a good place for Oil Modified Poly, with it's greater moisture sealing properties. Much better than any oil/varnishes, or water based finishes. And in most climates you might consider applying at least one coat of this good finish on the underside of the boards before a week before you install them. The slower the moisture migration in and out of these boards the better. This would be especially important if you have a damp basement or crawl space under your plank floor. Cupping and crowning problems occur when the two surfaces of the board have different moisture content. The more moist surface will curl up on itself. Moisture from below will cause a crown or convex effect. Excess moisture on the surface (too little finish and too much mopping) will cause a cupping effect.

You may not be able to avoid this slight warping of the surface with these wide boards. So I would suggest you use a satin or low luster polyurethane finish instead of a high gloss. In any case the lower luster finish will look better, longer and within the same brand it has just about the same durability.

Now the last and worst plank floor installation is the prefinished plank floor. Most people assume that you can install this floor without screws or pegs and a 3/4” subfloor is quite adequate. And that is what all the manufacturer’s suggest. But nothing could be further from the truth. The standard 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.

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.

Next, once this is done use a regular nailing schedule (I'm so glad you are not using flooring 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). 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 (half the time the RH is higher than this figure and half the time lower) 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. A prefinished floor 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.

I do not advocate this last type of installation with prefinished wood. 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.

Here’s one more suggestion with this wide wood flooring. Most people find it difficult to prevent these boards from forming gaps over the dry winter heating season. So in this case you might consider installing a V grooved plank floor instead of a square edge. The deeper the V groove the more the gap will be disguised. But the deep grooves will also be dirt and dust collectors, so be careful what you choose.

Be sure to read the section on subfloor and nails in the Strip Floor article, There are warning in this about OSB subfloors, and those lousy flooring staples. A lot of people will consider this all too complex and go ahead and just get by with what ever subfloor they have.

So remember to choose a thick plywood or softwood subfloor, beef it up if need be. Choose the unfinished plank floor for best results. You can see in the pictures that the plugs in the plank ends are very subtle, and do not detract from the appearance of the finished floor. And please turn down imploring salesman at those wood floor boutiques, when they want to sell you prefinished plank floor. it’s just asking for trouble.