We wrote the book "HARDWOOD FLOORS" and the two accompanying video tapes/DVDs (“Laying Hardwood Floors”) & (“Sanding and Finishing Hardwood Floors”) published by "Taunton Press and Fine Homebuilding Magazine" perceived by many throughout the wood flooring industry as the definitive text for the last 20 years on the installation, sanding and finishing of wood flooring. The book HARDWOOD FLOORS can be found in nearly all public libraries throughout North America. It can be purchased directly from us, or through the publisher, Taunton Press/Fine Homebuilding Magazine, or from any of the various wood flooring associations, or at book resellers including those online such as 
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Wood Flooring over OSB

I’ve seen trade associations refer to OSB and waferboard as “panel products made from strands or wafers of various different wood species bonded together under heat and pressure using a waterproof phenolic resin or equivalent waterproof binder.” When you look deeper, you see the binding adhesive referred to more specifically to as phenol-formaldehyde resin.

On a personal note, I feel the use of the term “waterproof binder” in the description somewhat misleading. One gets the impression all OSB is weatherproof. It’s not. I might go as far as using the term “water resistant” when referring to some special types of OSB, but certainly not waterproof. Formaldehyde is extremely responsive to moisture by nature. So is wood. Modifying the resin and sealing the wood can make the product more water resistant. Coating & sealing the faces, edges and ends can make it repel moisture. But, I have a hard time believing it can be classified as “waterproof”.

I’ve noticed trade associations and others frequent use of the term “commonly used” or “successful uses” when referring to applications for OSB, composite board or waferboard as a subflooring material for hardwood flooring. Why not declare it “rated for use” or “recommended for use” as a subflooring material for all hardwood flooring. If all makes and brands of OSB and composite board are without deficiency as a subflooring material under nailed down wood flooring, why not say so?

OSB or oriented strand board (like most things) comes in a variety of different qualities. Standards vary depending on intended use as well as intended destination. Many of these appear to be changing year by year, especially in the USA.  OSB manufactured in and intended for use in the USA may be rated, but at best will conform only to “voluntary product standards” compared to that intended for use in Canada, which must conform to set CSA (Canadian Standards Association) product standards.

Some independent testing was conducted at the collegiate level a number of years ago at the request of and paid for by one of our wood flooring associations.  It was to determine, among other things, OSB's feasibility, suitability and overall reliability as a subfloor or underlayment for solid wood flooring. Without getting into a whole lot of specifics and details about the sampling methodology, testing procedures or the findings, the results were such that it caused the wood flooring associations to approve (more or less) the use of ¾-inch-thick OSB as a subfloor or underlayment material for hardwood flooring in the absence of other more definitive data or restrictive constraints pertaining to a specific installation specification. I think it’s interesting to note that given the same circumstances, 5/8” plywood remains approved as a subfloor for solid nail down wood flooring by these same organizations.

I would like to go on record as saying once again -- I personally recommend as a MINIMUM subfloor for solid nail down wood flooring ¾” underlayment grade plywood (minimum 7 ply construction) glued & screwed or glued & nailed to 2 X 12 or better joists set 16” on center or closer. This is only if the flooring is set perpendicular to the joists. If the wood flooring is to be set parallel to the joists OR if the flooring boards have an average length of under three feet OR if the flooring is to be laid in a pattern (parallel with or diagonally to joists) OR if the flooring is to feature borders, inlays, insets or other unsupported objects, a minimum additional ½” of underlayment grade plywood (minimum 5-ply construction) should be installed by ring shank nails or screws over the ¾” plywood subfloor insuring that all underlayment and subflooring seams are offset by a minimum of 18 inches.

One strong caveat to OSB’s use as a subfloor or underlayment for hardwood flooring that came out of the study was that the OSB must not be allowed to “get wet” prior to its installation or prior to the installation of the hardwood flooring over it. This disclaimer came as a result of observations made during testing that showed a substantial reduction in the holding power of fasteners driven or shot into the OSB when it was wet or after it had been saturated with water and then allowed to dry. These observations also showed that the OSB tested exhibited a tendency to swell more at the seams when allowed to get wet. This was particularly pronounced along cut edges. Further, this swelling did not appear to subside significantly, once the material was allowed to dry. Finally, these “swollen” areas showed a marked reduction in holding strength compared to other sections of the OSB panels.

Since OSB has come onto the market (roughly 25 years at this writing), I have enjoyed both good and bad experiences with hardwood flooring installed over it. I have rarely seen out-and-out failures, but on a number of occasions have observed what I consider to be below standard results partially or entirely due to the use of an OSB product, no doubt of a lower or inferior grade compared to some others on the market.

To the best of my knowledge at this writing, the resin formulations and the processes utilized in the fabrication of OSB and composite panels vary considerably by manufacturer and as a result the quality and suitability of the panels they produce for use as a subfloor or underlayment for hardwood flooring.

Let me quickly make two comments about this. To my knowledge, at this writing, and as a general rule, the majority of “industry standard” OSB manufactured in the good ole USA still does not carry a grading stamp that specifies or qualifies its functionality specifically or indicates the specific formulation of resins utilized in its manufacture. Rather, panels are “performance rated” allowing manufactures to toy around with the methodologies, procedures and products utilized with and in its manufacture.

Currently, the Canadians hold themselves to a bit higher standard. The OSB manufactured there must meet CSA standards, unlike OSB products made here that have only to meet “voluntary” levels or standards of manufacturer. Of course, this alone does not insure a better product – just a more consistent one.

A QUICK STORY

A couple years back one of my installers (let’s call him Bill – not his real name) called me on my cellular phone and said, “Boss, we have problem.” Bill was working on a high end residence where he had just finished installing a border and was preparing to put down the initial portions of an inlay. As Bill began dry setting the interior inlay, he was not happy with the appearance of a couple of flooring planks he had already nailed down on the perimeter border. He began prying up the disagreeable boards with his crowbar when he noticed that they popped up way too easily. He reached down with his bare hands and easily pulled up several more planks. All of the boards he removed still gripped their fasteners. There were plenty of nails to secure the boards. They should have held firm. Removing the boards, even with a crowbar, should have been difficult. But he was using his bare hands and had no difficulty whatsoever!

Bill said the face ply of the subflooring looked like plywood so he assumed that’s what it was. He went down below the subfloor and looked up. The bottom face of the subfloor also looked like plywood. Bill was confused. He had never seen a nailed down wood floor pull up so easily. After searching around the house, he found a couple of large holes had been cut through the subflooring in the kitchen area, probably to allow for plumbing or electrical lines. Using a flashlight he saw what looked like a particleboard core sandwiched in between the top and bottom layers of the subflooring. That’s when he called me.

I finished what I was doing as quickly as possible and drove to the jobsite. Sure enough, I could pull up virtually any board Bill had nailed down using just my bare hands. I was appalled. I immediately called our primary contact for the project -- the homeowner and advised him of our concerns. To my surprise, the homeowner, also a general contractor (a commercial builder) told me he had already noticed what his builder was using as subflooring. But, when he questioned him about it, his builder told him that his lumber supplier said the product was now rated for use under hardwood flooring. Our client said he didn’t think that it was right, but chose to go with what his contractor had elected to use.

We refused to warrant our floor over the existing subfloor and pulled off the job. In the meantime, the builder contacted his lumber supplier and they in turn contacted their subflooring supplier. The builder said his lumber supplier called the product a composition board and that it was rated for use as a subfloor under hardwood flooring as was standard OSB material. I told him, it didn’t matter to us how it was rated or by whom, if we could pull up our flooring boards with only our bare hands, we were not going to install our wood flooring over it.

The owner and builder waited for several weeks to have the subflooring inspected and tested by the vendor’s supplier, but to no avail. Finally, the owner had the entire subfloor removed and replaced at his own expense. He may have made his builder pay for it later, I don’t know. The new subflooring the builder installed was a good ¾” 7-ply ACX plywood. This time however, another ½” 5-ply plywood underlayment was installed on top. From that point on, things went extremely well and in the end, our customer got a superlative wood floor on a sound foundation

I’ve become aware of several “quality oriented” OSB manufacturers attempting to differentiate their products from lesser products by using names that sound like they’re superior (and no doubt charging more for them). Of course most of us are aware that this might be nothing more than a marketing ploy. “New and improved” has sold an awful lot of product over the years. Without indicating on the panels just “how” one manufacturer’s product is superior to others, I don’t see the point.

On a positive note, I have noticed that some “principled” OSB manufacturers (who also make plywood) do not recommend their OSB for subflooring under hardwood flooring. They indicate instead the use of their plywood products for hardwood flooring subfloors. I congratulate them on that. Still, I know, as do they, that many builders simply opt to buy OSB panels for use where hardwood flooring is going to be installed regardless of anything but price. They can’t force their customers to follow their recommendations.

An impromptu polling of many of our customers found that the normal purchaser of OSB panels was a general contractor, developer or their purchasing agent. What was surprising was that the purchaser had little knowledge about the overall “quality” or “suitability” of the OSB they purchased for use under hardwood flooring. They purchased what their lumber or materials supplier suggested or what was available at a given price point befitting the budget. By the time a wood flooring contractor came onboard or visited the site, the OSB panels were in place (and in our part of the country, had often been rained on repeatedly).

Let’s quickly pop the “OSB is greener” bubble. I’m not buying into it. I know the size trees it takes to make OSB vs. plywood and I know the chemicals going into both products. It makes more $$$ for the lumber companies I’ll grant you. Like the over-used saying goes…at the end of the day and like the once popular, now ex-Seattle athlete playing back East put it: “It’s not about the money. It’s all about the money.” Now I have nothing against making money. I wouldn’t own a business for profit (well it does sometimes) if I did. I simply think the more we know about what we buy the better our buying decisions. It really disturbs me to see a small wood flooring contractor eat an installation simply because a poor choice was made (not by him) on the proper subfloor.

Back in the days when many of the US plywood manufacturers were developing their own versions of OSB, I tested several different products from a number of different companies on their behalf and at the request of one association. What I discovered still intrigues me.

Virtually all the products I tested not only proved the equivalent of 7-ply or better underlayment grade or marine grade exterior plywood, they were far superior. Water didn’t seem to faze them. Left for weeks submerged in water, test samples show no signs of moisture impregnation even when cut or with fasteners driven into them. Each retained fastener holding strength even when left to soak in water for weeks (months in one case). My own personal assumption is that the resins used in those early products proved too expensive for production run products (at least for the average manufacturer).

It wasn’t long before I noted the quality of OSB products produced by most manufacturers failed to equal or even come close to those initial test results. The MDI resin formulations I was told was utilized in the manufacture of those early products quickly degenerated into resins formulations of lesser resolve. I found that formaldehyde-based resins formulations were quickly adopted by almost everyone. The pendulum had swung way backward in my estimation.

Formaldehyde has long been utilized as an inexpensive (yet generally effective) resin base for many wood composites. Unfortunately, like wood, it is also extremely moisture sensitive

A dry (never allowed to get wet) ¾-inch-thick “industry standard” OSB panel installed by gluing and nailing (or screwing) over 16” on center joists or closer, will provide adequate holding power for nailed down solid wood flooring -- in some instances. However, more movement and loosening of flooring boards must be expected over time compared to the same or similar installation over “industry standard” ¾” 7-ply plywood. This is particularly true over I-joists and/or where joist spans exceed 16 feet.

Significant swings in relative humidity will subject OSB subflooring or underlayment to even greater stresses when solid wood flooring is nailed into it. Some common examples of this might be found in vacation properties, homes with in-floor heating or wood burning stoves/fireplaces, and structures located in especially moist or especially dry regions. The most profound effects are generally noted in areas where the indoor atmosphere fluctuates significantly (moist to dry or dry to moist with ± 40% or more relative humidity) between seasons.

OSB panels constructed with many of the more commonly used resin formulations should not be counted upon to adequately withstand the severe additional stresses placed on them by wide swings in moisture content within wood flooring. Once completely flooded, wood flooring installed over OSB should under most circumstances be considered “ruined” and not capable of drying and resurfacing as with comparable installations of solid hardwood flooring over solid wood subflooring or plywood subflooring.

For this and other reasons (e.g. potential for off-gassing), OSB must generally be considered a “poor choice” for subflooring or underlayment over in-floor radiant heating systems.

Swelling of OSB panels along the seams or along cut edges represents evidence of an inferior subflooring material and should be given strong consideration for replacement prior to installing any solid wood flooring. Flattening the raised or puffed edges of the OSB by sanding or other methods will become necessary to make such a subfloor installation even workable. Still, the fastener holding power in affected areas (if not the entire panel) must be considered highly suspect. To my knowledge, no conclusive testing has ever been conducted (or at least made public) on the overall fastener worthiness in water damaged OSB panels.

Some evidence exists that the “all or nothing” holding characteristics of staples affords some enhanced properties when used to fasten solid wood flooring to OSB or wood composites. A portion of this may be due to the overall mass or bulk of most staples in comparison to that of nails, cut fasteners or cleats. There appears to be a reduced tendency for staples to “fracture” or “crumble” the fibers of composites when they are shot or driven into them compared to what can sometimes happen with larger fasteners. Unfortunately, the holding power of staples, nails, cut fasteners or cleats appears to be universally and dramatically assailed when the composite has been damaged by water or exposed to moisture.

It is important to note that the foregoing observations are based on OSB and composite products previously and currently (as of this writing) available on the market in North America -- primarily the USA. Not all OSB panels have proven, at least to this observer, even intermittently deficient as subfloors under hardwood flooring. Further, construction products manufacturing of these types of products should be considered a “real time” effort. As advances in resin formulations, processing procedures, etc., come about or are discovered by these firms, real and substantial changes can occur. Movements away from certain processes, techniques or resin formulations (especially given the primary grading authority’s stance of “performance rating” such products) could happen without conspicuous notice.