Glue-down solid wood floors over slab?


Old 01-23-09, 08:23 AM
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Glue-down solid wood floors over slab?

Hi all,
I've lurked here for a while and we are starting to think about wood floors now. We have a solid slab (no basements here in TX) where we are primarily thinking about installing wood floors. Glue-down engineered seems to be the best fit, but in researching, apparently sold wood glue-down is an option as well (see : NOFMA : Find Publications : Tip Sheets, Guides, Manuals, and How To's about wood flooring ). We are interested in quartersawn white oak floors, so most of the expansion will be vertical. Thus, I believe I can be even less concerned about failure than with flat-sawn boards, if we do a solid glue-down installation.

The only negative points I see are:
1. Bigger height difference between neighboring floor materials.
2. More issues with a prefinished floor, since the substrate has less give than plywood, causing edges to sometimes not line up. Careful board selection and surface preparation would reduce/eliminate issues here I'd assume.

Am I missing anything? What do some of the professional installers who are on this board think? Would you install such a floor for a customer? Is this a doable diy project compared with a glue-down engineered floor?

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Old 01-24-09, 07:11 AM
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my understanding is that solid wood cannot be used on a slab. otherwise thats what i would have done

Old 01-24-09, 10:50 AM
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Solid 3/4" wood floors can be installed on concrete above grade. Go to and download for free by clicking Publications and How To and Installing Wood Floors and PDF.

Solid 3/4" wood floors are a nail down flooring product, not glue down. As indicated in the NOFMA literature, solid wood floors can be installed if vapor retarder and 3/4 exterior grade partical board are installed over concrete or a screed system used.

Most flooring manufacturers have transitional pieces to make transition to adjacent floor coverings.

"More issues with a prefinished floor, since the substrate has less give than plywood, causing edges to sometimes not line up. Careful board selection and surface preparation would reduce/eliminate issues here I'd assume." Whether glue down or nail down or whether over concrete or wood, the subfloor needs to be smooth and flat without high spots or low spots. High spots are ground. Low spots filled with self-leveling compound. There should be no "give," which can result in movement of flooring and flooring failure. Edges do not line up because of poorly milled or manufactured flooring that is not square or faulty installation, inadequate acclimation, or other issue, not because the flooring is "prefinished."

Whatever flooring is installed, it is important to follow the manufacturer's instructions for subfloor prep, vapor retarders, acclimation, and installation. Whether installing over concrete or wood floors moisture test of both concrete or wood subfloor and the wood flooring are important.

If installing 3/4" solid wood, there should be no greater than 4% difference in moisture content in subfloor and solid wood. No more than 2% if installing widths 4" and up.
Old 01-24-09, 02:01 PM
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Hi twelvepole-
The pdf at the NOFMA link I included in my original post actually states that NOFMA supports 3/4" solid hardwood glue-down installation directly on slabs at or above grade without plywood or plywood/cribbing in between. You can click on the pdf at the original link and read the doc w/o a purchase.

I don't know how many manufacturers intend their solid hardwood for direct glue-down on slab, but my research shows that junkers and internetlumber both say its ok and provide directions.

I am trying to determine if going with direct glue-down is that much more risky, and what pros like twelvepole and carpentsdonewright would think about such an installation. I am specifically interested in thoughts on solid glue-down installation with quartersawn white oak, which should be relatively stable and most movement will be vertical, not across the width/length, so I'd think failure at the glue joint would be greatly reduced.

Old 01-24-09, 05:05 PM
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So, NOFMA has changed its position on glue down of solid wood to concrete, citing improvements in adhesives, consistency in manufacturing re: moisture content by NOFMA certified manufacturers. In the past, NOFMA did not recommend solid wood glue down on concrete because of the known fact that solid wood is dimensionally unstable, expanding and contracting as temperature and humidity levels change. And, the potential for vapor emissions through concrete can be catastrophic for an expensive wood floor.

NOFMA does, however, despite its change in position about glue down of solid wood on concrete, recommend contacting the manufacturer of the wood floor product. If manufacturer's installation instructions are not followed, then warranties are voided. NOFMA emphasizes throughout their Tips to follow manufacturer's instructions.

The above referenced link did not work for me, so I am providing this one:

NOFMA points out that the glue-down solid hardwood on concrete will behave similarly as wood installed over plywood subfloors, but they point out that homeowners can expect:

• Occasional minor gaps to
3/16" wide resulting from crooked pieces present at installation
• Occasional slight seasonal cupping of wider widths (3-5") is acceptable

If using glue-down method on concrete, you therefore have a higher percentage of waste. Bowed boards and not perfectly flat boards should not be used. In nail-down method these problems are remedied by pulling boards and nailing, so there is less waste

NOFMA cautions to pay attention to details. Job site conditions, moisture tests, acclimation, condition of slab, etc. NOFMA also emphasizes following the adhesive manufacturer's instructions.
You will likely find the following: The manufacturer is not responsible for the product failure caused by inappropriate job site conditions, subfloor deficiencies, or poor installation.

From what I read, glue down on concrete is not for the faint of heart. It is well-known that solid hardwood is not as dimensionally stable as engineered wood. Dimensional stability also varies among different wood species, with some species, particularly harder species, being more sensitive to changes in temperature and humidity. Also, the wider the board, the less dimensionally stable.

Even if it is stated that expectations of solid wood installation on concrete are similar to that of solid wood on wood subfloor, but to expect cupping, that would be unsettling to many homeowners. Over the years, we have had many posts here wanting to know what is causing the cupping of their wood floors installed on wood subfloor. Of course, it's moisture issues. Wood flooring tends to expand more so across the boards than along the length. When moisture and humidity rise beneath the board, it gets trapped and cupping or crowning can occur.

It is well-known that glue-down on concrete can pose problems even for even engineered wood. Over the years, professional installers have posted that glue down installation of engineered flooring on concrete is often prone to failure where there are moisture problems, poor subfloor prep, failure to do moisture tests, acclimate, and failure to follow all the manufacturer's instructions. Too, many installers use the wrong adhesive or old adhesive.

Having sold solid hardwood for a decade, despite the claims of improvements in adhesives, the thoughts of gluing 3/4" solid wood on concrete makes me faint of heart. Despite NOFMA's change of heart, I just checked the website of the manufacturer I represented to see if they had changed their installation instructions. Nope. Still the same. Vapor retarder over concrete + 3/4" exterior grade plywood + asphalt 15# minimum underlayment + 3/4" solid wood. Or, install using the screed system.

Years ago, this manufacturer, because of the known increased problems with cupping and crowning of boards 4" or wider, changed the installation instructions from nail down to skipping the asphalt and using urethane adhesive to glue directly to subfloor in addition to the traditional nailing into wood subfloor. With urethane adhesive acting as a vapor retarder as well as extra assurance to hold the boards flat and along with the nails to hold boards in place, this method tended to eliminate the cupping/crowning problems with plank floors.

When I posted this information in a post long ago, installers, ready to criticize, jumped up and down about these instructions for plank floors. What could I say? Like NOFMA, "It's always best to follow manufacturer's instructions for installation."

Quarter sawn oak has vertical graining, thus the expansion and contraction will be vertical rather than horizontal, making it more dimensionally stable. It is more expensive than flat grain, and it is more difficult to find in some areas. As NOFMA recommends, check with manufacturer before glue down on concrete in order not to void warranties.

To the best of my knowledge, we have not had a post here where someone glued solid hardwood directly to concrete. It would be interesting to have feedback from those who did and how their wood flooring is getting along. Perhaps an installer who is using the glue-down method of solid hardwood on concrete can provide some input to this discussion.
Old 01-26-09, 11:26 AM
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I won't be installing a solid over concrete, any time soon, if ever!!

You walk a very fine line installing solid over concrete!

Then what no one mentions is cool concrete and dew points!!!

Too risky for my liability!! I'll pass!

Now, a good engineered floor, I would give a go at it.
Old 01-26-09, 12:49 PM
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Thanks, CarpetsDoneWright for the affirmation of my feelings about glue down of solid hardwood on concrete.
Old 01-26-09, 04:18 PM
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Thanks twelvepole and Carpets Done Wright for your thoughts. I guess I'll have to go back to looking at engineered flooring, since I am not up with dealing with failure of a floor.

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