The short answer to “Can I glue solid hardwood over concrete,” is “yes”, but there are caveats. All things being equal, an engineered floor will expand and contract much less than a solid hardwood floor. This must be taken into account before, during, and after installation to avoid serious issues.
Last year a multifamily developer I work with fell in love with one of our solid hardwood floors for the amenity spaces in one of their developments; however, a raised subfloor was not an option so they could not nail it down. To use this floor, they would need to glue solid hardwood over concrete. This prompted lots of conversation and research from all parties involved. Eventually, it was decided that we should proceed with the selected floor. Thanks to very good work by the contractors everything turned out well. The client was happy, and the floor looks amazing a year later.
Here are a few tips we shared with the contractor that you can use as well:
Always follow the manufacturer’s guidelines when installing any hardwood floor
At the end of the day it’s the hardwood manufacturer that will resolve any warranty issues with the floor. It’s important to follow any guidelines they publish so that if issues do arise they will be willing to help solve the problem. This obviously applies to the recommended installation method. If they don’t recommend gluing the hardwood, then don’t do it!
Always follow the adhesive manufacturer’s guidelines
Adhesive isn’t something you see after installation, but it’s incredibly important to remember. Adhesive manufacturers all have different approaches in terms of the installation method they recommend. Many of them are very specific – even requiring certain trowels so that adhesive is spread at the proper rate.
Make sure the slab is completely cured
When you glue solid hardwood over concrete it is not something that can be rushed. It’s very important to moisture test the slab multiple times over a period of weeks to ensure that moisture levels are consistently below the thresholds recommended by the hardwood and adhesive manufacturers. It’s not enough to test it one time – a significant rain the next day could very easily increase the moisture content of the slab and result in problems.
Make sure the HVAC is up and running and can continue running before, during, and after installation
HVAC is the first line of defense against the two primary causes of a solid hardwood installation failure: temperature and moisture fluctuation. It’s very common for flooring crews to start installing materials when the HVAC is turned off, or for the HVAC to be turned off after they leave. Obviously it’s advantageous to avoid the expense of heating or cooling an unoccupied building; however, it’s not worth the risk with a solid hardwood floor. Be sure it runs before, during, and after hardwood installation.
When in doubt, refer to NWFA guidelines
The NWFA has been around for a long time and is a trusted source for unbiased installation knowledge. They are an excellent resource for any hardwood installation question – especially in unique situations.
Design credit for pictured project: Design South – http://designsouthstudio.com/