The installation of hardwood floors on concrete offers several types of applications. Over the last twenty years newer forms have all but replaced the older, more dated sleeper on slab system. Now with advancements in adhesive and manufacturing technology, floating floors and engineered hardwood floors glued direct to concrete (right) have become viable alternatives.
For those seeking to install solid hardwood flooring on concrete without a wood sub floor system, changes are slowly taking place when considering gluing directly. Upon closer examination, thinner solid hardwoods, shorter length products, and parquet patterns have been successfully used for years. Unfortunately the explosion of hardwood manufacturers in recent years trying to stimulate sales of solid hardwood on concrete created a plethora of problems, namely by inexperienced installers and salespeople.
The Older Method. Sleeper On Slab System
The sleeper system that entailed fastening 2' X 4's to the concrete laid on their side (image right courtesy of The National Oak Flooring Manufacturers Association - NOFMA)¹ has faded from use. The application involved adding a poly film and/or #15 asphalt felt laid into messy cutback mastic over the slab for moisture protection.
Finished Floor Was Too High
While a subfloor over concrete such as this application is not widely used today, it created vertical height concerns. Final heights are in the 2 1/4" range after the actual hardwood floor is nailed. This can cause problems with exterior door entries and other fixed objects. In some cases 3/4" plywood is used above the sleepers, adding another 3/4" or a total overall height of three inches is created. By using this method in new construction, one should plan well in advance to avoid vertical height concerns from one floor covering to another.
Recent Methods. Plywood Only On Concrete
Over the most recent thirty years the most popular method of installing solid 3/4" hardwood floors on concrete was a plywood subfloor attached to the slab (illustration left). Using a minimum 5/8" CDX plywood, the material is installed over the same moisture barriers but it is attached by way of concrete fasteners. A variety of fasteners are used, including concrete cut nails, tapcons (concrete screws), and Hilti types. Methods vary from one region to another, or what professionals are accustomed to.
Update 2010- The above procedure is losing credibility due to the loss of moisture protection through direct fastening. Read more.
Newer Methods. Floating Subfloors
Other methods of using 3/4" solid hardwood on concrete would be a floating subfloor. Yea...really...what's a floating subfloor you ask? The same moisture barriers (without mastic) are used but with two layers of 3/8" plywood. Plywood is installed opposite of one another, overlapped at the seams, then stapled or screwed together with shorter fasteners The preference here is not to puncture the moisture barriers covering the concrete.
Or. Glue Plywood To Concrete
If the floating subfloor doesn't work, another method calls for gluing the plywood direct to concrete. In this situation a premium urethane adhesive (troweled) is recommended. Keeping the plywood adhered or flat can cause problems. The solution here is establishing kerf cuts on the backside of the plywood with a circular saw giving it better flexing properties.
Traditional ¾ Inch Hardwood Glued. No Plywood Sub Floor
For more traditional hardwoods or ones that are longer in length, the largest disadvantage of gluing has always been material that is not milled straight or may not lay flat. This creates problems trying to close up gaps, whereas nailing to a wood sub floor forces gaps to close by the sheer force of the hardwood nailing tools used. Other common problems include wrong adhesives used or incorrect amount used.
Other methods of installing solid ¾ inch hardwood without a wood sub floor system include a product named Elastilon, currently sold through Lumber Liquidators. One more product to the mix is the Sika Acoubond system that also provides some sound dampening properties for apartment or condominium dwellers. As mentioned previously, any successful installation using the these products requires very straight material.
• Floating Hardwood Floors. Introduction
• Installation of Hardwood Floors On Concrete
• Engineered Hardwood Floors
• Hardwood Floor Glue
• Locking Hardwood Floor Systems
¹ Merged with the National Wood Flooring Association
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