While I rarely have to install a wood strip floor on concrete here in the North East, a lot of you have houses built with a concrete slab as a subfloor. I must say, you are lucky to lack a basement, it has just been a depository for junk in my case. A slab on grade subfloor presents a lot of concerns when trying to install some types of wood floor. And if it’s a fairly new home it may have one of those hydronic-heating systems installed right in the slab. I’ve decided to write this article siting the best choices, down to the worst choices, in each category of floor types. You may find that some of the worst choices are the most commonly used methods in the construction industry, but I cannot help that.
The first things to consider if you want a wood floor on the concrete are flatness, and moisture incursion. Now don’t confuse flatness with levelness, you cannot simply put a 2-foot level on a small patch of bare concrete and say it’s flat enough for a wood floor. If you are planning on installing any type of glue down hardwood floor product you must see the entire concrete surface, and make sure that it is flat within 3/16″ in a 10 foot radius or 1/8″ within a 6 foot radius. The way you do this is to take a 6 or 10-foot straight edge and place it in various areas on the subfloor. It will be very clear where the dips are. If there are just a few dips, you should fill them in with a non-shrinking mortar, available at you local concrete supply store.
The Quikrete ® company make a variety of repair products that have a good track record for this use. Go to their web site or buy their book Build and Repair with Concrete. I highly recommend the book if you want to do any concrete repair work yourself, and Don Knotts, one of my favorite actors is on the cover, to give the book that friendly reassuring look. The Quikrete web site and the book also give detailed instructions on how to apply a floor-leveling compound. These floor levelers made by Quikrete are cement-based. Don’t confuse them with the inferior gypsum based products that have a checkered past (lots of cracking and adhesion failures). You can see why a good flooring contractor wants to see the entire concrete, in order to give you an accurate estimate. We don’t have X-Ray vision, we can’t look through carpet.