So after the floor is filled and patched and these compounds are cured, what choices do you have in hardwood? Not so fast, you still have to do a moisture test if your concrete subfloor sits right on the earth. I wouldn’t consider a solid wood floor unless the slab is at least a foot above the highest outside grade. Moisture can creep into the concrete and by capillary action rise through the minute cracks and get into and swell the wood. A well-sloped and drained outside grade will prevent this. A solid wood floor you are about to lay on that slab is hygroscopic, meaning that it will absorb moisture from surrounding materials. You will need to do a moisture test on the concrete in various areas of the house. You will need to do this during the wettest of your seasons. I’ll bet you won’t hear any of this when you purchase your wood floor from one of those big box stores. But I hear about it later when the floor has failed, and numerous strips are buckled off the subfloor. The easiest way to test for moisture is to duct tape down a 2-foot square piece of clear heavy plastic on all sides. Suspend a 100-watt light 18″ over it and wait 3 days. If there is any moisture in the slab it will show up as condensation on the plastic or as a darkening of the concrete. If this is so, you will then need to do a more quantitative test and that is the calcium chloride measure. The easiest way to get the materials and instructions for this is to go to www.vaportest.com. Or try www.sinakcorp.com (this company offers professional and warranted solutions to moisture incursion problems). The results of this test will give you an idea of just how much moisture is coming into the slab. The guidelines for wood floors are no more than 4 lbs. per 1000 sq. ft. over 24 hrs. This will make more sense when you see the vapor test web site. Oh, and by the way if this is new construction, you must wait at least 60 days for the slab to fully cure, and do these tests also.
Of course it should go with out saying that a concrete subfloor that has a basement under it or is on the upper level of a modern building will only experience moisture problems at the edges of the floor. This may be due to wind driven rain and should be only a rare occurrence. On those temporary damp conditions, good glue shouldn’t fail much. And good glue means flexibility for the life of the floor. Dri Tac 6200 has a proven record in this ability, to flex even 5 decades later, if the wood swells a bit out of place during these temporary moist conditions. Later when conditions dry the wood will settle back to it’s original spot, as if it remembered where it was installed.
The very best choice of wood floor on concrete is solid wood parquet, which comes in a variety of patterns and styles, so don’t dismiss this elegant flooring just because you think it will give your floor a cheap look. In fact some of these 3/4″ block floors in the Montecello pattern are more expensive than most strip floors, and are better suited to a glue on slab method. The plain mosaic pattern you see in most stores is a poor representative of this durable solid hardwood. These 6″ by 6″ tongue and groove prefinished squares don’t install very well except on the flattest of subfloors. Where as the smaller unfinished 4″ slats of a square edged parquet easily follow slight undulations of the concrete. All parquet has opposing grain patterns that are much less likely to fail, if some excess humidity gets to them. You can read more about the special considerations of this type of floor in the Hardwood Authority section.