More about installing by-the-book: Standards Q & A
My last column about the importance of industry standards raised a number of questions, and I hope it answered some questions as well.
I just started my 40th year in floor covering, and the industry has done a good job establishing a large number of written guidelines that cover the variety of flooring materials and job conditions.
The important thing to be aware of is that the standards that are set by the manufacturers of a floorcovering, adhesive, or setting materials will overrule any industry standards that might be published. So, if the industry standard and the manufacturers guidelines don’t agree, go with the manufacturer.
For many of these standards and test methods, a lot of research goes into the process that the standards writing organization goes through, so it benefits the manufacturers to use these documents or reference them rather than develop their own from scratch.
That being said, here are some questions that came up from the column and some answers that might be helpful.
One note: a document I refer to often here is ASTM F 710, Standard Practice for Preparing Concrete Floors to Receive Resilient Flooring. Despite the title, a lot of F710 is referenced in the installation of other floor covering materials besides resilient, to the point that section 1.6 was added, “Although carpet tiles, carpet, wood flooring, coatings films and paints are not specifically intended to be included in the category of resilient floor coverings, the procedures included in this practice may be useful for preparing concrete to receive such finishes.”
Q: Your column said moisture testing is mandatory. Is there an industry standard for the allowable level of moisture in the concrete?
A: ASTM F710 says “All concrete slabs shall be tested for moisture regardless of age or grade level. For the preferred moisture testing methods and limits, consult the written instructions from the floor covering manufacturer, the adhesive manufacturer, the patching/ underlayment manufacturer or combinations thereof.” The methods most commonly referenced are ASTM Standard Test Method F2170 for the relative humidity test and F1869 for the calcium chloride test. However, these standards tell you how to do the test correctly, but don’t set any limits on what is it is acceptable. The allowable limits are almost always set by the manufacturers.
Q: How clean does a slab have to be prior to installing floor coverings?
A: The day before I was writing this I got called on to a jobsite to answer this exact question. In this case, they were preparing to install terrazzo tile using thinset mortar. The previously installed floor covering was glue-down carpet. The general contractor had removed the carpet and scraped the floor, and thought it was acceptable. The tile contractor felt there was too much residue left behind and wanted to scarify the floor to be sure the slab is completely clean. The architect wanted my opinion. I sided with the installer in this case. Even though the carpet adhesive residue was flat, it was till there. They were preparing to install a very expensive tile, so why take chances?
Tile Council of North America (TCNA) addresses this specific situation on tcnatile.com. “The adhesive (often called carpet mastic) used under most glue-down carpet is usually water-soluble and tends to prevent thinset from properly adhering. Should the tile get wet, the mastic may soften, negatively affecting the bond of the tile to the substrate. Thinset manufacturers recommend removing all traces of carpet adhesive if you plan to bond the tile directly to a surface that previously had glue-down carpeting.”
Regardless of the type of floor covering, when you install over adhesive residue, the entire flooring system is only as strong as that adhesive layer. That includes cases where patching or leveling is done over an adhesive layer. It’s a very common practice to remove an old floor covering and “skimcoat” over the adhesive. That may create a smooth substrate, but it’s not strong. If the adhesive lets go or breaks down, the whole floor fails.
Q: Is there a standard for the thickness of a wood subfloor?
A: This is a common question, and varies with the type of flooring. Deflection is the enemy of all floor coverings, so the subfloor needs to be designed so as to minimize movement. When I was growing up in the 1970s, standard subfloor for carpet and resilient was two layers of plywood, total 1 inch thick over joists 16 in. on centre. Three-quarter in. wood floors in the same situation were a nail down over a single layer of ¾ in. plywood. However, building practices have changed so the standards have as well. Now, joist spacing can be up to 24 in. and there are alternates to plywood subflooring such as OSB (Oriented Strand Board) and other materials.
For wood floors, National Wood Flooring Association (NWFA) has standards for subfloor thickness that are based on the joist spacing and what type of wood is being installed (solid or engineered). For example, for 24 in. joist spacing, 7/8 in. thick tongue in groove OSB subfloor is recommended. Visit NWFA.org to learn more.
For tile, it’s most common to have a two layer system with a wood subfloor under cement backerboard that the tile is adhered to. However, it’s not at all that simple. There is a long list of variables based on the thickness of the tile being installed, the spacing of the joists, the use of the space (commercial versus residential), and many other factors that have to be considered. Obviously, the worst thing for a tile floor is any kind of subfloor movement, so this is a critical consideration. The TCNA Handbook is a MUST READ for these considerations so get to TCNATILE.com if you don’t have a copy.
For resilient, ASTM F1482 Standard Practice for Installation and preparation of panel type underlayments to receive resilient flooring. Says only “A combination of the wood subfloor and panel underlayment shall be of double layer construction.” Refer to the resilient flooring manufacturer’s recommendations and local building codes for references to minimum thickness. Flooring failures continue to abound because of failures to go “by the book” — often because people don’t know the book exists. “I’ve always done it this way” is not acceptable as there are so many variables today. Educate yourself!