How come I didn't get the coverage I thought I was going to get with my adhesive over this concrete slab?
Sam F. Biondo Jr., national technical presenter for Deerfield Beach, Fla.-based MAPEI Corp., answers:
There are a two main reasons you might not get the coverage you expected from an adhesive: Lack of surface prep and using the wrong trowel.
Let's talk about surface prep. The shortest distance between two points is a straight line. If the floor has undulations, unevenness, roughness or texture in it, then the floor will need to be filled in order to be flat. In some cases, contractors fill those gaps with glue, but that's not the right way. You will wind up with too much glue in spots that could end up bleeding through the floor, and you won't get the coverage that you expected. On small jobs, this might not be a problem, but on a large job, if I'm 50 feet short for every bucket and I have 100 buckets, I'm buying a lot more product than what I estimated for the job.
Your first step on the job should be to read the adhesive instructions so you understand the required concrete surface profile (CSP) for that adhesive. CSP is determined by the International Concrete Repair Institute Inc. (ICRI), and it basically describes the slab's roughness. Most adhesives require a CSP of 1 or 2, which is about the texture of an average sidewalk.
Using the wrong trowel size or profile can also cause issues with your spread rate. The directions from the adhesive manufacturer will recommend the correct trowel. Also read the flooring manufacturer's directions so you understand what contact is required between the back of the wood floor and the adhesive. And remember that some adhesives are designed to be multifunctional adhesives—they are a moisture membrane, a sound membrane and an adhesive—so you have to understand what that specific adhesive manufacturer wants in terms of coverage in order to maintain the warranty.