Installing a floor is a breeze-if you don’t care about how the floor looks and performs over the long term. But if you want it exactly right (as very professional installer who takes pride in his work should) and you want a floor that will withstand traffic over the long haul, remember this: There is no detail too small for your attention and no problem to big for you to solve Follow the manufacturer's instructions to the letter and don’t let anyone compromise your work. With this in mind, I wanted to use this opportunity to field some of the questions sent to me by NFT readers. The questions reflect a wide range of concerns and a number of issues that pop up on a regular basis.    



We store our underlayment in a heated warehouse. Do we still have to acclimate on the job site?

Not necessarily. If you keep the underlayment in the skid, yes it should be acclimated.  If the underlayment is taken out of the skid and stored on edge it will acclimate to the warehouse conditions quickly.  If you leave the underlayment in the skid it can take 30 – 45 days to thoroughly acclimate.



We are located in an area that has high summer humidity. Is it possible to ever get concrete dry enough (3 pounds per 1,000 s/f per 24 hours) for a vinyl-backed sheet product?

A lot depends upon the quality of concrete that is being placed. If it has a water-to-cement ratio above 0.50 it will be difficult to get to 3 pounds. Concrete mixes above 0.55 water-to-cement ratio will be extremely difficult and if it is a 0.60 water-to-cement ratio the answer is “never.”  Remember, the higher the water-to-cement ratio the more moisture there is in the mix. Water is non-compressible and when it evaporates it will leave larger capillaries that will allow humidity to move in and out of the concrete..

Why do adhesives dry faster over a Portland-based patch or self-leveling underlayment?

It is simple chemistry. Portland –based patch and underlayment manufacturers use an accelerator in their products. This accelerator is water starved and will rapidly absorb the water in the adhesive being applied over its surface. This causes the adhesive to dry much faster than normal. The faster water is removed from the adhesive the more detrimental it is to the adhesive, leaching the plasticizers out of the adhesive, causing it to crystallize.



What are your thoughts about installing over gypsum concrete?

Like it or not, this is an underlayment that is widely used. Even so, resilient manufacturers have a lot of challenges to cope with when gypsum is used. including:

• The tendency of applicators to over water the gypsum product.

• Extreme absorption. This weakens the adhesive by drying the adhesive too fast.

• High sensitivity to moisture. Will expand and soften when exposed to moisture.

When confronted with an installation over gypsum concrete make sure the compressive strength indicated by the underlayment manufacturer is being met. Also, consider using a primer to harden the surface and slow the absorbency of the gypsum concrete.



What causes VCT tile to gap when installed over concrete, especially in corridors?

Moisture, or more specifically excess moisture. When you see gapping there’s a good chance the tile was installed over concrete with a moisture content over the limit recommended. When the moisture vapor emissions are above the limit recommended for installation of VCT tile (normally in the 6 – 8 lbs. range) the adhesive will remain soft. This will allow the tile to slide creating the gaps you are seeing. When VCT tile is installed over a damp substrate the tile will expand slightly.  Once the concrete starts to dry the VCT tile will revert back to its original size. Unfortunately the adhesive is the last to totally cure and harden.



What are your thoughts on calcium chloride testing?

It can be a tricky business. Calcium chloride testing has many variables that affect the outcome of the test, so the results are marginal at best. My  guess is that about 80% of the tests are done incorrectly. The calcium chloride test measures only the top 1/2 – 3/4” of the concrete. If the slab is not in equilibrium the test results will yield a false positive.  I think we will see the use of the hygrometer (in situ) probe test in the near future, with greater accuracy. The hygrometer probe measures the internal relative humidity at the apex of the drying curve and is not as affected by temperature, humidity or slab equilibrium.



Does lightweight concrete dry as fast as normal weight concrete?

No, it takes about twice as long for lightweight concrete to dry compared to normal weight concrete.  Why? The reason is the lightweight aggregate is extremely porous and holds moisture for a longer period of time and is more difficult to get moisture to migrate out.



When installing over a slab that has been treated with a moisture remediation treatment and skim coated with a cementitious underlayment coating is the substrate porous or non-porous?

Most cementitious coatings are nonporous until they reach a minimum thickness of 1/8” and for some manufacturers 1/4” is the minimum. When installing over a slab that has had a moisture remediation coating, check with the manufacturer of the underlayment to see at what thickness their product is porous.  If in doubt treat the substrate as though it is non-porous.



I have had several instances where the seam sealer I have been using has turned “milky.”  What causes  this?

In most cases it is the moisture that causes seam sealer to turn white. Usually this happens when you are wiping down the seam with a wet rag prior to the sealing operation.





How deep should a heat welded seam in a sheet vinyl be grooved?

It depends on the type of material being welded.  The rule of thumb is half to two-thirds the thickness of the wear surface.  This is true of vinyl backed material as well.