What level of flatness ...?

Received an email with an interesting technical question:

What level of flatness do we need in a concrete slab for a ceramic tile installation – using the F-number system?

Note:

Measuring concrete levelness and flatness using the F-number system for ceramic tile & stone installations may be over and above what is needed. Typically measuring FF/FL tolerances would be for projects such as warehouses where forklifts or machinery requires tight concrete levelness and flatness tolerances.

And our Laticrete Technical Support reply:

Please note that the American Concrete Institute's (ACI) Floor Flatness (FF) / Floor Levelness (FL) are typically used by concrete specifiers / masons to pour their floors to specific tolerances.

Typically, the following are the FF #'s which roughly correlate to the respective floor tolerances:

FF 25 is roughly equivalent to a single ± 1/4" defect in 10'
FF 50 is roughly equivalent to a single ± 1/8" defect in 10'
FF 100 is roughly equivalent to a single ± 1/16" defect in 10'

Tile Council of North America / ANSI Standards typically require substrates receiving thin set applications to be within 1/4" in 10' (and 1/16” in 1’-0”). For large format tiles and running bond patterns, a tighter tolerance of 1/8" in 10' may be necessary.

Acid Washing of Concrete Slabs

There is a terrible misconception that "Acid Washing" will remove sealers or curing compounds from concrete slabs.

Acid will not clean or remove grease and oil and acid will not remove many types of curing compounds and sealers.

One way to remove grease and oil is with a strong detergent (TSP found at paint or hardware store) or floor stripper (this is available from janitor supply companies).

Another way to remove concrete sealers & curing compounds is to mechanically remove them by grinding, sanding, scarifying, sand blasting or high pressure water blasting.

We DO NOT recommend or approve "Acid Washing" as a method to prepare treated or sealed concrete prior to thin bed installation of tile.

Many problems can arise when the contractor tries to 'Acid Wash' a concrete slab to remove curing compounds, grease or oil before installing tile and stone.

We do NOT give an approval or the IMPRESSION of an approval for this 'Wash the concrete slab with Acid' technique. We do NOT recommend "safe" acids such as Sulfamic acid for removing grease, oil or curing compounds!

Note: Occasionally a contractor asks about using Acid to "etch" a concrete slab before installing tilework to "get a better bond." Do NOT recommend this procedure! It will etch the concrete surface. Do you really want to install/bond tile over an etched & weakened, concrete surface? NO!

Not only does the acid weaken the surface of the etched concrete, the acid will remain within the concrete ( neutralizing is never 100% - who on a jobsite does a thorough neutralization??), and weaken any cementious adhesive applied over this substrate.