What is the Acceptable Moisture Content in a Concrete Slab for a Tile Installation?


Quick question, do you have any indication as to what the 'acceptable' moisture content in the concrete slab should be prior to a thin-set installation?

For your information, this tile will be installed at the bottom of the parkade, about 77FT down street level. Currently the bottom level is very humid, the contractor needs to dry to whatever acceptable level for the tile.


Answer -

The ASTM F1869 Calcium Chloride membrane was the dominate test for years and depending on the tile installation manufacturer they would request a value of anywhere from 3% to 12% per 1,000 sf per 24 hours.  This test has some flaws as the environment conditions can give misleading results.

The most reliable moisture test method is the ASTM F2170 Relative Humidity (RH) test where they insert a probe into the concrete slab about 1/3 down and let it acclimatize for 24 hours or more.  Most of the installation product manufacturers state in their literature for about a 75% to 80% RH, but we find that is not realistic.  We often see 90% or more RH on suspended slabs without problems.   If you are installing over crack isolation membrane moisture is more of concern, but thin-setting directly to properly prepared slab is less of a concern as the moisture can migrate through the grout joints; unless it is an epoxy grout system.   There are epoxy based vapor reduction membranes designed to reduce the moisture that can be installed first.

If there is a continuous source of water due to a high water table or underlying water drainage towards the building you could get negative hydrostatic water pressure that actually exerts a force that can cause tile debonding.   Short of that, if you have continuous moisture you can develop an efflorescence problem.  If you are setting and grouting with epoxy then you might be resistant to a problem as long as there isn’t hydrostatic pressure.

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