A general rule-of-thumb for concrete drying is one month for every one inch of concrete thickness when the slab is 3" thick (under good drying conditions). For each additional inch of slab thickness, dry times increase exponentially, i.e. a 4" slab might take 5 months, an 8" slab 12 months. Add to this wet, west coast weather, and dry times are definitely not on the construction schedule’s side.
One thing is for sure - if the concrete is not dry in time for the scheduled installation start date, a decision will have to be made. The choices are these:
1. Delay installation and wait for the concrete to dry to recommended levels.
2. Install a moisture barrier.
3. Switch the flooring system and or products.
All too often, a fourth option is taken…
Proceed with the flooring installation (against manufacturers’ installation guide lines), waive the warranty and risk moisture related problems later on.
Substrate testing is often pushed onto the flooring contractor who then fails to take the tests according industry standards. Taking tests according to industry standard (ASTM), in a commercial setting, can be a complicated and expensive process.
National standards call for 3 tests for the first 1000 sqft of installation area and 1 test for every 1000 sqft thereafter, with strict rules that govern test placement and test area preparation.
The risk for the flooring contractor is obvious. He cannot protect against multiple tests being spoiled on site by other trades disturbing (spoiling) the tests and if the concrete is too wet and tests fail, who will pay for the next round.
The National Floor Covering Association (NFCA) Floor Covering Reference Manual states:
- Moisture and alkalinity tests shall be conducted by an independent third party testing agency using testing methods and devices in accordance with NFCA requirements and the floor covering manufacturer's recommendations. In multiple story buildings each floor level shall be tested. All test locations shall be marked on As-Built Drawings.
- It shall be the responsibility of the General Contractor and or Building owner to provide and pay for such testing in a timely manner.
What can be done to address these issues?
a. Raise awareness of how sensitive many flooring products are to small amounts of moisture.
b. Understand how long it takes concrete to become dry enough to receive flooring.
c. Create acceptable site conditions early enough so that substrates have a chance to dry.
d. Use a third party testing agency.
e. Include cash allowances for concrete moisture barriers (should they be required).
f. Register for access to the Online version of the NFCA Floor Covering Reference Manual and use the 10 specification guides available in it. Click here for registration information.