Vapor Barrier needed under Concrete Slab?


Vapor Barriers under Concrete Slab - We are undertaking a large tile project on Florida and need your advice. We are laying concrete in a large enclosed area by building up the heighth with dirt/sand, then compressing the soil,digging/building footers, spraying for termites, then laying ribar.

Qs: Do we need a moisture barrier? Would the moisture barrier be applied after termite but before ribar? While the concrete will be inside we do intend to have non load bearing wall installed over part. will a 4" depth of concerete be sufficient? How long should be allow the concrete to cure before installing cermaic tile? Any pointers on ceramic tile installation?
This project is on an Island so moisture IS an issue! Thanks!


ANSWER - Yes, you always need a vapor retarder (barrier) per ASTM E1643.  The vapor retarder should continue up the sides of the foundation to prevent lateral migration of moisture.  Sometimes a sand bed can be applied over the retarder and sometimes not depending on the situation.

The thickness of the concrete depends on the intended use and condition of the soil below it, but generally 4" thickness is ok for light traffic residential applications over a structurally sound landscape.

The general rule is to wait 28 days or curing before installing tile, but it depends on the recommendations of the installation products you will be using.

We do provide a complimentary copy of some general installation guidelines for owners to use to help make sure they get a successful tile installation.  Visit our website at and go to this Expert Answer.

As a consultant CTaSC does provide customized installation specifications and guidelines, and provides quality control services during the installation. Cost is based on the current CTaSC fee schedule.

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