CSLB General Building (B)Site Work, Foundations and ConcreteEasy
A contractor is placing a concrete slab-on-grade over native soil in a residential slab pour. What is the primary purpose of installing a polyethylene vapor barrier beneath the slab?
- ATo provide additional reinforcement against cracking
- BTo reduce moisture migration from the soil into the slab
- CTo act as a thermal insulator for the foundation
- DTo increase the compressive strength of the concrete
Show answer & explanationAnswer & explanation
Correct answer: B. To reduce moisture migration from the soil into the slab
A vapor barrier (typically 6-10 mil polyethylene) is placed under slabs to prevent ground moisture from migrating up through the concrete, which can damage flooring, coatings, and cause mold issues.
Why the other options are wrong
- A. Reinforcement against cracking is provided by rebar or mesh, not the vapor barrier.
- C. Thermal insulation is a separate function performed by rigid foam insulation, not poly sheeting.
- D. Vapor barriers do not affect concrete's compressive strength.
Vapor Barrier
A polyethylene sheet placed under a concrete slab to block moisture from the soil from entering the slab.
- Typically 6-10 mil polyethylene sheeting
- Placed directly beneath the slab, often over a gravel base
- Prevents moisture damage to flooring and finishes above
Memory trick: Vapor barrier = umbrella for the slab, blocking ground moisture.