What is a Moisture Barrier?

Moisture barriers are horizontal or vertical barriers that prevent water from moving from one area to another. Some objects, such as driveways, can act as moisture barriers, although they are designed for other purposes. Purpose built moisture barriers are usually membranes or sheets of plastic, rubber, or similar compounds. In residential construction, moisture barriers are most commonly used to stabilize the water content of soils under foundations, which helps to reduce foundation movement. Another common use is to waterproof basements.

How do Moisture Barriers work?

Moisture barriers work by keeping water where it is wanted or keeping water out of areas where it is not wanted. For residential foundations resting on expansive soils (the kind of soils that move up and down and move houses), moisture barriers work by keeping excess water from flowing under foundations, where the water will cause soils to expand, resulting in a foundation moving upwards. At the same time, moisture barriers act to trap water that is already in the soils under a foundation. When water is trapped, soils can not dry out and shrink. This prevents foundations from sinking because shrinking soils cause foundations to go down.

What is a Root Barrier?

A root barrier is a physical barrier that roots can not cross. Metal edging strips around gardens act as root barriers, keeping grass out of flower beds.

How do Root Barriers work?

Root barriers work by stopping roots from growing into areas where roots are not wanted. Roots can be stopped physically, by barriers such as plastic or rubber sheets, or by chemicals, such as copper sulphate. Root barriers must extend far enough into the ground to intercept any growing roots. For homes, root barriers must go at least 12 inches below the bottom of a foundation, which is typically at least 24 inches deep.

How do Root Barriers differ from Moisture Barriers?

Root barriers can be porous and allow the passage of water. So a root barrier will not necessarily act as a moisture barrier. Moisture barriers prevent water from passing, and stop all roots too. Moisture barriers also act as root barriers.