The Dry Floor Begins Beneath The Concrete

Ground Moisture Has Two Routes:

A concrete floor may look solid, but it contains tiny pores. Liquid water in the soil can move through these spaces by capillary action. This is similar to water rising through a paper towel, though the movement through concrete is much slower.

Water can also enter as vapor. Damp soil continuously releases water vapor, which may travel through a slab toward the drier indoor air. Liquid movement and vapor movement are different processes, so a durable floor assembly must control both.

Stone Interrupts The Liquid Path:

Builders commonly place a layer of clean, coarse aggregate over compacted soil before pouring a slab. The large spaces between the stones break the network of tiny passages that would otherwise help liquid water climb upward.

The aggregate must be suitable for drainage and free of excess fine particles. Sand, silt, and stone dust can fill the open spaces and create new paths for moisture. The required material and depth depend on the building design, soil, and local rules.

Plastic Slows The Vapor:

A continuous vapor retarder is usually installed above the aggregate and directly beneath the concrete. This tough plastic sheet limits the amount of water vapor that can diffuse from the ground into the slab and the rooms above it.

The membrane should not be confused with the stone layer. Aggregate mainly interrupts liquid-water movement and provides drainage space. The vapor retarder controls vapor diffusion. Used together, they form a stronger moisture-control system than either layer provides alone.

Small Openings Can Defeat The System:

Seams in the membrane must be overlapped and sealed according to the project specifications. Tears should be repaired before concrete is placed. Pipes, posts, and other penetrations also need careful sealing because water vapor can pass through small gaps.

Workers must protect the sheet while installing reinforcement and moving equipment. A puncture may be difficult to find after the pour. Planning pipe locations and using stable supports reduces foot traffic and sharp contact on the membrane.

A Healthy Slab Needs More Than One Detail:

Under-slab protection cannot replace good site drainage. The ground should direct rain away from the foundation, while gutters, drains, and waterproofing manage water around the building. Plumbing leaks and condensation require separate solutions.

For an existing damp floor, the first step is finding the moisture source. White mineral deposits, loose flooring, failed adhesive, or a musty smell may provide clues, but they do not identify the cause by themselves. A qualified building professional can test the slab and recommend a repair that matches the problem.