Concrete moisture meter. What 8% really means.

Someone holds a meter on the slab, it reads under 8, and the membrane goes on. Sometimes that is fine. Sometimes the membrane blisters a month later, and everyone points at the same number.

Three ways to measure, three different numbers

Surface meter

A percent reading from the top of the slab. Good for comparing one spot with another.

In-slab probe

Relative humidity inside the concrete, from a drilled hole. The wet-area standard's guide is 80% or less.

Calcium chloride test

Moisture vapour coming off the slab. The guide is 15 g/m² per 24 hours or less.

A slab can read dry on a surface meter and still be wet inside. The top dries first, and the meter only sees the top.

What the standard actually requires

The requirement is short: the surface is dry, clean and smooth, and its moisture content suits the membrane. The 8% figure is offered in a note as a guide for concrete and toppings, with the words that follow it: or whatever suits the membrane. So the membrane maker's limit comes first.

More on test methods and drying times: moisture in concrete.