Testing a Concrete Slab for Moisture

Most people think of a concrete slab as solid and dry, a sort of stone. It is closer to a hard sponge. Concrete is full of fine pores, and it sits directly on soil that holds water, so moisture is always making its way up through it and leaving from the top surface as vapor.

On a bare floor you never notice. The vapor escapes into the air and that is the end of it. The trouble starts when you put a dense coating over the top.

Why does moisture in concrete cause epoxy to fail?

Epoxy forms a tight film that vapor cannot pass through readily. When moisture keeps rising from below, it collects at the underside of the coating, builds pressure and carries salts from the concrete with it. The result is blisters, a hazy or white bloom under the film, and eventually sections that lift away from the slab entirely.

This is a different failure from the one caused by poor surface preparation, and it is more frustrating, because the work can be done perfectly and still fail if the slab was too wet to coat.

Which slabs are most at risk

Some floors are far more likely to have a moisture problem than others.

Older slabs. Today a sheet of plastic called a vapor retarder normally goes under a new slab before it is poured. Many slabs from the post-war decades may have been poured without one, and there is no way to tell from above.

Garages cut into a slope. On hillside lots in Bonita and Spring Valley, a garage is often built into the hill, with soil banked against one or more walls. Water draining down the slope meets the back of the garage.

Planters and irrigation against the garage wall. A flowerbed watered every morning on the other side of a garage wall is a steady supply of moisture into the slab edge.

Low ground near the coast and the bay. In low-lying parts of Imperial Beach, near the estuary and the bay, groundwater can sit close to the surface, and a slab there may stay damp from below for much of the year.

The signs worth noticing

You can often spot a moisture issue without any equipment:

• A white, powdery or crusty deposit on the floor, especially along walls or near the door. That is efflorescence, salts left behind as water evaporates.
• Dark patches that appear and fade with the weather or the irrigation schedule.
• Cardboard boxes that are soft or stained on the bottom, or tools that rust where they sit on the floor.
• An old coating that has blistered or turned white underneath.

Any of these is a reason to test before coating, not a reason to give up on a coated floor.

How a slab is tested

There are several methods, and they answer slightly different questions.

1. The plastic sheet test. A square of clear plastic is taped tightly to the floor and left for most of a day. If droplets form underneath or the concrete darkens, moisture is coming through. It is a yes-or-no indicator, useful as a first look, but it does not give a number to compare with a coating’s requirement.
2. The calcium chloride test. A small dish of calcium chloride is sealed under a dome on a cleaned area of the floor for a few days, then weighed. The gain tells you how much vapor the slab is releasing over that period.
3. Relative humidity probes. Holes are drilled into the slab to a set depth, sensors are sealed in, and after they settle they report the humidity inside the concrete. Its strength is that it reads the slab itself rather than just its surface.
4. Handheld moisture meters. These are useful for mapping wetter and drier areas across a floor, but they are not a pass or fail on their own.

Whichever method is used, the result is compared with the limit stated on the data sheet of the coating you have chosen. Different systems tolerate different amounts of moisture, so there is no single number that applies to every floor.

Timing matters as well. A slab tested in late summer after months without rain may read differently from the same slab in February. A careful contractor keeps that in mind and tells you where the reading sits relative to the limit, not just whether it passed.

If the slab does not pass

A wet reading is information, not a dead end. The options usually include:

• Fixing the source first. Redirecting a downspout, changing the irrigation next to the garage, or regrading soil away from the wall can make a large difference, and it often belongs to a landscaper or another trade.
• A moisture-tolerant primer. Some floor systems include a primer designed to go under the epoxy on slabs with higher vapor emission.
• A breathable alternative. For some floors, a penetrating sealer or a breathable coating is the honest recommendation, because it protects the concrete without trapping vapor.

What should not happen is coating over a wet slab and hoping. That is how a floor that looked perfect in the first month ends up blistered by the end of the first winter.

What Ebenezer does

Ebenezer looks at the slab at the estimate and tells you what he sees, including any signs of moisture. If a test is warranted, he explains which one and why before the coating date is set, and if the result changes the plan, he tells you the steps and how it affects the work before going further.

For the full picture of how a floor is coated, see the epoxy floors service, or read about the slopes and older housing on the Spring Valley page. Then ask for a free estimate, and he will tell you whether your slab is ready for a coating or what it needs first.