Surface preparation is key to laying down a durable, beautiful floor. It’s easy to want to rush through the surface preparation phase. All general contractors and building owners want the floor installed as fast as possible, but speeding through surface prep is a trade-off. You’d be taking short-term time savings at the long-term risk of flooring failures. That’s why one of the biggest mistakes you can make in surface preparation is focusing only on the surface.

You already do many of the essential steps to create the right concrete surface profile for your tile assembly: filling in cracks; cleaning away dirt, dust, chips and other material remnants that create an uneven surface; and removing any surface obstruction that could impact bonding.

But what about what’s lurking below the surface?

There is always moisture in the concrete slab. If the concrete is new, it may not have had enough time to cure and dry. If the concrete is old, some of the damage you cleaned away could have been the sign of a bigger issue: excess moisture. Signs like salt deposits or efflorescence could all be due to moisture issues.


How Does Surface Prep Take Care of Excess Moisture in the Concrete Substrate?

Concrete holds most of its moisture below the surface and that’s the moisture that can kill your work. The only concrete moisture test that gets below the surface is an in-situ relative humidity (RH) test. Consequently, measuring the concrete moisture below the substrate surface with an RH test needs to be part of your surface prep routine.

This excess moisture evaporates over time from the surface of the concrete. That’s why a surface moisture test can’t tell you the real moisture condition of the concrete. It only tests the surface and ignores all moisture waiting to evaporate over time.

When the floor covering goes down, the moisture moves upwards and gets trapped under the low permeable finish installed on the surface.

Concrete moisture contains alkaline salts. With tile, these salts have the potential to transfer to the surface through grout lines, causing unsightly issues. Additionally, with the increased usage of large-format tiles with smaller grout lines, there is less area for excess moisture to escape, potentially getting trapped and causing issues to the entire system.

At times, these effects are more than ugly. They can be dangerous and they might not show up until months after the installation. That’s when you'll be called back in to fix it all -- at your expense.


Moisture Testing is Surface Preparation

The moisture condition of concrete isn’t something you can eyeball, though wouldn’t that be nice. You can’t wait a designated amount of time and then be certain the moisture RH is low enough. No, instead, the only way you can know whether the moisture condition is in the right RH range for your materials is if you measure it.

Independent Floor Testing & Inspection (IFTI) lists the top causes of flooring failure and one is the application of flooring to a concrete subfloor with too much moisture.

IFTI states that, “The only solution to this issue is to invest in adequate concrete moisture testing before applying flooring to the substrate. This allows your team to understand the risks of moisture-related failures, and make a plan to navigate around them.”

If you worry only about surface moisture, you’re missing the bigger picture. The moisture isn’t evenly distributed in unsealed concrete. Most of the moisture is held deep in the slab. It’s this deep moisture below the surface that slowly seeps upwards and gets trapped below your installation. If you only measure the moisture condition on the surface of the concrete, you miss all the moisture waiting underneath it.

The only concrete moisture test that tells you the moisture condition within the concrete is an in-situ RH test. Surface-based moisture tests only tell you what moisture is present on the surface right now. They can’t tell you anything about how much moisture is below.

If you want to do a thorough surface prep, you need to know the concrete’s moisture condition below the surface. It’s the only way to know whether the moisture condition is appropriate for the proper installation of your tile project.

An RH sensor, like the Wagner Meters Rapid RH® L6, is inserted into the concrete to test the moisture condition below the surface. Only the RH test has been scientifically proven to provide an accurate measurement of concrete moisture. An RH moisture test gives you the information you need to ensure that the concrete meets the moisture tolerance of your products.

Each project is different. The conditions of the concrete are unique. The exact tile assembly is unique. And since you can’t base one project off another that makes moisture testing an essential piece of your surface preparation. You don't want to find out months after installation that you have a failure because of excess moisture. How frustrating would that be?