In preparing concrete subfloors to receive new floor covering materials, all good mechanics know how to flash patch and skim coat areas to provide the flat, smooth surface required to install the new flooring. However, when it comes to the variety of construction joints and cracks in the subfloor, there are a number of decisions to be made as to which ones can be successfully filled and which must never be patched. The following is intended to clarify the types of cracks and joints you may run into on a project and a recommended way to handle them.

Types of Joints: For all intents and purposes, there are only two types of joints in concrete subfloors - those that move and those that don’t! While perhaps a bit overly simplistic, the reality is that if a joint (or crack) is dormant, it can and should be filled. If it moves or is designed to accommodate movement, it should never be filled with a cementitious patching compound. A more detailed description of the types of joints you encounter on a concrete slab, and how to deal with them, is provided below.

Expansion or Isolation joints: These joints are typically the most easy to identify in a concrete slab. Expansion joints are long, straight joints between sections of concrete slab that have an expansion (flexible) material present in the joint. These joints are all the way through the concrete slab sections and are designed to allow for differential expansion and contraction of adjacent concrete pours. Isolation joints are also easy to identify in a concrete slab. These are found around columns in a concrete slab and are configured in the shape of a “diamond” around the column. Their purpose is to also allow for differential movement because the concrete into which the column is set is a completely different pour than that of the concrete floor slab.

Both of the above types of joints are moving or active joints designed into the building itself. Never install any cementitious topping or patching product over a joint that is designed to allow differential movement between concrete pours. HONOR ALL MOVING JOINTS IN THE SLAB UP THROUGH THE UNDERLAYMENT OR TOPPING.

Cracks: There are also two types of cracks that we typically encounter on concrete slabs. Control joints, also known as saw-cuts, are cut into the freshly placed concrete to provide a place for the concrete to crack in a straight line as it shrinks and cures. The second type of crack is a random crack that develops where a saw-cut should have been, or cracking that occurs as a result of settlement or deflection in the slab itself.

If cracking is active, structural defects must be remedied before attempting to repair the cracking. Consult with an engineer on the project or request the services of a structural concrete repair professional to deal with cracking repair methods such as gravity filling small cracks (1/4” maximum width) with epoxy. If the crack is larger or extends entirely through the concrete slab, the use of an epoxy injection following manufacturer’s instructions is often recommended.

Dormant hairline cracks (those that are 1/32” wide or less) typically do not pose a threat to the performance of the underlayment, topping or floor covering to be installed.

To maximize your success when dealing with concrete floors, follow these two rules: Joints in a concrete slab that are designed to allow for movement must never be filled with a cementitious patching material. Covering these will result in an underlayment and/or a flooring failure. Dormant cracks greater than a hairline should be cleaned out and filled with a cementitious patching material to minimize their potential to telegraph into your floor covering selection.