First make sure the concrete is sound. A dusty concrete substrate should be vacuumed and then damp mopped. Priming is a good idea.


Although surface prep does not receive a lot of attention, it can make the job. I have often heard that the best floor covering can not compensate for inferior substrate prep. There are a variety of patching products on the market that help prepare the substrate. Defining which to use, how to use it, and when will dictate your success.

As a way of discussing patching, I have pulled together the most often asked questions along with some of the most common problems in the field.

What options are there when working on a porous substrate?

Porous substrates quickly pull the moisture out of the patch reducing working time and workability. In some cases the moisture is pulled so quickly that the patch does not develop the desired strength and hardness. There are a couple of ways to provide the expected workability and working time. One option is to prime. Check with the manufacturer to use a compatible primer. The primer will fill the pores of the substrate keeping the water in the patch. The primer will also increase adhesion. Another option is to pre-dampen the substrate. Use cool, clean water to dampen the substrate. Do not, however, apply the patch over standing water. The substrate’s pores are now filled with water and will allow the moisture to stay in the patch.

Some moisture reduction products include a sand broadcast. Here a self- leveling product is pumped to cover the sand broadcast epoxy moisture barrier to prepare for floor covering.

How does sealing or priming the seams in a wooden subfloor help the installation?

The edges of the wood are open areas. It allows the moisture from the patch to absorb into the wood, especially where the wood has been cut, increasing the chance of seam telegraphing by causing slight expansion. Priming or sealing that edge with a compatible material will restrict the moisture from the patch from entering into the wood.

Should patch be applied to a concrete substrate when it is emitting excessive moisture? If a moisture reduction barrier is used, when should the patch be applied?

Underlayment patch should not be applied to a substrate expected of excessive moisture. In fact, if the building is not enclosed and the environment is not acclimated, patching should not be done. Most patch manufacturers will abide by the floor covering manufacturers’ restrictions on moisture. Non-breathable coverings will cause excess moisture to collect under its surface affecting the patch and the adhesive. If a moisture reduction barrier is used, underlayment patch is applied over the barrier. Underlayment patches (cement or gypsum based) are not designed to be applied in a wet environment. Check with the individual manufacturer of the moisture reduction barrier for compatibilities with the patch.

What is the difference between re-tempering and re-whipping?

Re-tempering refers to the practice of adding more water to the patch mix once the product starts to set and/or loses workability. Regardless of the type of patch, when additional water is added at this time, it drastically weakens the product. Strength is greatly reduced, permeability increases and dusting and cracking are probable.

Re-whipping, on the other hand, is simply referring to the practice of re-mixing (without additional water) the patch as it loses workability. Some cement-based patches can be given more life when they are re-mixed. Re-mixing does not negatively affect the strength or performance of the patch.

A cement-based patch is forced into the properly spaced seam.

Can underlayment patches be force dried?

Some patches may be and some may not. It is never a good idea to force dry cement-based products. Robbing the product of water at this early stage will lead to shrinkage, cracking, dusting, increased permeability and an overall weaker product.

Gypsum products, on the other hand, may be force dried once they have hardened. Fans, or heated air, may be blown directly across the surface of the patch to help dry the gypsum product without a change in performance.   What can cause underlayment joint telegraphing? There are many factors that can lead to joint telegraphing, including:
  • Inadequate acclimation
  • Unprotected crawl spaces
  • Excessive rainfall
  • Panel direction/installation
  • Panel underlayment joints
  • Fasteners: length + number
  • Green floor joist/Crowned or concave joist
  • Basement moisture
  • Improper underlayment board choice
There are a few common mistakes regarding the use of underlayment patch and joint or seam telegraphing. The first is applying the floor covering before the patch has dried. Moisture from the patch can cause problems with the performance and bond of the adhesive. The adhesive does not obtain a proper bond and a ridge soon appears. In this instance the ridge is hollow. Injection of adhesive into these hollow areas may attach this ridge.

It is recommended that the boards are left with a slight gap (1/32” or thickness of matchbook cover). The patch is then forced into the seam, anchoring it between the boards and then creating a smooth surface. The mistake is often made when the boards are butted tightly together so that there is no space to place the patch. The patch is then installed over the seam in a thin layer to create a smooth transition. As the boards move, normal expansion and contraction, the patch is forced up (because it is not anchored) creating a ridge. This ridge is not hollow, instead it is solid, because of the patch that has let loose. It is unlikely that injection of adhesive will correct this problem.

Mold, Mildew and Staining

Although it doesn’t seem as much a concern as it once was, discoloration due to mold/mildew still causes problems and raises questions. First, keep in mind that mold needs water, nutrients, favorable temperatures, and oxygen to grow. All four are needed, and are present almost everywhere. Nutrients include items like wood, paints, and adhesives. Patches contain latexes or other materials that are a food source, but a cement patch that is highly alkaline fights mold/mildew growth, where as gypsum has no natural or inherent protection and requires mildewcides to protect them. If using gypsum patch in an area where mold growth is a concern, make sure it contains the proper mildewcides. For reference, look to ASTM G3273 for proper testing to ensure protection, or ask the manufacturer.

Patching products are often viewed as either being cement based or gypsum based. Gypsum-based patches have been on the market for quite some time – 50 plus years. There are many reasons why gypsum products have remained popular for so many years. They are low in alkalinity, are chemically less reactive than cement products, and fast drying in thin applications. Gypsum products are typically less complex than their cement-based counterparts. There are fewer ingredients, meaning that they are generally more consistent in workability and performance. There is also less of a chance of problems occurring.

Also keep in mind that as soon as gypsum products are dry, they achieve their maximum strength, where as, cement products continue to cure and develop strength after they have dried for days or even weeks.

Cement products are alkaline, and as previously stated, this gives them a natural way to defense against mold and mildew. Gypsum products require an additive to protect against mold and mildew. There are many different types of cement based products - care must be taken to know the product’s capabilities and performance characteristics. Newer technology incorporates a blend of cements that provide some unique benefits such as internal drying (or drying from inside out) and improved finishability.



There are a range of products in each category. It is wise to use the products for their intended purposes. It is also wise to use additives when suggested and to use the additives manufactured for that specific product.