Self-Leveling Underlayments

Self-Leveling underlayments have been used in the flooring industry for many years now. Flooring contractors have found self-leveling underlayments (SLUs) to be convenient, time saving and cost efficient. Their use, however, have not been without problems. Most problems seem to relate to the substrate or not understanding the product’s requirements or characteristics.

Substrates
A wide variety of substrates exist both on residential and commercial applications. Plywood, stripwood, vinyl, ceramic, concrete, lightweight concrete, gypsum, metal, terrazzo, and epoxy. The list can almost be endless. We will deal with the common substrates and the issues that surround them.

Wood
The requirements for wooden substrates are well documented. Along with being tightly secured and having proper support, the board being used should be an exterior grade. Most interior grade, OSB, and chip board generally do not have the water resistance needed to be used for a self-leveling application. The water attacks the glue holding the board together causing the board to warp. Once you are satisfied with the substrate, follow the manufacturer’s instructions.

Due to their rigidity, Portland cement-based SLUs tend to require metal lathe or plastic mesh to help secure them to the wood and to provide additional flexural strength. If movement occurs, the SLU may crack but the lathe will help hold it in place.

Gypsum-based materials are a popular option for use over wood. Gypsum materials offer good fire resistance and sound deadening. Standard gypsum materials usually require a minimum thickness of ¾” – at proper water demand these materials are flowable but do not offer the same heal qualities as standard SLU’s. Compressive strengths can vary between 2,000 – 4,000 psi.

There are some gypsum-based, however, that are designed for thin applications and offer the same or better workability and heal as their cement-based counterparts. Due to the additives used, these materials offer greater flexural strength and flexibility than cement-based SLUs and therefore may be applied in thinner applications over wood without the installation of lathe or mesh.

Concrete
Many applications deal with on-grade concrete – retail, residential, institutional, etc. For new concrete, most manufacturers recommend that it be at least 28 days old. The concern here is with drying shrinkage of the concrete. As the concrete moves and shrinks cracks will telegraph into the leveling material. Moisture is always a concern both on new and existing concrete and it should be tested. The calcium chloride test (ASTM F1869) or relative humidity test (ASTM F2170) are most often referenced. If moisture treatment is needed, then leveling can occur over the moisture treatment system.

If moisture is not a concern, then select the product that best fits the situation. Many will select a cement-based SLU in these cases. For most cement-based SLUs sufficient texture of a clean, sound concrete is required. Cement-based SLU’s tend to be reactive, generate heat and create tension on the substrate. Therefore most manufacturers will recommend mechanical preparation to open the pores of the concrete and provide a sufficient texture to ensure bond to a clean sound substrate. Although many abrasive methods exist, shotblasting has proven to be the most cost efficient and readily available. Depending upon substrate conditions, shotblasting may be combined with other methods to achieve the desired substrate. Shotblasting, as opposed to other methods, leaves the concrete with open pores allowing penetration of primers to provide a secure bond of the leveling system.

Chemical cleaning / preparation can be troublesome. Acid etching is difficult to control and requires neutralizing and testing of the concrete afterwards. The acid actually attacks the cement paste and may weaken the matrix causing problems down the road. Sweeping compounds, which are widely used, can leave oily, waxy film on the surface which will act as a bond breaker. Chemical cleaners, degreasers, and adhesive removers can also contaminate the substrate. In most cases the treatment calls for a thorough rinse which can drive the material (oil, grease, adhesive residue, or stripping agent) into the pores of the porous concrete.

Any material that may prevent bond, oil, grease, curing compounds, sealers (topical or penetrating), adhesive residue must be mechanically removed. We are all called into renovation work and the re-use of existing buildings. Be aware of the condition of these slabs along with the work or processes that occurred on them. Some manufacturing facilities will use various chemicals in their process which can adversely affect the concrete. Printing operations use inks and solvents, rubber processing use silicones, machine shops use cutting solvents, bottling facilities’ products are acidic all penetrate the concrete causing bonding issues.

Elevated slabs may require slightly different treatment. Elevated concrete slabs will tend to be more porous and lighter (less dense) than standard on-grade concrete. Standard concrete has a density of 140 – 150 lbs. per cubic foot, where as lightweight concrete may have a density of 70 pounds per cubic foot. When leveling elevated slabs the leveling product should be less reactive and more flexible (higher flexural strength). Very reactive materials can pull apart slabs with low tensile strength and little integrity. Deflection is always a concern on elevated slabs and very rigid products can crack and de-bond. Also, if additional weight to the structure is a concern, lightweight SLU’ are now available that offer very good compressive strengths.

Gypsum
Gypsum concrete, which offers qualities previously mentioned, has been used for many years. Although gypsum concrete has been very popular for residential buildings it has been used in many different applications. Gypsum concrete has been used over wood, pre-cast and concrete to provide a lighter weight, economical finish with sound deadening and fire resistance. In order to smooth or level these floors a compatible material is required. First, remove any unsound or loose material. Use a gypsum-based material that is designed for thin applications. These materials are polymer modified and will typically offer the desired placement characteristics like flow and heal but are also less reactive and more flexible than cement-based products. Good compressive and flexural strengths will help provide a lasting, durable repair.

Other
As previously mentioned, the list of substrate types is almost endless. In most cases the keys to success are a clean, well bonded, and properly supported substrate. If you are relying on another material’s bond to the substrate, be cautious and critical. Remove anything that may be loose which will affect your bond and your floor’s performance. In bonding to these dense substrates, a less reactive material requires less bond, and may be a good option. Special primers may also be used to help achieve a secure bond. Check with the manufacture on their recommendations for dealing with these substrates.

Material Selection
Selecting the right material for the application is very critical to the success of the job. It is important to thoroughly investigate the job and get all the facts.

Is the goal to simply smooth the floor or is their a levelness specification that is required? There is a large difference and you should be paid appropriately for the work done. Sufficient space does not exist here for full details on leveling, however, proper measurement and gridding the floor are necessary to achieve desired results.

What floor covering will be installed? What is the maximum thickness required? What is the average depth to be poured? How quickly does the floor need to be installed? What is the substrate? How large is the job? Is the job one large room or a lot of small individual rooms? Will traffic occur over the material before floor covering? These types of questions will help get you started.