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According to the National Wood Flooring Association, 3/16” in ten feet, 1/8” in six feet.  According to the 2011 TCNA Handbook for Ceramic, Glass, and Stone Tile Installation, 1/4” in 10’, 1/16” in one foot.  For tiles with at least one edge 15” in length, maximum allowable variation is 1/8” in 10 feet from required plane. Did you know that these are the flatness requirements for wood and tile? Not much variance is there.  So, are you missing the following? An opportunity to add more income? Having a better installation because your finished floor will be flat?  Having a customer who will be happy to bring you more business because your installation looks better than the installer who just tried to use a floor patching compound, or build up of thin-set (which by the way should be no more than 1/4” for thin set mortar)? Floor patch and thin set mortar are used too much in my opinion to try to create a flat installation.

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With more and more of the industry seeing the advantages and ease of application of self-leveling underlayments (SLU), the use of these products has increased dramatically over the past few years. Even though the products are fairly easy to use, there are some important installation guidelines that need to be followed for a successful installation. Though most all self-leveling underlayments seem to be installed the same way, there may be a few slight differences with each, so make sure to read the instructions for each manufacturer. So let’s go through the basics of a self-leveling project. You will need to establish whether the substrate needs to be level or flat. In the flooring industry, we typically require a “flat” substrate, so make sure you identify this in your contract. Self-leveling underlayment implies that the product will be level; this has caused some big headaches with contractors who have not covered this at point of sale. Next comes estimating the thickness of the self-leveling product. The use of string lines, lasers, or a straight edge, to check the undulation of the substrate can be used to determine the thickness the SLU needs to be; 1/4” to 3/8” is commonly used.

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Product thickness for manufacturers goes from 1/8” to no thickness restrictions; some require aggregate (pea gravel) for thickness of one inch or more. Estimating the amount of product is important, as you want to give as accurate a bid as possible. There are manufacturers that have a product calculator, or coverage charts on their website to assist in estimating the amount of product necessary.  Now that you have the necessary amount of product, let’s start with a concrete substrate. It will be necessary to make sure that there is no paint, curing compounds, sealers, or any contaminants that may affect the bond of the SLU.

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Scarification and even shot-blasting may be required (Photos 1 and 2). SLU products will flow; what that means is that if there are any cracks, holes in the substrate, or gaps along walls, the material will flow right through. If you are above grade or only doing certain areas, you will need to do a couple of things.  First, use a sealant around the perimeter filling in the voids (Photos 3 and 4). For cracks and holes, use either a floor patching compound or an epoxy, as shown in Photo 5.

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If you are stopping at a doorway, you need to create a dam. A strip of 3/8” rubber sport flooring was used to create the dam for this pour (Photo 6). Also, a self-stick weather strip available at home improvement stores works well; it also comes in different thickness.

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The weather strip works great to dam around drains or a radius. Next is the primer; this is where I have seen too many installers take the shortcut approach. A primer is needed for almost every SLU on the market; why? For a better bond, plain and simple. Here is a photo of a SLU failure due to no primer on the concrete substrate (Photo 7).

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Primers may be rolled with a 3/8 nap roller, broomed, or sprayed and broomed; follow the manufacturer’s recommended procedure, broom application (Photo 8).

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Some manufacturers may require a second coat and a dilution of water to primer depending on type or condition of substrate. Allow primer to dry, typically three hours but this will depend on site conditions. Now for the self-leveling process.

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First of all, self leveling underlayments  don’t completely “self” level themselves. The material must be assisted with a screed bar and then will level within itself.  SLU products can be pumped (Photos 9 and 10) for large pours.

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They can also be mixed in mixing barrels available from SLU manufacturers (Photo 11) or flooring supply/concrete tool distributors.

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The mixing barrels will hold two bags of SLU and can be purchased as a kit with a mixing blade, adjustable screed bar (Photo 12), smoothing trowel, and pre-measured buckets (Photo 13). A heavy duty 1/2” drill is recommended. Prepare a staging area by setting up in a well ventilated area and lay down a tarp.

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If doing a residential pour, you will want to lay down drop cloths in all the walk areas of the home as you will have drips from the barrel. If you are in a commercial pour, make sure you have access to water; if not, you may have to fill a large barrel and cart it to your mixing station.

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I had the opportunity to help a friend in Tucson, Arizona last summer and we did a 200-bag pour in a home. The outside temperature over 100 degrees, water temperature out of the tap, 82 degrees measured with an infrared thermometer. We ended up placing our mix water in two large garbage cans. We filled these with water and ice just to keep the water cool, and ended up using 225 pounds of ice. The cool water is necessary to allow the product time to flow. For those of you in hot regions, don’t forget the ice!

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  How much manpower is needed? For 50 bags or more, I like to use four applicators, one to be on the mixer, two to carry the barrels, and one person using the screed bar and smoothing trowel. If you can use a hand dolly to transport the barrel, the extra hand can follow the screed person with the smoothing trowel. You will also want to have some shoes that you can discard or a pair of non-metallic cleats. So what about pours with control joints, expansion joints, or isolation joints? Manufacturers state that these types of joints need to be honored, meaning that the joint needs to extend up through the SLU. If this is not practical for the type of flooring being installed, contact the manufacturer of the SLU to determine if there are some alternate installation methods that can be utilized.

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If you are going to be using the barrel mix application, depending on the size of the pour, you may want multiple barrels so that you can maintain a constant pour, I recommend at least two barrels. When you purchase the kit, there are pre-measured buckets that you can use. You can also make your own, measure the amount of water required for two bags, pour it in the bucket and mark with a marker. Empty the bucket and cut out a notch. This gives you a pre-measured bucket that will take care of two bags (Photo 14). Make sure everyone has a protective dust mask. Fill the barrel with water first, then add the bags of SLU. Many manufacturers have a recommended mixing time; use a watch as two to three minutes seem a lot longer when mixing (Photos 15 and 16).

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Start the pour in the opposite corner of your exit (Photo 17).

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Set the gauge depth on an adjustable screed bar prior to pouring, and rake across SLU (Photo 18).

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Use a smoothing trowel if recommended (Photo 19). Most SLUs have a working time of 10 to 15 minutes. When finished, the substrate should have a nice smooth sheen (Photo 20). Allow for proper drying time, usually from 6 to 24 hours depending on the type of flooring to be installed, and finish with a nice flat floor.

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When installing over a gypsum based pour, it will be necessary to use a SLU that is compatible with the gypsum. The advantage that the gypsum-based SLUs have is a  higher compressive strength than the actual gypsum that is pumped in as the subfloor; these products are recommended over wood subfloors also. Make sure to follow the manufacturer’s recommended installation guidelines. The biggest difference between the Portland-based SLU and a gypsum-based SLU for the installer is drying time. A gypsum-based SLU requires more drying time, typically 3 days or more before any flooring can be installed over the top. Remember, if using a self-leveling underlayment, you cannot use fasteners; the flooring will need to be either a glue-direct or floating floor only. Thanks to the crew from Richardson flooring, Salt Lake City, Utah, and Ardex for assistance with photos.