Steps for How to Level a Subfloor the Right Way

By Jeffrey B. Johnson

Self-levelling concrete compounds are widely used by building professionals in flooring projects. For contractors, it can be tempting to use these products beyond their design specifications for below-grade applications, where they need to remediate floor depths that exceed the stated maximum limitations of self-levelling compounds. It’s always best to resist this temptation.

Applying multiple layers of a self-levelling compound at its maximum thickness level until the desired depth has been achieved might seem like a reasonable approach. However, it can be prohibitively expensive, in terms of time, effort and material cost.

Doing this may also create a variety of future structural problems, as self-levelling compounds add roughly 12 to 14 pounds of weight per square foot at a depth of one-inch.

If, for example, you need to fill a depth of six inches, it could require several layers of self-levelling compound, which would not only be laborious and time-consuming but also add more than 70 pounds per square foot.

Even in a small area, this additional weight load could be too much for the substrate to bear and lead to a structural problem down the road. As such, this approach amounts to a fool’s errand because even if it can attain a level surface, it would never be approved by a structural engineer.

To achieve deep-fill applications without the massive load addition, the best solution is to use high-density, rigid insulation boards secured with construction panel adhesive to fill most of the area, and then finish with a single pour of a self-levelling underlayment.

High-density Foam Insulation Board Flooring Applications
Insulating foam boards of various types are manufactured in different thicknesses, ranging from a half-inch to two inches. These products are often used in conjunction with a vapour barrier to insulate foundation walls but they also have great applications for floors.

XPS insulation boards are made through a process of continuous extrusion that produces a closed cell structure. EPS, in contrast, is made by expanding spherical beads in a mould and then fusing the beads together through the application of heat and pressure. Both products can be used to overcome low spots and excessive flooring depths. But before choosing one over the other, building professionals should first familiarize themselves with all the technical data available regarding each type of foam board.

Before placing foam boards for deep subfloor levelling applications, make sure the plywood substrate is clean and the room is free from dust, dirt, debris and excess moisture. Also, fill any floor joist gaps or voids with a patching compound to prevent the self-levelling compound from leaking out.

The processes for installing EPS or XPS foam boards are quite similar but there is one important difference. For EPS foam board, drill one-inch diameter holes, beginning in the centre and moving out at right angles every 12 inches throughout the foam board. These holes will be filled with the self-levelling compound, creating support pillars for the entire floor.

Insulation boards should be installed to reach one-inch below the desired finishing product.

For maximum stability and ease of installation, use a high-quality construction panel adhesive to install and secure your foam board of choice to the substrate.

Adhere foam tape around the edges of the wall columns, supports and equipment, as well as a bead of caulk along the walls and at every seam to keep the self-levelling underlayment from flowing into other areas.

Apply a primer to the surface of the foam board to enhance the durability and adhesion of the self-levelling underlayment.

Once the foam board panels have been properly installed, sealed and primed, any self-levelling underlayment can be used over them.

Jeffrey B. Johnson is the business manager for Mapei’s floor covering installation systems line. He brings to the industry more than 30 years of experience in the development and marketing of floor covering installation products. Founded in 1937, Mapei is one of the world’s leading manufacturers of chemical products for the building industry. The company currently offers 11 product lines in Canada, and has production facilities in Quebec, Ontario and British Columbia, and a distribution centre in Alberta.

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