Subfloors are defined in the construction industry as any type of flooring material that goes between the substrate floor and the finished floor material. Subfloors can be used with vinyl, hardwood, tile and natural stone installations as well as others. When dealing with concrete basement floors, many people assume that no subfloor is required, but the reality is that it depends on the type of flooring material you are installing as well as the condition of the concrete and other factors.
Integrity of the Concrete
The integrity of the concrete slab is one of the most important aspects of any flooring project, especially with hardwood, ceramic tile or natural stone installations. With carpet, it is less important as the movement of the slab does not affect an elastic material such as carpet, but with wood floors and tile installations, the cracks and seasonal movement of the slab can ruin the finish installation. In these cases, you will need to install an appropriate subfloor on top of the slab to protect the finish material.
Climate and Moisture
If you live in an area where there is constant moisture, such as humidity, or your basement floor is prone to moisture retention due to the surrounding soil having higher levels of moisture, you will need to install some type of waterproof or moisture-resistant subfloor to help the finish material breath and avoid rot over the years. Moisture barriers are vital to the integrity of any surface materials as rot will affect wood, carpet and other materials within weeks to months if your concrete slab has moisture issues.
Levelness of the Concrete Slab
Some installations, such as hardwood, ceramic and natural stone installations, require an absolutely flat surface. If for some reason your concrete basement slab has variations in the height, a subfloor may be required in order to build up the floor to a consistent, level installation area, so that the finish materials can be installed properly. Warped floors can cause hardwood floors to warp in the same direction and will create lippage (tiles that stick up further than others) with tile and natural stone installations.
Type of Finish Material
The type of finish material you choose also affects whether or not you need to use a subflooring material over your concrete basement slab. Carpet and sheet vinyl, for example, can be installed without a subfloor provided the concrete slab is moisture free, while hardwood will require some type of subfloor since it cannot be installed directly onto concrete. Ceramic tiles and natural stones only require subfloors with concrete if there is excessive cracking or moisture present, and then it is usually only waterproof or anti-crack membrane subfloors.
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