Let’s open this discussion with a vocabulary lesson. The word substrate means “an underlying substance or layer.” When a builder, designer or contractor speaks about a good substrate when it comes to flooring or tiling walls, we are referring to the surface in which we are building on. There are many different substrates in a home. It could be the concrete slab in the basement, the plywood subfloor in the living room or the cement board walls in the shower stall. Some of these surfaces are primed and ready to tile (more or less) but others will require changes to ensure you have a good solid substrate for tile to prevent cracking and shifting over time. Let’s dive in a little more.
Concrete slab as a substrate
Concrete is an acceptable substrate. It is solid and won’t move much over time. Concrete will expand an contract due to temperature and moisture so you should check for moisture prior to any install to make sure that water will not damage your newly installed tile or laminate. Check the seams and the corners and seal them if needed. Also, be sure to check that your slab is level prior to any tile install. An “uncoupling membrane” should be laid down prior to any tile install. An uncoupling membrane’s job is to isolate the tile from the substrate, allowing the two to move independently, preventing cracking that may occur as the substrate naturally breathes. These membranes also act as a moisture barrier to prevent moisture from creeping through. If you are going with a laminate or LVT floating floor on a cement slab, you don’t necessarily need an uncoupling membrane, but you will still want to use a moisture barrier at the very least to prevent water vapor from causing damage. Be sure to further review manufactures directions for laying the membranes, and which thin set they recommend with their product.
Plywood as a substrate
Plywood is technically an acceptable substrate to lay tile directly over, although there are a few things to consider. Plywood, like a concrete slab, will breathe over time. It will shift as the house settles and from foot traffic over time. Most thin-set manufacturers will recommend to NOT lay their product directly on plywood knowing the potential issues if not prepped properly. There are methods to making plywood more secure such as intervening layers of plywood, ensure proper board spacing, using long enough screws and driving the screws into the joists below. But with acceptable methods aside, you can consider using an uncoupling membrane as discussed earlier, a mud base, or simply lay cement board over the plywood and tile directly over that. A mud base or the cement board layer may increase the height of the floor higher than you may want, so be sure to consult with your contractor on this prior to the install. The uncoupling membrane is a much lower profile option if needed. If you are a DIYer, be sure to check all manufacturer directions prior to an install over a plywood subfloor.
Drywall as a substrate
Our first two sections mainly spoke of flooring surfaces. When we start talking drywall, we’re going vertically. Most of the time, vertical surfaces receiving tile will be bathroom shower stalls, maybe the walls in the rest of the bathroom, or a kitchen backsplash. It may be safe to say that 10/10 contractors will install cement board in the shower stall (Durock, Hardiebacker, WonderBoard or Permabase to name a few). Hopefully, if the design calls for it, there will be these cement boards on any of the half walls or full walls that are planning to receive tile. But lets say there isn’t. There is only that green board (mold resistant) or regular drywall. If you don’t want to rip out the drywall and replace it with cement board, you can lay the tile directly over the drywall in areas that won’t get too wet. For areas that may receive more moisture than others, such as behind a sink, it may pay to replace the drywall with cement board. The drywall may soak up moisture over time and cause the tile to fall off. To potentially prevent this from happening, prime the drywall prior to the thin-set. The primer will seal the drywall leaving a better substrate for your tile. Thin-set adheres best to porous surfaces, so if the walls have been painted, use sand paper to remove the gloss and be sure to clean off any excess dust. Prime the wall and then go for your thin-set.
Cement board as a substrate
Last but not least, the old reliable cement board to the rescue. As stated earlier, you can use cement board in almost any application, where it can be screwed down. It is the ideal substrate in high moisture areas and vertical surfaces. It can provide a better bond on flooring surfaces over plywood, but may raise the floor height too much. Cement board does not need to be primed or sealed prior to tiling. An extra step that some contractors take, is to paint roll another moisture barrier (such as Red Guard) over the cement board in a shower stall, to provide added protection against water damage. Besides that, screw up your cement boards (or down) and get tiling!
Now that you have had the proper substrate education, you should hopefully feel better about hearing the word come up in conversation. Don’t be afraid to as your contractor questions regarding what substrate prep he will be doing, or if it is a viable substrate to begin with. If the contractor becomes defensive with your questioning, maybe you don’t use them, or give it a shot yourself as a new substrate professional! Or just call Gotham Tile, that works too.
If you decide to DIY, good luck and be safe!