Ceramic floor tiles resist water, making them appropriate for use in bathrooms and kitchens where spills ruin hardwood flooring or carpet. These tiles require a special adhesive to hold them firmly to the subfloor. Only a few underlying materials offer the right surface for adhesive use, and installing ceramic tile over the wrong underlayment leads to broken tiles and a weak floor.
A properly cured and solid concrete slab works fine as an underlayment for ceramic tile. Installing ceramic tile over the surface of the foundation saves time and money over adding an additional subfloor. Check that the surface of the slab is completely flat and level before trying to glue ceramic tiles on it, warns Home Time, or some tiles will be lower than others. A concrete-compatible levelling compound helps you level out depressions that naturally form in the surface of a large slab.
Softer subfloor materials bend when you walk across the floor, and even the slightest give in the underlayment cracks the grout between ceramic tiles. Cement backerboard is a strong, rigid board that offers enough support for tiles of all kinds says, Old House Online. Cement is mixed with fine aggregates like sand, then covered in a fine mesh to create the rough surface adhesives needed to hold tiles down. These boards come preformed and ready to use.
Before concrete slabs were commonly used and when cement backerboard wasn't invented yet, ceramic tile installation required an underlayment of a thick layer of mortar. For the most durable ceramic tile flooring, lay a level bed of mortar over wire mesh and roofing felt, according to Home Time. Smoothing and perfecting the mortar is difficult even for seasoned tile installers. Most homeowners who install their own ceramic tile underlayment do not attempt the messy process of building a mortar bed.
The thin, flexible plywood subfloor used under carpet installation won't support the heavier ceramic tiles. Install another layer of plywood if your existing underlayment is under 1 1/8 inches, says Home Addition Plus. Normal interior-grade plywood absorbs most of the moisture used in the ceramic tile adhesives and grouts, becoming weak. Use a plastic barrier designed for use under ceramic tiles or choose exterior grade, water-resistant plywood for the top layer.