Sizing Up a Suitable Substrate
The two publications most commonly consulted for this information are the American National Standards Institute (ANSI) A-108 and Tile Council of America (TCA) handbook. Other valuable sources of information include the National Tile Contractors Association (NTCA) and the Ceramic Tile Institute of America (CTIOA). All of these organizations will steer you in the right direction.
Before we get into suitable substrates, we should first examine unsuitable or questionable substrates — of which there are many.
Questionable substrates are those that appear to present problems in bonding, but which nonetheless may be prepared to still provide a good base for the tile installation. Substrates that most commonly fall into the problem category are those that were previously covered with vinyl composition tile (VCT), vinyl sheet goods, wood flooring and paint, as well as oily and contaminated concrete, concrete with curing compound, gypsum concrete and cracked concrete. Also among this category are cork and gypsum underlayments.
Most if not all of these substrates can be suitably prepared for the installation of ceramic tile or natural stone. Manufacturers of setting materials are aware of these potentially problematic substrates and they can offer helpful advice or supply products that will overcome existing problems.
Unsuitable substrates. Certain materials have proven to be unstable for use in a direct-bond application of ceramic tile or stone. Many of these are acceptable for resilient or carpet installations, but are trouble-prone when it comes to the ceramic tile and stone products.
Among these unsuitable products are particle board, lauan plywood, oriented strand board (OSB), any plywood that has been chemically treated, masonite, concrete with deficiencies such as contaminants, curing compound, over-watered and weak surfacing, and substrates with excessive deflection values.
Suitable substrates. Gypsum board is a suitable substrate in dry areas. Water-resistant (WR) board is necessary for use in wet areas and in walls. However, cementitious backerboard are becoming more popular for use in wet areas because WR board is subject to deterioration when exposed to water. In addition to cementitious backerboards, other approved substrates include masonry walls, brick, ceramic tile and stone, concrete and Portland cement mortar.
All substrates have deflection limitations where tile or stone is bonded directly (i.e: not using a cleavage membrane). The deflection may not be greater than 1/30 of the span. Specifiers should make allowances for not only dead load, but live load and tile weight as well.
Plane. Variation in the plane is set at ¼ inch in 10 feet for mortar beds, and 1/8 inch in 10 feet for tile or stone bonded directly to the substrate with dry-set mortar, latex Portland cement mortar and tile-setting epoxy. Steel plates may be acceptable when using a manufacturer recommended tile-setting epoxy.
Waterproof membrane. Many commercial applications require the installation of waterproofing membranes prior to the installation of ceramic tile or stone. In many cases, these waterproof membranes — which may be a liquid-applied product or a flexible sheet — are installed by waterproofing contractors.
Any such membrane that’s used should comply with ANSI A108.10. If it complies with the ANSI standard, a mud bed or thinset bonding method may be used. With the thinset method of installation, the membrane must be firmly bonded to the substrate. If not firmly bonded, you may experience hollow-sounding tile sometime after the installation is complete.
Crack-isolation membranes. Again, you may be required to install ceramic tile or natural stone over this type of product. Any crack-isolation membrane used must also be firmly bonded to the substrate.
A suitable concrete substrate requires proper placement of joints within the slab. Joint placement is the architect’s responsibility. The TCA handbook addresses guidelines for the contractor’s benefit. Various joint definitions, as prescribed by the American Concrete Institute and as printed in the TCA 2000 handbook, follow:
Construction joint — The surface where two successive placements of concrete meet, across which it may be desirable to achieve bond and through which reinforcement may be continuous.
Contraction joint — Formed, sawed or tooled groove in a concrete structure to create a weakened plane and regulate the location of cracking resulting from the dimensional change of different parts of the structure.
Expansion joint — A separation provided between adjoining parts of a structure to allow movement where expansion is likely to exceed contraction.
Isolation joint — A separation between adjoining parts of a concrete structure, usually a vertical plane, at a designated location such as to interfere least with performance of the structure, yet such as to allow relative movement in tree directions and avoid formation of cracks elsewhere in the concrete and through which all or part of the bonded reinforcement is interrupted.
In my experience, the vast majority of problems occur because of unsuitable or questionable substrates, or due to improper prepping of suitable substrates. So, be aware of potential problems before that become actual ones.