The true concern in residential applications should be the glaze rating. Under industry standards, a "3" is the minimum glaze wear rating recommendation. Because a tile is porcelain, it does not mean it will automatically achieve a superior glaze rating. We are talking about two separate things here. Unless you are walking on an unglazed porcelain surface, the body of the tile is not relative to the glaze. A glaze has an entirely different and unrelated function, to provide wearabilty, not durability. After all, the glaze is the surface you are walking on. So just how does this glaze rating system work? Both American and International standards use the same test; the American version is called C1027 Surface Abrasion. This test is for glazed tile only. It is based on the P.E.I. rating system (Porcelain Enamel Institute). A number of sample pieces are subjected to rotation with an abrasive material on their surface, which is contained in a cup. After a certain number of cycles, a specimen is removed. This is repeated after eight subsequent rotations (100, 150, 600, 750, 1500, 2100, 6000, and 12,000). The tile that shows no wear at 12,000 cycles is then subjected to a staining test. If it passes that test it is classed as a grade V tile. Each test piece is then compared to a new piece and if visible wear is seen in a viewing box, the tile is considered to have not passed that level. There are six ratings that can be achieved by this method (0 to V). The recommended areas of use for these levels are shown in chart below.
1. Decorative only with no use on floors
I. Light foot traffic without scratching dirt and direct outside access, such as in a residential bath
II. Residential areas but without exposure to abnormal footwear or outside access
III. Normal residential areas and normal footwear and traffic
IV. Commercial foot traffic such as entrances, kitchens, offices and sales rooms
V. Areas subjected to severe pedestrian traffic such as airports, shopping centers, hotel foyers, public walkways, and industrial applications
All wooden panels, when placed over conventional floor joist or other systems, should be of a thickness as recommended by the setting material manufacturer or setting material selected based field conditions. Industry methods are designed to maintain a substrate deflection not to exceed 1/360th of the unsupported area, including live and dead load. It is not possible to accurately calculate this in a field application. Pay attention to the recommended panel thickness for both subfloor and underlayment. The 2003-2004 edition of the Tile Council of America Handbook (TCA) for the first time recognizes OSB as a suitable SUBFLOOR, not underlayment. All TCA setting methods and manufacturer recommendations are based on tongue and groove subfloors. The subfloor should be 19/32-inch thick plywood or OSB when joists are spaced 16 inches o.c. The overlay surface should be 15/32-inch minimum (19/32 commercial) C-C plugged exterior grade, non-fire treated plywood. Areas to receive plywood underlayment for ceramic tile should be exterior grade plywood only. Nails or screws should be located at 6 inch centers along panel edges and 8 inch centers each way throughout the panel, with underlayment panels offset from the subfloor below.
These recommendations change to a 23/32 or commonly known as 3/4-inch subfloor panel for 19.2- and 24-inch centers. Underlayment fasteners should not penetrate the framing supporting the subfloor. A gap of 1/8th-inch is recommended between sheets of plywood and 1/4-inch at all other surfaces such as walls they abut to allow for expansion. These recommendations come from manufacturers of wood panels and have been adopted by the tile industry. Some setting material manufacturers require the gaps to remain empty when the installation is complete. The installation of the ceramic tile should be performed in accordance to manufactures instructions which typically include reference to ANSI A108.12. As always, the substrate should be dry, clean, free of oil, grease, dirt, old adhesive residues, gypsum patches, curing compounds, sealers, waxes and paints including overspray. Problems can occur in plywood substrates when subjected to moisture.
We know we can bond to plywood, no problem. However, such was not always the case. For many years, the only industry method for installing tile over plywood was either epoxy mortar or mastic. Most mastics are not suitable for today's large size tile, even fewer for porcelain. Mastics tend to have long drying times and offer little support. Epoxy on the other hand has a tenacious bond, excellent support but is expensive and very unforgiving of excessive movement. It can be argued quite successfully that the structural movement to cause debonding is great. However, once it does, though it is a rare event, it can be explosive. Epoxies also tend to be a bit pricey for most end users and installers. If bond strength is your only criteria, nothing beats epoxy. In wood construction, that is not the only consideration. The issue with achieving adequate bond, support, and flexural value was a challenge for a number of years using all but the very best thinsets. Standard polymer thinsets meeting the basic industry standard (ANSI A 118.4 ) can offer "basic" performance. One the other hand, most liquid latex products will offer a "better" level of service.
However, if you want the ultimate in long term performance, thinsets made for installation of tile over plywood have their own standard known as ANSI A 118.11 or EGP thinset. What in the world is EGP Thinset? Exterior Grade Plywood. Products meeting this designation are designed specifically for plywood floors. They come in both latex and polymer formulations. EGP thinsets tend to command good prices but as with everything else, you get what you pay for.
As roughly 60 percent of the homes today use engineered floor joists or trusses, it would not be fair to not specifically address that in this article. There has been some rather extensive cutting edge research on this type of system by both truss and setting material manufactures. While systems seemingly have abounded for dealing with 19.2 centers and to a lesser extent, 24-inch centers most manufacturers of these systems would recommend 16-inch centers if they knew tile was going to be installed. But often tile is an upgrade or the flooring selection remains unknown at time of design hence the proliferation of 19.2 and 24-inch centers. These systems can be dealt with on a successful basis. There are numerous membrane manufacturers that provide written warranties for use of their product over various panel thicknesses.
Personally, every time the span between the joists grows greater, so does my consideration of using a membrane. Barring use of a membrane, panel thickness and type used for the subfloor and underlayment on 19.2 and especially 24-inch centers becomes absolutely critical to performance. Various setting material manufacturers make different recommendations for thickness when using their products. The Tile Council of America handbook contains numerous drawings making recommendations based on supporting field experience and test data. Please don't use your own engineering department (we never had a problem) to change these recommendations. We know what works and what doesn't. By the way, 1/4-inch wood anything is not on the list of approved panels at this point. Yes, numerous ones have been tested; sorry, because I know you have never had a problem. Please email me your number so when we get those calls we can forward them to you; maybe you can find a way to make it work.
The use of quality materials, proper coverage and movement accommodation has been key to the success for tens of thousands of these types of installations. If I were to speculate, I would guess over the course of the years, my own firm had been back to possibly 20-30 of these projects out of thousands. All repairs were related to cracked tile or grout. I am unaware of any tenting or unbonded tile. Very fine hairline cracks in the grout joints are also common in this type of installation, again with a somewhat higher incidence rate than conventional methods and in most cases, unnoticed by the customer. This leads me to the following conclusion. Our firm did not experience warranted claims when using approved methods over dimensional lumber and plywood floor systems. We had experienced limited claims when following tile industry accepted installation practices on engineered products. I believe most currently used industry accepted installation practices related to wood construction provide for more than adequate protection from all but occasional failure.