Ceramic Tile Flooring - Remodeling Over Wood Subfloors
By Clay Carson

Before you can install a ceramic tile or stone floor, you need to know if the subfloor is even capable of supporting tile. Simply put, tile can be a durable, low maintenance, beautiful floor choice…if it s on a solid substrate. Or it can be an expensive mistake that cracks, breaks and requires multiple repairs that may never work if the subfloor is not prepared correctly. What factors do you need to look out for to decide if tile is right for your project, and what steps can be taken to insure a trouble free installation?

For tile to be successful, it needs rigid support, with very little tolerance for movement. The more rigid the substrate, the better chance the tile has of remaining crack free throughout its life. Most problems with tile floors over wood come from excessive bounciness of the substrate. Carpet can handle some bending, vinyl tile can flex and bend a bit, hardwood floors can bend a little too, but if tile or stone is subjected to forces that push in 2 different directions at once, it doesn t know how to bend. Instead, it cracks, first in the grout and then in the body of the tile. Consumers who have just paid thousands of dollars for a tile floor do not find these cracks appealing, to say the least.

In residential settings, the most common substrates [surfaces to be tiled] for flooring are wood and cement. In this article we ll deal with deal with wood subfloors. In new construction, it s often possible to see the structure of the subfloor and joists and usually communicate with the carpenters who built them or the contractor in charge of the project if there are any questions. In remodeling, however, sometimes one can only guess who installed the floor and how strong it is. Maybe it s as strong as a battleship, or maybe it s about to fall through to the basement. If a property owner is trying to install the floor himself, he or she may wonder how to know if the subfloor is strong enough. Let s start with the technical and then translate it to the everyday way to tell.

There are formulas used in the industry to determine if the subfloor has excessive deflection [bounciness, lack of rigidity]. The most cited one is the Tile Council of North America standard for deflection, which is stated as L/360 as a minimum, before tile underlayment is installed. L/360 means that the floor should not bend under weight more than the length (expressed in inches) of the unsupported span divided by 360. For example, if the span between supports runs for 20 feet then the deflection should not be more than 2/3″ between the center and the end. L=20 x 12″ = 240″. L/360 = 240″/360 or 2/3″. So 2/3″ is the maximum amount of movement the center of the span should be allowed to move.

Fine, but how do you know if your floor meets the L/360 standard? We face this in the field all the time, but in remodeling, there s not always a clear answer. There are published tables for calculating deflection, (including a really cool online calculator at http://www.johnbridge.com/vbulletin/deflecto.pl ) but they assume you have full knowledge of how the floor was built. To be able to use the engineering tables, you d need to know how far apart the joists are, the length of the unsupported span, how thick the joists are, what type of wood and in what condition the wood is in, as well as how thick the plywood is, if any. Realistically, if all of this flooring is hidden by finished ceilings below and covered over by old flooring layers above, educated guessing takes center stage. The following questions help to determine floor stiffness using common sense guidelines:

1. What floor covering was on the floor before? If it had ceramic tile or stone, and the floor received reasonable traffic for years with no cracking or broken grout, it s a pretty good bet that the subfloor is up to the job.

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2. Does the floor feel bouncy? If so, it is. Trust your instincts. It s not ready for tile. A well built subfloor feels very stiff underfoot. Squeaking can also be a bad sign, but it may also solvable by screwing down the planks or plywood better into the joists.
3. How thick is the subfloor and what is it made of? In new construction,

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