Ceramic Tile Flooring - Remodeling Over Wood Subfloors

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. If it was vinyl, carpet or hardwood, we are still in the dark.

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, ¾ inch plywood or Oriented Strand Board is a standard subfloor over joists that are 16 inches on center apart. We find that is almost never enough to meet the deflection standards in most homes. Other times there is old plank flooring beneath a layer of plywood. This is a wild card, since the engineering tables usually don’t include the value for planks in their calculation, but common sense says it does add some stiffness.

4. How tough is the tile to be installed? Fairly thick quarry tiles, for example, may be rated for heavy duty industrial applications, although they are often installed in homes. Because they are thicker than normal tiles and able to withstand heavy traffic, they may be less prone to cracking than a sensitive, thinner tile. For that matter, natural stone such as marble and granite are on the other end of the spectrum – they crack even easier than ceramic tile and should not be used in settings where any excess deflection is possible. Intuition may tell you they are stronger than ceramic, but in fact they are more brittle and prone to cracking. They need twice as rigid a floor as ceramic.

5. What condition does the wood appear to be in? Even if the amount of wood support seems adequate according to the tables, if it appears to have been water damaged, if sections of it look moldy or corroded due to rot or decay, it’s not doing its job. Options include replacing or reinforcing it, but not just ignoring it. Also, has it been cut into in various spots, such as a plumber cutting sections of the joists for positioning pipes? All of these problems can make the wood less effective.

6. What’s the property owner’s risk tolerance? Does he/she want to be rock solid sure of the stability of the floor? Even if that means spending extra money and/or time to reinforce the floor, and accepting a floor that may sit higher than surrounding floors? Or is some risk of failure acceptable if the floor is not built to the righteous standards of the TCNA? Sometimes the extra effort is not worth the cost to the property owner, who should be fully informed on all options. Contractors who install flooring shouldn’t assume that clients don’t care enough to solve the problem: in the last year we’ve had two clients who spend thousands of extra dollars to reinforce subfloors in a kitchen and laundry room when we explained that their floors were too unstable for tile. They really wanted tile, and were willing to make the subfloor ready for it, even if it cost more.

7. Is there an unfinished ceiling below to look up and measure the distance between joists and the condition of the wood below and how long the unsupported span is? A few minutes in the basement with a flashlight and tape measure can let you know if you have a winner (thick and deep joists, spaced closely together, in good condition, with a narrow span), or a loser (thin and shallow joists, irregularly spaced or spaced far apart, in bad condition, with a long span).

8. Can you cut into the layers on top to get a cross section of the existing floor? If there is a heating grate that you can remove, that may show the layers the floor is composed of. What will be reassuring to see is a thick layer, ideally over 1 ½ inches thick of plywood. Alternatively, with the property owner’s permission, we sometimes cut in to it to check what it’s composed of.

If a subfloor displays excessive deflection, it can usually be remedied by installing more plywood on top of it before tile is laid, and by reinforcing the joists from below. While it may make the floor higher than before, think of it as a sort of ‘insurance policy’ against flooring failure.

Contractors who address these issues with their clients beforehand are only doing the client a favor. The industry as a whole benefits when tile installations are done correctly and excessive deflection is avoided in the beginning.

Clay Carson works for Act One Marble and Tile in Arlington Massachusetts, a tile and stone contractor serving the Boston area. http://www.tileboston.com