We ran into this situation on a recent project and it seemed like a good time to talk about it.
Even though personally I would rather listen to Miley Cyrus talk about politics than to allow anyone tile over hardwood, the long and short of the answer is yes, you can.
The real question is; Why would you want to?
You have to remember that the same rules apply for tiling over hardwood as do for tiling over any suitable substrate.
This is a picture from one of our jobs where ceramic tile had been installed over 2 1/4" oak flooring. We discovered it when removing the tile to install an updated tile.
As you can see, the tiles came up with little of the thinset mortar still attached to the floor. This is a perfect example of a poor bond. When we look at the tiles themselves, you can see that the thinset had no problem staying attached to the back of the tile.
After the tile had been completely removed from the area, this is what the floor looked like with no major scraping.
Barely any real adhesion had occurred.
So let's talk about these "rules" for tile installation. There are three very important things you always want to be aware of when you are installing a tile floor.
Rule #1: Motion is your enemy.
Anytime you install a tile floor, you want the surface you are tiling over to have minimal motion. If a floor moves, tiles can loose their bond and/or crack along with the grout lines.
Things that may cause your wood-frame floor to move:
- Undersized framing: Older homes may have been built smaller framing than code currently requires. It bounces and flexes more than the beefier framing of today.
- Wider framing layout: Another situation we often find in older homes is that the framing is wider than 16" on center. This forces the subfloor to span a longer distance and allows it to flex more than in a newer home.
- Unsuitable subfloor sheathing: In older homes you may find wide plank floor boards, newer homes usually require 3/4" plywood as their subfloor. When it comes to installing tile, the rule of thumb is to have at LEAST 1 1/4" of subfloor sheathing underneath to provide the necessary strength. That means that if you have 3/4" plywood, you need to add another 1/2" layer of substrate over it to create a thick enough subfloor (always run additional plywood layers in the opposite direction of the previous for added strength). I like to fasten additional layers with 1 1/4" galvanized screws at 6" on center, 2 1/2" screws along the framing lines. This gives the entire floor frame every opportunity to lock itself together tightly.
- Improper fastening: Sometimes the subfloor is not properly anchored to the framing and can move which can result in tile bond failure. Don't be afraid to dance around on your subfloor before you tile. Spreading your legs apart and leaning side to side like your trying to tip a boat over is an easy way to spot creaks and squeaks which can often be cured with additional fasteners.
Rule #2: Surface must be clean for a strong bond.
Hardwood flooring is usually finished with some form of polyurethane. The nature of this finish will tend to reject things that are trying to bond to it (including your tile adhesive). To tile over hardwood, you'd need to clean it first, preferably with mineral spirits to remove any build up you may have on the surface. Any waxes or soap that have been used to maintain the floor over the years will leave a build-up that is awful for adhesion.
Sanding the floor with a low grit sand paper (30 grit or lower) would create a better bonding surface. Rough wood makes for a much better adhesion. Also, the less porous a substrate is, the harder it will be for thinset to bond to. Another reason a finished hardwood floor isn't the best candidate for tile installation.
The natural surface of cdx plywood provides a rougher more porous surface to allow thinset to adhere to.
Rule #3: Use a premium modified Thinset mortar
This is extremely important. All your other steps will be for nothing if you don't use a high grade latex/polymer modified thinset mortar. These modifier additives help improve the bonding and flex characteristics of the mortar. You can purchase liquid additives though I've found the high end, pre-mixed powders have excellent bonding strength when properly applied. Make sure you don't mix your thinset too dry. A drier mix can have poor adhesion. You're looking for a consistency somewhere between pudding and peanut butter.
In conclusion: Why we don't recommend tiling over hardwood
Having read the steps it would take to even consider hardwood flooring as a suitable surface, it's usually just easier to remove and replace it with a decent cdx plywood or cementitious backerboard substrate. The grain of cdx plywood is perfect for creating a long lasting floor tile bond. There are those that would say using a tile backer board is preferable but when it comes to floors, I still prefer to put down a material with the ability to carry more weight. Although backer board has excellent moisture resistant qualities, I can snap most backer boards without a lot of effort. Backer board is much better suited for non-traffic tile installation in moisture heavy areas such as shower stall walls or counter tops in my opinion.
So in conclusion; Follow the three steps for any tile installation, don't tile over your hardwood because it's just silly and always remember to floss! (That has nothing to do with flooring but my 8 year old son can't seem to remember to do it so, I find myself constantly saying it.)
Do you have a flooring nightmare project on your plate? Drop us a line in the comments below and we'll see if we can help!