A friend of ours is asking about a bathroom upgrade that they are planning and they are wondering about the best method for installing a tile floor in their bathroom over an existing board subfloor. They are wondering if they should remove the old subfloor or supplement it with a plywood subfloor before installing their cement backer board and then their tiles.
This is a common situation for any home that was built in the early 1960s or before. Subflooring was made out of 1 inch lumber that was not structural grade like a stud in a wall or beam. It is a lower but good quality wood that would be installed like decking on an outside deck. Over the decking there was normally a hardwood finished floor installed.
In bathrooms where tile would be put down for both the floor and for the shower stall a thick mortar bed was applied over this board subfloor. It was normally about an inch thick and the tile would be laid directly in the mortar bed rather than coming back and applying it after the mortar bed cured. It is a great method and it is somewhat easy but it does take a lot of labor.
In the 1990’s concrete backerboard started to replace the mortar beds in most new homes. You might still see a mortar bed but it wasn’t as common as years went past. This transitional period had some problems because the OSB that was being used for subfloors in the rest of the house was not really up to the duty of direct attachment. Even common laminated plywood was not much better however with a 1/8th inch luan plywood addition to these subfloors you could lay a number of sheet goods directly or you could install concrete backer board or a mortar bed.
So today when you are faced with all of the options that contractors had from the 1960’s through even today you are not exactly sure what you are going to find when you rip up a floor.
The Primary Goal Of A Subfloor Under Tile Flooring
Under tile flooring the subfloor has one duty and that is to provide stability. Tile is strong but it is very brittle so it will not take flexing of any level. The subfloor under a tile floor must be stable and not flex.
To achieve the necessary stability for tile floors you normally have to reinforce it by following a flow of methods. First the subfloor must be attached to the floor joists with nails and then it must be supplemented with decking screws that are coated or zinc plated so they will not rust. Some people use stainless steel but it is not really necessary. The idea is that you are taking the flex out of an existing floor with screws that won’t move the way nails will move during the year when there are different levels of humidity in the home. You would do this with a plywood, osb or board subfloor.
Next you want to separate the subfloor from any moisture so you need to install a layer of luan plywood with screws. Again use screws that will not rust.
When the luan is down you have the choice of using a leveling agent and installing vinyl sheet goods with a mastic adhesive or you can continue to prep for tile.
If you are using tile you should install concrete backer board that can be cut to size and then installed with more screws. It is important to follow the manufacturer’s directions but normally two layers of concrete backer are required and the joints should not overlap. Never have the seams of the lower boards stacked by the top sheets even if you have to start out with a half sheet to make sure the joints are staggered. This will reduce any moisture flowing through the seams into the luan and then down into the subfloor. Joints should be taped with a fiberglass tape as is used in drywall installs and the backer boards should be installed with a small amount of mortar or mastic so they become one full unit and not individual sheets. Mud all the joints too.
At this point you should have zero flex in your floor. Relying on tile to cure any remaining flexing will not work. If you have flexing you must cure it now.
Now you can install your tiles and enjoy a long life flooring without problems.
Final Note
Subfloors have one duty to provide a stable base for your finished flooring. It could be wood, vinyl, tile, stone or even carpet but you want to make sure that your subfloor is stable before you move on to the finished floor.
If you have some bad boards you can fix the area by replacing them with plywood or board material available at your local supply center but don’t use common white wood for this use boards that are rated.
Using screws is extremely important especially in areas that you won’t have access to again. Don’t skimp and get a bucket of cheap drywall screws and expect them to last more than a handful of years in a moist environment they just won’t. Anyone that has worked on a deck knows that the screw heads look fine but the shaft of the screw rusts even when you are using coated deck screws. This is why many contractors will go with stainless steel to reduce callbacks.