Ceramic tile installations are one of the most solid types of flooring material you can choose from, but there are strict regulations for the installation, including floor deflection. Deflection is the up and down movement of the floor, or its “bounce.” The more deflection there is, the more likely the tiles are to break free of the thinset mortar and pop up from the installation surface, which is why fixing such deflection issues is a must prior to installing the tile.
One of the quickest solutions for a floor with too much deflection for tile is to double up on the plywood subflooring. Rather than install it in the same pattern as the first layer, your goal is to stagger the second layer so that none of the joints line up with the first layer, either by installing it perpendicular to the first layer or starting in a completely random part of the room. While tile installations can be installed over 5/8-inch plywood, it’s always best to have two layers installed on top of each other for stability. Mount the second layer of plywood to the same floor joists as the first layer.
If you have ready access to the joists beneath the subfloor, you can bolster the joists with blocking. These are boards of the same joist material cut down to length and slotted perpendicularly in between the joists, then attached to the sides of the joists with screws or nails. Either stagger the blocks, or line them up across the joists in a single line of blocks that stretches out between the joists. This braces the joists from side-to-side deflection, which in turn strengthens the floor from up-and-down deflection.
A secondary option, if you have access to the joists, is to double up on the joists themselves, or to install what is known as sister joists. These are joist boards placed directly alongside original joists and then nailed into place against the first one. This is particularly useful if you are working with large-bodied tiles, and can be paired with double layers of plywood for greater durability and strength.
Beams are another option if you want to maximize support from underneath the joists of the floor. This is particularly useful for second-storey applications where you are bolstering an upper floor from below. It is a more challenging addition if you are working on a first floor area and only have access to a crawlspace. In both cases, a structural engineer is required to help determine size and placement of the beam, as well as installation methods. As a general rule, crawlspace beams are installed and held in place with special beam jacks, while second-storey beams need to be either built into the home or held up with pillars of some type to brace the floor above.