Stabilize a Floor for Tile
Subfloors for tile installations require a little more attention to detail than other flooring materials. A floor might be solidly constructed and adequately supported but still have a degree of “bounce” or vibration when people walk across the room. This is usually not a major problem with resilient flooring, carpet or even some tongue-and-groove systems. Stability and rigidity should be addressed, however, if you plan to install a tile floor, because any minor amount of deflection in the subfloor could result in cracks or damage to the brittle material.
That was the problem faced by pro remodeler Ritchie Hamilton during a recent flooring job. Over time the house had settled and the floor joists had sagged. Not only did this result in a noticeable dip in what should be a flat surface, but the floor lost rigidity, and the surface would slightly bend and bow when you jumped up and down on it. Installing mortared and grouted floor tile over the unstable floor would invite cracked grout lines, loosened tiles, and maybe even broken tiles.
If you can gain access beneath the subfloor, you can construct a perpendicular beam with piers to level the joists and stabilize the framing with additional support that connects to the ground. The beam serves two purposes. First, it bridges across the bottom of the joists so they can all be leveled to the same height to reduce sag. Second, if it’s located midway through the span of the joists, the beam essentially halves the length of the unsupported framing, which takes some of the flex out of the flooring. That was the solution that Ritchie chose to strengthen the subfloor and ensure a long-lasting tile floor installation.
Here’s how it went.
The surface of the existing floor was covered in resilient flooring. A little jumping around was all it took to determine the subfloor was in no shape for tile in its current condition.
Ritchie’s first step was to level some precast concrete pier footings laid in a row perpendicular to the floor joists he planned to support.
After locating the joist that sagged lowest in the crawlspace, we attached a 4x post to its underside with a metal construction tie. In this case the beam was a 4×4, but a 4×6 would work even better.
Ritchie placed a bottle-jack on the footing beneath the lowest joist and used a 2×4 extension block to slowly raise the joist and beam. As the sagging joist goes higher, the gap shrinks between the beam and the bottom of the neighboring joists.
Once the space between the beam and joists closed, Ritchie installed a 4×4 post cut to fit between the adjusted beam and the concrete footing.
The new posts provide a secure connection all the way to the ground and to help take the shake out of the floor.
A plumb bob will help determine the right spot to locate the bottom of the 4×4 posts beneath the joists.
To span the entire floor a couple of 4×4 beams were installed end to end in the same manner.
Positioning the posts snugly between the joists and footing will take a little encouragement from a hammer.
Install each post perfectly plumb, and toe-nail with a framing nailer to secure them from movement.
If you have any problem joists that simply refuse to align with the others, use wood shims to close the gap and provide a solid connection that will reduce movement.
Shown here is the completed subfloor girder (lower right).
Once you’ve stabilized the floor framing, your subfloor will probably be in less-than-perfect condition for tile, which requires a very flat, solid surface. Ritchie decided to go with cement-board underlayment, specifically using 1/4-in. HardieBacker.
After stripping the floor covering to the plywood subfloor, Ritchie installed the HardieBacker. Although it’s common to see remodelers fasten cement board directly to the subfloor with nails or screws, that is not recommended.
The best practice is to first apply a supporting bed of modified thinset mortar to the subfloor using a 1/4-in. square-notched trowel.
Then, embed the cement board firmly and evenly in the wet mortar.
Stagger all the cement-board joints, and avoid aligning them with the subfloor joints. The four corners of cement boards should never meet at one point. Leave an 1/8-in. gap between board edges for expansion, and keep the edges 1/8-in. back from walls and cabinet bases.
To make cuts to the HardieBacker, you can use a razor utility knife to score and snap the boards to size and make necessary cutouts.
Fasten the cement board with the manufacturer’s specified screws or roofing nails (minimum 1-1/4 in. and corrosion-resistant) every 8 inches over the entire surface. Keep fasteners between 3/8 and 3/4 inch from board edges and 2 inches from corners. Set all the fastener heads flush with the surface without overdriving.
Then, embed 2-in. wide high-strength alkali-resistant glass fiber tape in the mortar.
Level the tape and mortar with a drywall trowel.
Follow these guidelines to stabilize the joists and prepare a proper substrate, and your subfloor should pass the bounce test so the tile installation will go smoothly and last for years to come.
Shown here is the finished tile floor.