Best Subfloor for Ceramic Tile

Scott writes: I have a galley style kitchen that is approximately 10×20 ft. The floor joists are 2x10s, spaced 24” on-center and running lengthwise. The maximum span is 14 ft., as a steel I-beam breaks the floor into two sections of 14 ft. and 6 ft. The basement ceiling is finished in drywall so I do not have access from below. The subfloor is 23/32 T&G OSB, with ¼ inch lauan and sheet vinyl over top.

I would like to remove the lauan and sheet vinyl then lay 12×12 porcelain tiles over the entire floor.

My concern is with weight on this floor. Instead of cement backboard, should I use ½ inch plywood to provide extra support for the tiles? Is there too much weight for the joists? It would be a lot of work to double up on the joists because of perpendicular wires, copper pipes, and PVC through some of the joists and some round duct running along between two joists.

Steve Bliss, of BuildingAdvisor.com, responds: Wood framing is not an ideal substrate for tile since wood tends to flex, shrink, and expand, while tile and grout are very inflexible. Excessive deflection in a wood floor system is a common cause for cracking in thin-set tile installations. The Tile Council of America (TCA) recommends a max. deflection of L/360 for ceramic tile and L/720 for natural stone tiles.

With the joists 24 in. on-center, you also need to worry about deflection of the subflooring between the joists. You can address this by using a double layer of plywood, laminated as described below, as your setting bed. You can also reduce deflection between joists by adding additional joists or solid blocking from below.

Residential floors are typically sized for a live load of 40 psf and a dead load or 10 psf. This is fine for wood, carpeting, and resilient flooring, but not for ceramic tile. Tile floors are likely to weigh 15 to 20 pounds or more when you add the framing, ceiling below, subflooring, tile, and setting materials. Kitchen cabinets or bathroom fixtures above can also add to the dead load.

Typical values are shown in the table below from lumber supplier Boise Cascade.

Thin-set tile dead load

So sizing for a 20 lb. dead load or greater is necessary to prevent cracking in a tile floor.

Residential span tables for 2x10s, 24” on-center, show a max. span of 11 ft. 6 in. for No.1/No. 2 SPF or 12 ft. 3 in. for  hem-fir. With Select Structural grades, you could span up to 13 ft. 8 in. for SPF and 14 ft. 0 in. for hem-fir, but these grades are less commonly used.

To stiffen the floor, you would need to remove the drywall and beef up the framing from below. Since adding  additional joists (or sistering the existing joists) would be impractical, you could improve the stiffness somewhat by adding solid blocking every couple of feet.

For greater strength and stiffness, you could add a layer of subflooring  to the bottom of the floor joists on the 14 ft. span. If you glue and screw structural plywood to the bottom of the joists with construction adhesive, you turn the floor system into a very sturdy box beam the should handle the load. I’ve used this technique successfully after a plumber did his best to destroy the floor framing in a bedroom ceiling by notching his way down the center of the span in nearly every ceiling joist.

Once you’ve stiffened the floor framing, I’d recommend adding plywood underlayment rather than cement board. Plywood is lighter and much stronger. If fact, cementitious panels are not considered structural at all.

Use at least 1/2 in. underlayment-grade plywood, but I’d recommend  5/8 in. or ¾ in. plywood in your case as the stiffness of the framing is in question . Also you need to stiffen the plywood over the 24 in. span between joists.

Ideally, the plywood underlayment should be glued and screwed to the subfloor using Type 1 or Type 2 yellow carpenter’s glue brushed over the entire surface of the plywood. The joints in the underlayment should be offset from the joints below and the screws (or underlayment nails) should penetrate the subfloor but not the floor joists.

Add a good quality crack-isolation membrane – Ditra, Stratamat, Nobleseal, etc. – to this two-layer plywood base and you will have one of the best substates available for thinset ceramic tile. This is the preferred detail on wood-framed floors for tile expert Michael Byrne and other expert tilesetters I have worked with. It also one of the preferred methods recommended by the Tile Council of America, as shown in the illustration below.

John asks: For a ceramic tile floor, do you recommend a second layer of plywood over the 23/32″ plywood  subfloor.  Your drawing suggests that it should be two separate layers. Could you please clarify? Thank you. (Posted at our sister website InspectApedia.com at Cement Backerboard Installation regarding Figure 6-34).

Steve Bliss, of BuildingAdvisor.com, responds: The drawing your refer to shows a layer of ½ in. backerboard, such as Hardibacker, over nominal ¾ in. (23/32) plywood, which is an acceptable detail for a countertop, as shown, or a floor. However, backerboard does not provide any structural support, so many tile setters prefer two layers of plywood on a floor.

Double-layer plywood subfloor for ceramic tile with crack-isolation membrane

CLICK TO ENLARGE    Plywood underlayment, glued and screwed to a nominal 3/4 in. plywood subfloor, makes a sturdy substrate for ceramic tile. The uncoupling (crack isolation) membrane helps prevent cracking in the tiles and grout.

The two-layer floor shown here is the preferred substrate (in wood-frame houses) of tilesetter Michael Byrne, author of Setting Tile, and Tiling for Contractors. Michael, an industry expert who I’ve worked with on several projects, goes further and glues the underlayment to the subfloor with yellow carpenter’s glue and puts solid blocking under all joints in the subfloor to increase the stiffness. A crack-isolation membrane is used under the tile. His reasoning is that wood-frame construction moves around a lot (deflection, shrinkage, seasonal moisture) while tile and grout are very inflexible and prone to cracking. Michael does a lot of failure inspections and feels that this detail is pretty bulletproof.

Tile installation standards are established by TCNA, the Tile Council of North America. The detail shown is essentially TCNA’s F150 method – with the addition of an uncoupling (crack-isolation) membrane. The TCNA spec shows this detail with the tile bonded directly to the plywood, but few tile experts recommend doing this. Another TCNA method (F147) uses a min. 3/8 in. plywood underlayment over a 3/4 in. subfloor with a crack-isolation membrane under the tile.

The crack isolation membrane is critical as it protects the tile from small movements in the wood subfloor. Most tile membranes are also designed to provide waterproofing if installed according to specifications.

TCNA does still show a single-layer ¾” T&G subfloor with an uncoupling membrane (F148), but I don’t know anyone in the tile industry who advocates a single-layer plywood substrate for ceramic tile or stone.

While some consider Byrne’s a approach overkill for ceramic tile, a more common method is to use nominal 1/2 in. plywood underlayment instead of 3/4 in. over the 3/4 in. subflooring.  While few contractors add the carpenter’s glue between the plywood layers, I would consider it cheap insurance against future problems. Another option for tile is to use 1/2” Hardibacker instead of plywood underlayment although this is not as strong as plywood. In all cases, use a membrane under the tile. Some key details to keep in mind:

  • The floor framing should be designed for max. deflection of L/360 (stiffer for stone).
  • Both t&g subflooring and underlayment should be gapped 1/8”
  • Underlayment should be exterior-rated “underlayment grade” plywood with no voids.
  • Joints in underlayment should be offset min. 2” from joints in underlayment.
  • Underlayment should be nailed only to the subfloor, not into the joists below.
  • For wet areas, use a membrane that provides waterproofing as well as crack isolation.
  • Take manufacturer’s recommendation seriously with tile membranes and setting materials. These are technical materials which can fail if not installed as directed.
  • Don’t mix and match. Stick with one manufacturer for setting materials – thinset, grout, colorant, sealer. Mix two products and no one is responsible if there are problems.
  • Never grout tile joints that abut other materials, planes, or restraints such as : wood, fiberglass/acrylic, tile base, recesses, etc. These joints should be caulked with a high-quality sealant. Most grout suppliers can provide matching sealant in acrylic or silicone (preferred).
Schulter Ditra uncoupling and waterproofing membrane for ceramic tile

Ditra and similar membranes protect ceramic tile from movement in the wood sublfoor and framing. The membrane is bonded to the underlayment and tile with thinset. Courtesy of Schluter Systems.

There are many choices for the uncoupling membranes. I’ve used both Ditra and Strata Mat – the main difference is that Strata-Mat uses modified thinset to bond the tiles while Ditra requires unmodified thinset. Both use modified thinset to bond to the plywood – tile installation gets pretty technical nowadays. I’ve used both of these membranes in remodeling projects, going over various substrates including new plywood and well-bonded existing flooring.

Grout joints are the bane of tile jobs, which often get badly stained on floors over time. The trend today is to use larger tiles and smaller grout joints of 1/8” or less where possible (not possible with irregular shaped tiles). The latest thing is stain-resistant grouts that supposedly don’t need sealing, a cheaper alternative to epoxy grout, which is truly stainproof, but very expensive and difficult to install. I used Laticrete Permacolor on a recent bathroom remodel, but it is too soon to say how it holds up. Another contractor I know likes TEC Power Grout, a similar stain-resistant grout.

The tile industry is rapidly evolving with new setting materials and membranes coming on the market all the time. It takes some effort to keep up with it — even for tile contractors.

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Comments

  1. John Cooke says:

    Minimum Subfloor For Tile?

    After having removed vinyl, and then particle board, I’m left with a 5/8″ plywood subfloor for a rarely used bathroom. I’d like to add minimal height to this so that when I add ceramic tiles the height will be equal to the hallway hardwood floors. Calculations on the John Bridge site show I have a deflection over L / 1700, on 16″ joist spacing. Is it then acceptable to add 1/4 ” marine plywood and just thinset over that? Thanks.

    • buildingadvisor says:

      The calculator you are using is for deflection along the length of the joists. With ceramic tile, you also need to limit the deflection between joists, which is a function of the subfloor thickness and how it is installed.

      It sounds like your joists are well within the acceptable limits of L/300, so your main concern is the deflection of the subfloor between joists. Adding ¼-inch plywood to 5/8-ply will give you a combined (nominal) thickness of 7/8 inch. The bare minimum recommended by most tile setters is a total subfloor thickness of 1-1/8 inch. Most prefer a minimum thickness of at least 1-1/4 inch, such a ¾-inch t&g subfloor with ½ inch plywood underlayment.

      You might be OK with your installation, but are risking cracked tiles or grout joints. You’ll be better off with ½-inch plywood if you can make it fit.

      • To get the best performance out of any combination of subflooring and underlayment, make sure you do the following:
      • Add solid blocking to the underside of the floor at subflooring joints, unless they are t&g.
      • Offset joints in the underlayment and subfloor by min. 2 inches and avoid screwing the underlayment into the joists below. Place screws 6 to 8 in. o.c. in both directions.
      • Use a crack isolation membrane between the underlayment and subfloor. If height is a concern, you can use a very thin membrane such as Nobleseal or a liquid-applied membrane. There are many options.
      • Where you are concerned about rigidity, you can stiffen the floor by spreading an even layer of Type I or Type II yellow carpenters over the surface of the subfloor before installing the plywood underlayment.

      Whatever you do, don’t omit the crack-isolation membrane, which is designed to protect the tile from small movements in the subfloor. If you are unable to build up a thick enough subfloor, you can reduce deflection by add more joists or adding solid blocking between the joists at 16 inch intervals. This is assuming you have access from below the floor.

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