Floor framing for ceramic tile installations:

This article discusses floor framing and subfloor recommendations for use under ceramic tile, stone, granite, marble, and similar floors. We discuss the stiffness of floor framing needed to avoid cracks or loosening of ceramic tile, stone, slate, and similar flooring and we describe alternative methods of floor preparation for tiling. We address the floor framing and subflooring details for each of these tile types.

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Recommended Floor Framing under Tile to Avoid Cracks

This article series discusses current best design practices for kitchens and bathrooms, including layout, clearances, work space, and accessible kitchen and bathroom layout, clearances, turning space, grab bars, controls, etc.

We include advice on choosing and installing kitchen countertops, cabinets, and kitchen or bathroom flooring, sinks, and other plumbing fixtures and fixture controls such as faucets.

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Tiles, stones, and grout joints crack easily from stresses imposed by movement. For a successful installation, the structure underneath must be very stiff.

On walls, 16-inch on-center framing with 2x4s or steel studs is usually adequate. Floors must be level and subject to minimal deflection under uniform or point loads.

Our photo (above left) illustrates an exploration and repairs we made in the crawl area under a bathroom before installing ceramic tiles on the floor above.

Floor Deflection Problems in Tile Floors Lacking Stiffness

An insufficiently stiff floor will crack ceramic or stone tiles. The Tile Council of America (TCA) specifies a maximum deflection for floors of L/360 under a 300-pound concentrated load. While building codes limit deflection in living spaces to L/360 under uniform loads, code-compliant floors may still have too much flexing between joists under point loads. Many natural stone tiles require stiffer conditions, ranging from L/480 to as stiff as L/720.

Subflooring Requirements for Tile Floors

To meet TCA stiffness requirements, floor framing should be no more than 16 inches on-center with minimum 19/32 -inch plywood subflooring. Upgrading to 23/32- inch plywood will stiffen the subfloor by almost 80% and provide a more solid feeling floor. The subflooring should be level to 1/8 inch in 10 feet. (TCA specs now permit 1/4 inch in 10 feet, but this can be problematic for the large tiles popular today.)

To avoid tile cracks caused by tight-fitting plywood joints, it is best to use square-edged subflooring under tile and leave an 1/8-inch gap between sheets (unless the setting material specifications require tight joints). Lay the plywood with its long dimension across the joists and use solid blocking at all open joints.

Two-Layer Subflooring System for Ceramic Tile Floors

To meet the stiffness requirements for natural stone floors may require two layers of subflooring screwed and glued together, with the upper layer serving as the underlayment.

Two layers of 19/32 inch plywood glued and screwed together on 6-inch centers is several times stiffer than a single layer (and over four times as stiff as a single layer of 23/32-inch plywood). Offset the upper layer so the joints do not line up with the joints in the lower layer or the joists.

Also, screws in the upper layer, which serves as underlayment for the tile, should penetrate the subfloor only and not the joists. Use underlayment- grade plywood or plywood rated C-C Plugged or Plugged Crossbands, with a smooth face and no voids.

Floor Substrates for Ceramic Tile: Concrete, Cement Backerboard, Plywood, Drywall, Plaster

Ceramic tile can be installed over clean and sound concrete, plywood, cement backerboard, drywall, or plaster. Most substrates can be used with either organic mastic or thinset mortar, but the installer should always check the adhesive label for compatibility with the substrate.

Plywood Underlayment Specifications for Tile for Floors or Countertops

Because of its stiffness and durability, exterior plywood makes an excellent substrate for tile in relatively dry applications.

• Plywood type. The plywood should be free of internal voids such as underlayment grade, CC-Plugged, or Plugged Crossbands , and if tiling directly to plywood, it should have a sanded face free of voids, surface resin, or other surface defects.
  
  While TCA specs allow 15/32-inch plywood underlayment over a 19/32 inch subfloor in residential work, upgrading to 19/32 inch underlayment is recommended for a trouble-free floor (Figure 6-32).

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• Plywood installation. Leave a 1/8 -inch gap between sheets and make sure the edges of adjacent plywood sheets are no more than 1/32 inch out of plane. Overall, the surface should be level to 1/8 inch over 10 feet (1/4 inch is allowed by the tile industry but is not suitable for large tiles). Fasten with ring-shank nails or screws at 6 inches on-center.
• Glue and screw. Where greater stiffness is required in a floor, it is worth the effort to also glue the underlayment to the subfloor and upgrade from nails to screws.
• Isolation membrane. On large spans or where significant movement is expected in the floor due to wide moisture or temperature swings, a crack-isolation membrane is recommended as a precaution against cracking. Membranes, if sealed at seams, can also protect the plywood from moisture that seeps through the tile system. This is required in wet applications and recommended in applications subject to occasional wetting, such as bathroom floors.
• Countertops. Plywood also works well as a tile substrate on walls and countertops not subject to regular wetting (Figure 6-33). In applications subject to high humidity or regular wetting, cement backerboard or glass-mat gypsum are better choices.

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Kitchen and Bath Product Manufacturers, Sources, Associations

Ceramic Tile-Setting Material Manufacturer List

Bonsal American www.bonsal.com Setting compounds, grouts, preformed shower pans, curbs, and niches. Also, distributor of backerboards, isolation membranes, and other tile-setting products

Color Caulk, div. of Roanoke Companies Group www.colorcaulk.com Color-matched caulking

Custom Building Products www.custombuildingproducts.com Elastomeric and liquid-applied membranes, self-leveling underlayments, setting compounds, and grouts

Laticrete International www.laticrete.com Trowel-on membranes, self-leveling underlayments, setting compounds, grouts, and sealants

Noble Company www.noblecompany.com CPE sheet membranes, trowel-on membranes, clamping ring drains, and preformed slopes, niches, and curbs

Mapei www.mapei.com Trowel-on and sheet membranes, self-leveling underlayments, setting compounds, grouts, and color-matched sanded caulks

Ceramic Tile Backerboard Producers

Custom Building Products www.custombuildingproducts.com Wonderboard cement backerboard, Easyboard cement and polystyrene lightweight backerboard, and Rhinoboard fiber-cement backerboard

Georgia-Pacific Gypsum www.gp.com/build Denshield gypboard backer with glass-matt facing

James Hardie Building Products www.jameshardie.com Fiber-cement backerboard

National Gypsum www.nationalgypsum.com Permabase lightweight cement and polystyrene backerboard

Schluter Systems www.schluter.com Kerdi tile membrane goes directly over drywall or other substrates

T. Clear Corp./Fin Pan Inc. www.finpan.com Util-A-Crete lightweight concrete backerboard

U.S. Gypsum www.usg.com Durock cement backerboard

W. R. Bonsal www.bonsal.com Extruded polystyrene backerboard with fiberglassreinforced cement facing

-- Adapted with permission from Best Practices Guide to Residential Construction.

Ceramic Tile Articles

• BATHROOM MOLD REMOVAL / PREVENTION
• CEMENT BACKERBOARD INSTALLATION
• CERAMIC TILE, ASBESTOS / LEAD?
• FLOOR DAMAGE DIAGNOSIS
• FLOOR DESIGNS: KITCHEN & BATH
• FLOOR FRAMING & SUBFLOOR for TILE
• FLOOR, RESILIENT VINYL or CORK
• FLOOR, STONE, GRANITE, MARBLE, AGGLOMERATE
• FLOOR TILE, CERAMIC for K & B
• FLOOR TILE HISTORY & INGREDIENTS
• FLOOR TILE INSTALL on CONCRETE
• FLOOR TILE SEALERS, CERAMIC, STONE
• FLOOR TILE SETTING COMPOUNDS
• FLOOR TYPES & DEFECTS
• FLOORING MATERIALS, Age, Types
• GROUT INSTALLATION, TILE
• WALL, CERAMIC TILE INSTALLATION

...

Continue reading at FLOOR TILE, CERAMIC for K & B or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

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