Figure 6-33: Accessible Kitchen Design Specs: accessible sink and dishwasher work centers (C) J Wiley S Bliss Floor-Framing & Subfloor Details for Ceramic or Stone Tile Flooring

  • POST a QUESTION or READ FAQs about floor preparation for tile installation: framing, subflooring, underlayments, stiffness specifications or allowable deflection, and how to stiffen a floor that needs that improvement.

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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.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Recommended Floor Framing under Tile to Avoid Cracks

Crawl space inspection & repair before tile floor © D Friedman at 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.

[Click to enlarge any image]

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.

[Click any image or table to see an enlarged version with additional detail, commentary & source citation.]

Figure 6-32: Accessible Kitchen Design Specs: accessible sink and dishwasher work centers (C) J Wiley S Bliss Figure 6-33: Accessible Kitchen Design Specs: accessible sink and dishwasher work centers (C) J Wiley S Bliss

[Click any image or table to see an enlarged version with additional detail, commentary & source citation.]

Kitchen and Bath Product Manufacturers, Sources, Associations

Ceramic Tile-Setting Material Manufacturer List

Bonsal American 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 Color-matched caulking

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

Laticrete International Trowel-on membranes, self-leveling underlayments, setting compounds, grouts, and sealants

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

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

Ceramic Tile Backerboard Producers

Custom Building Products Wonderboard cement backerboard, Easyboard cement and polystyrene lightweight backerboard, and Rhinoboard fiber-cement backerboard

Georgia-Pacific Gypsum Denshield gypboard backer with glass-matt facing

James Hardie Building Products Fiber-cement backerboard

National Gypsum Permabase lightweight cement and polystyrene backerboard

Schluter Systems Kerdi tile membrane goes directly over drywall or other substrates

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

U.S. Gypsum Durock cement backerboard

W. R. Bonsal Extruded polystyrene backerboard with fiberglassreinforced cement facing

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

Ceramic Tile Articles


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