Tile backers: cement board, green board, other products:

This article discusses the types and installation details for backer boards used below ceramic tiles for countertops and splashboards, including cement backerboard, drywall, greenboard, and glass-mat gypsum backerboards. We address the installation details for each of these material types.

Cement Backerboard for Use Under Ceramic Tile: Installation Specifications

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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. A list of kitchen and bath product manufacturers and sources is included.

This article includes excerpts or adaptations from Best Practices Guide to Residential Construction (Steve Bliss, J Wiley & Sons) , by Steven Bliss, courtesy of Wiley & Sons.

Figure 6-34 (above) shows one layer of plywood with cement backerboard beneath a tile floor installation. A separate figure, Fig. 6-32 shown below, employs two layers of plywood for a more rigid subfloor design.

Developed specifically for use under ceramic tile, cementitious backer units (CBUs) are made of sand and cement and are reinforced by fiberglass facings or chopped fibers within the material itself. Cement backerboard is impervious to moisture, but may let moisture pass through, so it should always be backed by a waterproof barrier, such as polyethylene sheeting or asphalt-impregnated felt.

Because the material has little inherent strength, it relies on the structure beneath for stiffness when used on floors and other applications subject to significant loads. A variation on cement backerboard, Hardibacker (James Hardie Building Products), is made of fiber-cement. Because it is only 1/4 inch thick, it is usually installed over 1/4 -inch plywood, both for stiffness and to bring it flush with the surrounding drywall.

Cement backerboard should be installed with special backerboard screws (not drywall nails) that have an anti corrosive coating and oversized heads with ridges underneath designed to self-countersink and pull tight. Roofing nails are also acceptable with most backerboards.

Closely follow manufacturers instructions, which vary a little from product to product. Typical installations include:

• Backerboard on walls: CBUs can go directly over studs at 16 inches on-center or over drywall, and it is fastened with 1 1/2 -inch galvanized roofing nails or special self-countersinking galvanized screws at 6 to 8 inches on-center.
• Backerboard on floors and countertops: CBUs are bonded to the plywood with a layer of latex- or acrylic-modified thinset (Figure 6-34 above and Figure 6-35 below).
  
  Apply the thinset with the flat side of a 1/4 -inch notched trowel, then rake with the notched side to provide a continuous level setting bed.
  
  Seat the backerboard evenly with a beating block, then secure with roofing nails or special backerboard screws. Because CBUs provide little added stiffness, it is best to upgrade to a 23/32-inch subfloor.
• Joint details for Cement Backerboard Under Tile: Leave a 1/8 - to 3/16-inch gap between adjoining sheets of backerboard and at edges. Just before tiling, fill the joints with thinset and cover with 2-inch-wide fiber mesh tape (unless the adhesive manufacturer recommends otherwise). Then embed the tape in a thin skim coat of thinset. Reinforce inside and outside corners with three pieces of 2-inch tape or one piece of 4-inch tape.
• Obstruction Clearances at Concrete Backerboards: Leave a minimum 1/4 -inch gap where the CBU meets a tub lip, plumbing fixture, pipe, or any restraining surface and fill the joint with a flexible, waterproof sealant.

Above: Figure 6-35 offers design details for a wet-counter using cement backerboard.

Installation Guide for Drywall Substrate Below Tile

Regular 1/2-inch drywall over 2x4 framing or steel studs 16 inches on-center is a suitable substrate for dry installations.

Using 5/8-inch drywall or a second layer of 1/2-inch will provide a stiffer wall. Joints should be taped and one coat of joint compound applied to joints and nails. If tiles will be set in thinset mortar, use thinset with mesh tape at the drywall joints.

Using "Greenboard" Moisture-Resistant Drywall Under Tile

Commonly called “greenboard,” this offers moderate protection in moist conditions, but it is not recommended in wet areas, such as tub surrounds, unless protected by a waterproof membrane.

In our photo the greenboard has been coated with joint compound and sanded - those surfaces were being prepared for painting rather than for tile installation.

Coated Glass-Mat Gypsum Backerboard Use Under Tile

Competing with cement backerboard, gypsum products such as Georgia Pacific’s Dens-Shield® are designed as tile substrates for damp areas. The gypsum core is treated with silicone to make it water-resistant, and the fiberglass and acrylic facing acts as a surface vapor barrier.

While not as strong as cement backerboard, gypsum-based backerboard installs faster since it cuts and installs like drywall. Installation is with roofing nails or galvanized bugle-head drywall screws, which should be driven flush with the surface but should not break the waterproof skin.

Panels are butted tightly and the joints are taped and embedded with latex Portland cement mortar, also used to set the tile.

In use since 1987, Dens-Shield® is well-suited to tub surrounds and other light-duty wall applications, but it is not suitable for saunas, steam rooms, or other applications facing extreme heat and humidity. As with other tile substrates, follow manufacturer’s instructions closely regarding framing, installation, and tile application.

Subfloor Specifications for Installing Ceramic Tile

Reader Question:

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Above: an except cut from Figure 6-34 Plywood Subfloor with Cement Backer Board

2015/12/03 John said:

Regarding Figure 6-34:

I cannot tell if it is specifying one or two pieces of 24/32" plywood for the subfloor. There are diagonal lines in two directions with a horizontal line separating them, which strongly suggests that it's supposed to represent two different materials.

Could you please clarify? thank you!

Reply:

John,

Two layers of plywood is preferred by tile experts to minimize deflection and thus assure a crack-free ceramic tile job on floors.

Author Steve Bliss notes that our mutual tile expert Michael Byrne also glues the underlayment to the subfloor and adds solid blocking under all joints in the subflooring to further stiffen the floor against movement that occurs as temperatures and moisture change in the building. Certainly wood materials are subject to much more potential movement than ceramic tile, and the object is to avoid cracking.

Steve adds the following:

Tile installation standards are established by the TCNA, the Tile Council of North America.

The detail shown is essentially TCNA’s F150 spec – with the addition of an uncoupling (isolation) membrane. The TCNA spec shows this detail with the tile bonded directly to the plywood, but few tile experts recommend this. ...

TCNA does show a single-layer ¾” T&G subfloor with an uncoupling membrane (F148) and a double-layer detail with min.

3/8” plywood underlayment over a ¾” T&G subfloor with an uncoupling membrane (F147). These are minimum standards and I don’t know anyone in the tile industry who advocates a single-layer substrate.

... a more common recommendation is to use ¾” T&G plywood or OSB (glued and nailed or screwed) covered by a layer of ½” plywood underlayment for tile or ¾” for stone – with an uncoupling membrane under the tile.

Check out BUILDING ADVISOR.COM - Sister Site - hosted by Steve Bliss.

Comments from Steve Bliss 2015/12/03:

Your reader is very observant. The two-layer floor is the preferred wood-frame floor tile substrate of tile guru Michael Byrne, who is considered a bit extreme by some. Michael goes further and glues the underlayment to the subfloor with yellow carpenter’s glue and puts solid blocking under all joints in the subfloor.

His reasoning for this detail is that wood-frame construction moves around a lot (deflection, shrinkage, seasonal moisture) and tile doesn’t. He does a lot of failure inspections, expert witness work, etc., and feels that this detail is pretty bulletproof. Some contractors only go to this extreme with some types natural stone.

Tile installation standards are established by the TCNA, the Tile Council of North America.

Photo: Hardiebacker fiber cement tile board. See more photos below. [Click to enlarge any image]

The detail shown is essentially TCNA’s F150 spec – with the addition of an uncoupling (isolation) membrane.

The TCNA spec shows this detail with the tile bonded directly to the plywood, but few tile experts recommend this.

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

TCNA does show a single-layer ¾” T&G subfloor with an uncoupling membrane (F148) and a double-layer detail with min. 3/8” plywood underlayment over a ¾” T&G subfloor with an uncoupling membrane (F147).

These are minimum standards for tile installation, and I don’t know anyone in the tile industry who advocates a single-layer substrate.

Aside from Michael Byrne, a more common recommendation is to use ¾” T&G plywood or OSB (glued and nailed or screwed) covered by a layer of ½” plywood underlayment for tile or ¾” for stone – with an uncoupling membrane under the tile.

Another option for tile is to use 1/2” Hardibacker instead of plywood underlayment. Above: HardiBacker fiber cement tile backerboard, shown in more detail

at NON-ASBESTOS FIBER CEMENT BACKERBOARD

Key Details When Framing Floors for Ceramic Tile - from S. Bliss

· The floor framing should be designed for max. deflection of L/360 (stiffer for stone).

• Both subfloor 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 – sometimes it’s the same membrane but with different detailing.
• Always follow the manufacturer’s recommendation with membranes and setting materials. It’s always a good idea to stick with one manufacturer for setting materials – thinset, grout, colorant, sealer.
• 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).

There are many choices for the uncoupling membrane. I’ve used both Ditra and Strata Mat – the main difference is that Strata-Mat can use 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 in retrofits going over various substrates including stone and well-bonded vinyl.

FYI: Michael is author of Setting Tile (Taunton Press), which has sold over 100,000 copies, and Tiling for Contractors (JLC Books), which I edited and helped produce in 2009-10, so these details are seared into my brain. I specified the Byrne approach in a bathroom addition for a summer house I built 20 years ago in Cape Code.

The bathroom will be above an unheated porch, so I wanted the extra strength of this approach. Not surprisingly, a couple of contractors who looked at the plans said this was overkill, etc. I said, just do it and I’ll pay for the extra work. Looks like I’ve finally got an acceptable bid and should start work within about 30 days.

BTW, grout joints are the bane of tile jobs, which often get badly stained over time on floors.

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 great but very expensive and difficult to install.

We used Laticrete Permacolor on our recent bathroom remodel, but it is too soon to say how it holds up. Another contractor I know likes TEC Power Grout.

The tile industry is rapidly evolving with new setting materials and membranes coming on the market. It’s a lot to keep up with.

Hope that clarifies this murky subject which inspires a lot of debate. Please note that my TCNA Manual is from 2007, so it’s possible that some specs may have changed (generally they just add new methods, but leave the old).

Steve Bliss- BUILDING ADVISOR.COM - Sister Site to InspectApedia.com - Ed.

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 - see the HardieBacker® EZ Grid® fiber cement backerboard photographs just above.
• 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 (Steve Bliss, J Wiley & Sons) .

Fiber Cement Backer Board Products that Do Not Contain Asbestos

The fiber cement backer board shown in our photo above was used to support ceramic tile in an Australian home built ca 1986/1986. Thanks to reader KC, 2017/08/24.

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This non-asbestos fiber cement tile backer photo accompanied photographs of black ceramic tile mastic adhesive about which the reader worried of an asbestos hazard. Her question and our reply are

at ASBESTOS MASTIC IDENTIFICATION.

This photo of a fiber cement tile backer board stamped "MANUFACTURED WITHOUT ASBESTOS" reminds us that at the time your house was built, manufacturers of tile-backer board and cement board were dealing with customers who wanted to be sure they were not being sold old-stock of asbestos containing products.

(Though in fact cement-asbestos is not friable and so when intact is not high risk). So they made a clear stamping for people to see.

Below: James Hardie's HardieBacker® EZ Grid® 3' x 5' cement board showing the product label and a close-up of the material. This is a modern fiber cement board that does not contain asbestos.

...

Continue reading at CERAMIC TILE FLOOR & WALL (also used on walls) or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Ceramic Tile Articles

• ASBESTOS MASTIC IDENTIFICATION
• BATH & KITCHEN DESIGN GUIDE - home
• CERAMIC TILE FLOOR, WALL
• CEMENT BACKERBOARD INSTALLATION
• FLOOR FRAMING & SUBFLOOR for TILE
• FLOOR TILE SEALERS, CERAMIC, STONE
• TILED WALKING SURFACE SLIP HAZARDS
• CERAMIC TILE, ASBESTOS / LEAD?
• CERAMIC TILE BACKSPLASH INSTALLATION
• CERAMIC TILE DAMAGE REPAIR
• CERAMIC TILE FLOOR & WALL
• CERAMIC TILE GROUT INSTALLATION
• CERAMIC TILE SETTING COMPOUNDS
• CERAMIC WALL TILE INSTALLATION

Suggested citation for this web page

CEMENT BACKERBOARD INSTALLATION at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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INDEX to RELATED ARTICLES: ARTICLE INDEX to BATH & KITCHEN

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