You almost always need an underlayment for a porcelain or ceramic tile installation, sometimes even on a concrete subfloor, and while cement backer board is the most popular option, it isn't always the best choice, especially on concrete. Other options include:
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- Foam board: It looks like cement board but is much lighter.
- Glass-mat gypsum board: This type of water-resistant drywall has replaced "green rock" as a backing material for wall tiles out of direct water contact.
- Self-leveling underlayment: You pour this on the subfloor and allow it to seek its own level before it hardens.
- Uncoupling membranes: These are flexible plastic underlayment materials that you unroll on the floor and set in thinset mortar.
- Dry-pack mortar: This was the traditional underlayment for floor tile but is now seldom used.
Only three of these materials — cement board, foam board and glass-mat gypsum board — have the word "board" in their names, signifying that they provide rigidity as well as all the other qualities you need for good adhesion and moisture resistance. If the substrate is already solid, however, you may not need the extra rigidity, so it's good to understand the other types of tile underlayment because they have their own advantages.
The Three Types of Tile Backer Board
Cement backer board and the closely related fiber-cement board are the most popular materials when it comes to underlayments for a ceramic tile installation on a wall or floor, according to Construction Pro Tips. Cement board has a hard core of Portland cement sandwiched between layers of fiberglass mesh, and it's approved for use in wet areas, such as tub and shower walls. It's water-resistant but it isn't completely waterproof, so it's usually used in conjunction with a moisture barrier, which usually takes the form of plastic sheeting stapled to the studs before installation of the board itself.
One of the problems with cement board is its weight. A 3 x 5-foot sheet of WonderBoard, which is one of the leading brands, weighs about 45 pounds, while the same-size sheet of Durock, another leading brand, tips the scale at 36 pounds. Other popular brands, such as Permabase and Hardiebacker, fall in between.
Foam backer boards, such as Wedi board, Kerdi board, USG Durock foam board, Laticrete Hydeoban board and Johns Manville GoBoard, weigh less than 10 pounds per sheet, which definitely solves the weight problem. They are waterproof and can be used in wet areas, but installation is more complicated, requiring a special polyurethane sealer and sometimes special fasteners with washers, all available in the building products section of home improvement outlets that stock the boards.
Glass-mat gypsum board is the preferred choice for builders who want to use a product that cuts and installs like drywall. Like greenboard, it has a water-resistant gypsum core, but what sets it apart is its fiberglass mat facing, which makes the product mold- and mildew-resistant and suitable for use in wet areas. Leading brands include Sheetrock, manufactured by the United States Gypsum Company, which also makes conventional Sheetrock, Georgia-Pacific's DensShield and CertainTeed Glasroc.
Comparing Tile Backer Board Costs
Although you can buy cement board in 4 x 8-foot sheets, 3 x 5-foot sheets are more common, primarily because the product is so heavy. Prices per sheet vary from $10 for Durock to $14 for HardieBacker, making the average cost about 80 cents per square foot.
Glass-mat gypsum, like standard drywall, is available in 4 x 8-foot sheets, and the cost per sheet is about $20. The cost to use this product is about 63 cents per square foot.
Like cement board, foam backer boards also come in 3 x 5-foot sheets, and each one costs in the neighborhood of $22, making the cost of this product about $1.47 per square foot. Besides the fact that the sheets set you back almost twice the amount that other backer materials do, you also have to buy the fasteners and sealant.
Tips for Installing Tile Backer Board
When tiling a wall, you install all three backer board materials in more or less the same way — by screwing them to the studs. You use special rust-resistant fasteners to hang cement board, and you can also use these fasteners with certain brands of foam board. It's also advisable to use corrosion-resistant screws to hang glass-mat gypsum board, although regular 1 1/2-inch drywall screws will work.
The biggest difference among these backing materials as far as DIY tile installation is concerned is the cutting method. You can cut all three materials by scoring with a utility knife and snapping, but cement board quickly wears out conventional blades, so it's advisable to buy a carbide-tipped scoring tool.
You can also cut cement board with a circular saw and a six-tooth concrete blade, and you need a jigsaw and a carbide-grit blade to cut curves, whereas you can use your knife to cut curves in the other materials. Cutting cement board with a power tool raises clouds of silica dust, so be sure to wear goggles and a respirator.
Cement board is the only tile backer board that is suitable as a underlayment for a tile floor. After securing the subfloor to the joists to eliminate any squeaks, you trowel on a bed of thinset mortar and lay the sheets in the mortar while it's fresh. You then drive fasteners to secure the sheets and tape the seams with fiberglass tape and thinset.
Alternative Tile Underlayment Methods
In days gone by, the most common method for creating an underlay for floor tile was to trowel a 1- to 2-inch bed of dry-pack mortar. This gives the tiles a solid footing, but getting the bed smooth and flat takes a level of skill beyond that of most DIY tilers. An alternative is to pour a self-leveling underlayment, which by virtue of its being a liquid flows naturally to form a level surface, although you still have to coax it with trowels and rollers.
Uncoupling membranes, such as Schluter-DITRA, Laticrete Strata Mat and Mapeguard UM, are a great option when laying floor tile. They are completely waterproof and only 1/8-inch thick, so they don't raise the floor level by much. You can use them over concrete, wood and existing tile floors provided the subfloor is rigid enough to prevent cracking of the tiles or grout. You unroll the membrane in a bed of thinset mortar and then apply more mortar to the surface to hold the tiles.
You actually don't need an underlayment when installing floor tile on a concrete slab that is flat, smooth and in good condition with no cracks. Installing an uncoupling membrane is insurance against cracking, however, because by separating the tile layer from the subfloor, you guarantee that any movement of the concrete or any cracks that develop won't affect the tiles or the grout. If you choose not to use an underlayment, make sure you clean the concrete with trisodium phosphate, which removes oils and etches the surface to ensure it bonds with the thinset adhesive and holds the tiles firmly.