Your client has found the perfect tile, and now it’s time to install it, using tile industry standards and best practices. But what about the foundation for the installation? How level is that subfloor? And, one of the latest products on the market, gauged porcelain tile and gauged porcelain tile panels/slabs allows for only one attempt to set it correctly, since these tiles and slabs are almost impossible to slide or lift it once they’ve been placed.
If you want a long-lasting installation and a happy customer, your underlayment is critical.
Subfloor Surface Requirements for Tile
With today’s large format tile sizes, ANSI specifications can be tough to meet:
- For tiles with all edges shorter than 15 in. (0.38m), the maximum allowable variation is no more than ¼ in. in 10 ft. (6 mm in 3m) and no more than 1/16 in. in 1 ft. (1.6 mm in 0.3 m) from the required plane, when measured from the high points in the surface.
- For tiles with at least one edge 15 in. (0.38m) or longer, the maximum allowable variation is no more than 1/8 in. in 10 ft. (3 mm in 3 m) and no more than 1/16 in. in 2 ft. (1.6 mm in .6 m) from the required plane, when measured from the high points in the surface.
Getting the substrate within these tolerances is difficult with large format tile sizes. The solution is underlayment.
Products for Underlayment
There’s no standard underlayment for tile, and your choice of materials include:
- Backer board
- Mortar beds
- Patching compounds
- Self-leveling underlayments
- Liquid and sheet membranes
- Uncoupling membranes
Which one is right for the job?
It depends on the issues you find when you look at the subfloor or other substrate, and what problems you find.
- Is the floor flat enough?
- Are there low spots?
- Does the floor need to be level or sloped to a drain or perimeter edge?
These products may need to be used together to provide a suitable surface for a quality tile installation. The floor may need to be flattened with a patch or Self Leveling Underlayment (SLU), needing anything from just a skim-coat to as much as six inches.
Often, the floor you cover with backer board is irregular or wavy. The only way to fix this is to flatten the area of the floor with a patching product applied by a trowel before applying the backer board.
Also, Backer boards on a floor generally need a supporting layer of thin set mortar or other product, per the manufacturer’s directions, as well as fastening the board and seam treatment. Otherwise, you might void the manufacturer’s warranty.
Per the Tile Council of North America (TCNA) Handbook, the following materials are classified as backerboard:
For both interior and exterior:
- Cement Backer Board
- Interior installation specifications can be found in ANSI A108.11, while the material specifications are found in ANSI A118.9 or ASTM C1325.
- Exterior application specifications can be found under Handbook detail W202E. This is the only backer board rated for exterior use.
For wet or dry areas for use on ceilings, walls, and floors:
- Coated Glass Mat Water-Resistant Gypsum Backer Board
- Fiber-Cement Backer Board
- Fiber-Reinforced Water-Resistant Gypsum Backer Board
- Cementitious Coated Extruded Foam Backer Board
For dry areas only for use on ceilings, walls and floors:
- Glass Mat Water-Resistant Gypsum Backer Board
Backer boards on a floor generally need a supporting layer of thin set mortar or other product, per the manufacturer’s directions, as well as fastening the board and seam treatment. Otherwise, you might void the manufacturer’s warranty.
Designed to bond to the properly-prepared surface, they fill low areas and can flatten high spots.
- Trowel-on floor patch. This is for filling cracks, voids, rough surfaces and low areas or depressions. These products are usually latex-modified, which means they’ll need to be mixed with water.
- Trowelable underlayments. These contain cement or aggregate and depending on the manufacturer, they’re with either water or a latex additive. This product also requires sanding after it dries to either remove trowel ridges or to prepare the surface for an additional layer.
- SLU. This is designed to flatten or possibly level a floor. The floor must be properly prepared and primed with the manufacturer’s recommended product and allowed to dry. It can be a bit tricky to mix and use. Once it’s poured on the floor, you move it into place with a gauge rake that has adjustable legs to control the product thickness. Then, when that’s completed, move a surface smoothing tool across the surface to break surface tension and flatten the SLU.
Pourable, Self-Leveling Underlayments
Pourable underlayments are gypsum or cement based.
- Gypsum-based underlayments are often used with resilient flooring to patch small holes, cracks or to ramp up or down to an adjacent floor finish. They can also be used to flatten the large floor areas of properly prepared wood or concrete. Gypsum requires a certified poured gypsum installer, and depending on the thickness, has a drying time of three to fourteen days.
- Cement-based underlayments contain aggregate and chemicals that improve their strength and flowability. They are mixed with either water or a manufacturer recommended latex additive. When using this underlayment, the substrate should be treated with a latex primer to increase bond strength and to equalize the absorption rate.
A Word of Caution
It may be tempting to use the thinset left over from another job to save some money. But look at the label and the description of thinset.
You won’t see the following words: patch, fix, flatten, repair. Most manufacturers tell you NOT to use thin set mortar to level or flatten the floor or wall.
Use the right product for the job. And…
Use the Right Tool for the Job
Northside Tool Rental has been providing quality equipment to Atlanta contractors since 1953. We literally helped build the city. We’re committed to getting you the right tools and equipment for the best price possible.
We work hands-on with our customers, and our flexible rental solutions and loyalty program mean we’re there to help you see the job through ‘til the end.
Call or text us for immediate assistance from the experts at any of our four metro locations in Buckhead, Doraville, Gwinnett or Marietta.