How to install underlayment for tile

The first step to install underlayment for tile is to apply a bed of thinset to the subfloor with a ¼-inch square notch trowel.

Install underlayment for tile is a critical part of the prep work for a tile floor – no matter how well your tile is laid, bad prep will result in a bad floor.

If you remodel kitchens and baths you will invariably be laying tile floors. Tile floors have become ubiquitous with a custom kitchen or bath remodel. One of the most important steps in preparing a floor for tile is install underlayment for tile – a proper underlayment will provide a good bond and the strength to prevent cracking.

Problem: What’s the best way to install underlayment for tile?

Solution

There are quite a few options available to install underlayment for tile. One of the most popular and readily available is HardieBacker cement board, which is what we installed for this job. HardieBacker is comprised of Portland cement and ground sand, has no paper face and is mold and moisture resistant, making it a good choice for wet areas like kitchens and baths.

Use the notched edge to comb out the thinset. This will ensure the thickness of the thinset is consistent.

When installing HardieBacker, or any other underlayment, it’s important that the surface you’re installing it on is as flat as possible. Many times it’s necessary to sand plywood seams to remove humps or apply a floor leveler to fill low areas.

Note: Regardless of the underlayment you’re installing, be absolutely sure the floor framing is strong enough to support a tile floor – a weak floor system will result in cracking. The guidelines for acceptable floor deflection are available from the Tile Council of North America (tcnatile.com).

Once you have determined that your subfloor meets deflection standards and you have made the surface as flat as possible, you’re ready to install your HardieBacker.

Installing HardieBacker is fairly straightforward and only requires:

• Thinset
• ¼-inch square notch trowel
• T-square
• Razor knife, carbide cutter, shears, or grinder/saw to cut the board
• HardieBacker screws and cordless impact to drive screws
• Alkali resistant glass fiber tape

Cutting HardieBacker

The first step to install underlayment for laying tile is cutting the HardieBacker. HardieBacker can be cut with a razor knife or a carbide cutter by using a T-square and a score and snap method similar to cutting drywall. This works well but can be difficult for detailed cuts due to the strength of the material.

You can also use electric fiber cement shears, which is probably the easiest method and yet shears can be cost prohibitive for those who do not install HardieBacker on a regular basis.

A third option is a grinder or saw with a diamond blade. This also works very well but has complications – dust. If you use a grinder or saw to cut HardieBacker, your cut station must be outside and the person doing the cutting must wear the proper respiratory mask to protect from the silica dust.

Set the Hardiebacker into the thinset and embed it. Then install the HardieBacker screws working from one edge to the other set every 8 inches o.c.

Installing underlayment for tile

As with any other subfloor or underlayment installation, all of the joints on the HardieBacker should be staggered and joints shouldn’t line up with the joints of the subfloor it’s being installed over. HardieBacker also should have a 1/8-inch gap between all boards and a 1/8-inch space off of walls and cabinetry.

The first step of installing HardieBacker is to apply a bed of thinset to the subfloor with a ¼-inch square notch trowel. Spread the thinset out with the flat edge of the trowel and then use the notched edge to comb out the thinset.

Combing the thinset with the notch trowel will ensure that the thickness of the thinset is consistent – you may need to comb the thinset a few times to achieve this. You’ll see when the thinset is consistent after a few passes – the subfloor will be visible between the consistent ridges created by the square notches.

Once enough thinset is spread out for the HardieBacker, set the board into the thinset and embed it. I usually take a few steps on the board to set it firmly into the thinset then install the HardieBacker screws working from one edge to the other.

Once all the HardieBacker sheets are installed, install an alkali-resistant fiber tape over all the joints using the same thinset that will be used to install the tile. After the thinset has dried, vacuum to ensure a clean surface, lay out the pattern for the tile, and start laying tile.

Screws should be set every 8 inches o.c. With¼-inch HardieBacker (the thickness most commonly used for floors and countertops) this is made simple with a fastener pattern applied to the face of the board. The fastener heads should be set flush or slightly recessed below the face of the HardieBacker. Over driving the fastener will decrease the holding strength and create a weak spot.

Once all the sheets are installed, an alkali-resistant fiber tape must be applied over all the joints using the same thinset that will be used to install the tile. After the thinset used to embed the fiber tape has dried, I like to go over the HardieBacker with a floor scraper to remove any mushrooming around the heads of the fasteners and to remove any stray chunks of thinset remaining from taping the joints.

Once this is complete, go over the entire floor with a vacuum to ensure a clean surface, lay out the pattern for the tile, and start laying tile.

–By Chad McDade, Housewright Construction, East Palestine, Ohio