Your Checklist for Proper Mortar Coverage

By Chris Kain, Western Area Technical Manager, H.B. Fuller Construction Products

One mistake all tile installers can’t afford to make is not getting proper mortar coverage. It doesn’t matter if the installation is inside or outside, wet or dry, wall or floor, small or large tile or stone. If the bond isn’t strong and the coverage isn’t up to spec, the tile will develop problems sooner than later—and it’s going to come back to bite you in the wallet.

Here are some valuable tips to keep in mind before beginning any job.

Flatten the Substrate

If you find areas of the substrate that are uneven, you’ll need to patch the spots or use an underlayment to provide a level surface. This is especially important for large and heavy tile installations.

Mortar Makes the Difference

If you’re installing smaller tiles, such as 8″x8″ or 12″x12″, a traditional mortar should suffice. However, as more tile installations involve large and heavy tiles (15″ or more on any side), they require large format tile mortars. Tiles today can run as large as 5’ x 10’, so using wrong mortar is a costly mistake that professional installers will not want to make.

Be sure to pick the mortar that fits to the size of the tile. You can always use a large format tile mortar for smaller tiles…but not the other way around. The wrong mortar for large tiles will squish out the sides and not support the tile—on a floor or a wall.

You’ll want to ensure that mortar for floor tiles is non-slump so that the weight of the tile won’t squish the mortar. Likewise, wall mortar should be non-sag so the tile won’t slide down the wall. When you press the tile into place, the mortar needs to properly support the weight.

Follow the Manufacturer’s Directions

You’d be surprised how often professional tile installers will mix a mortar by eyeballing it to a certain consistency instead of following the manufacturer’s specifications. You want the mortar to perform up to specs. Take an extra minute to read the directions closely regarding the amount of water, how long to mix, wait time, etc.

Mortars have different water demands and mixing times. This is true not only for different brands of mortar, but even for different products with the same brand. Don’t assume to know the proper mix without checking first. Also, never use any product that’s past its expiration date, or if it’s in an open bag that might have already started to cure.

Use the Correct Trowel

There’s no science to picking the right trowel notch since most manufacturers will recommend using the trowel that will give you the desired coverage. In general, smaller tiles require less mortar and smaller notch trowels, while heavier tiles require bigger notches.

Picking the right trowel for the specific mortar is important. You can follow the guidelines provided in ANSI A108, which address the trowel notch for the appropriate amount of mortar needed to bond the tile to the substrate. The minimum amount of mortar thickness after the tile has been embedded is between 3/32″ and ¼” for smaller tiles, and a maximum of ½” for larger tiles. Specific trowels are also available for installing thin gauge porcelain panels.

Troweling Technique

Once you’re set to begin troweling, here are some basic reminders. Apply the mortar in straight lines in one direction, then put the tile down and apply pressure to knock down the ridges and push the air out. Follow the manufacturer’s recommendations to provide enough mortar to bed the tile, but not too much to cause mortar to squeeze out between the tiles.

When installing rectangular tiles, remember that your troweling lines need to run parallel to the short sides of the tile instead of the long sides. This allows the air in the trowel lines to be burped out once you begin to set the tile. Apply pressure to the tile and collapse the ridges to fill the valleys and release the air.

Once you put your mortar down and trowel it, begin setting the tile quickly. If the mortar sits for too many minutes, it will start to skim over and your tile will never bond. It might feel like it’s set, but the mortar has started to cure and the top layer is already hardening.

Another consideration is the environmental conditions on the job site. Warm temperatures and breezes blowing on the area will speed up the cure time. The point is to not get too far ahead of yourself or take any breaks once the mortar is applied.

Coverage is Critical

Your goal for any tile installation is 100% mortar coverage. This is particularly important along all four sides where full coverage right to the edges and corners is essential. Proper coverage is critical to the entire integrity of the job. Tiles that are installed with insufficient coverage are going to fail, it’s just a matter of when.

That said, here are some specific minimum coverage requirements to follow. When installing porcelain or ceramic tile in a dry area, such as an inside floor or kitchen backsplash, the minimum coverage is 80%. For outside or wet areas, such as a shower, the minimum coverage required is 95%.

When installing any kind of natural stone tile, as well as thin gauged porcelain tiles, only 100% complete coverage is acceptable for all applications—dry or wet, inside or outside. Transparent glass tile also requires 100% coverage since any voids in coverage will be visible to the customer.

Floor tiles with insufficient coverage can crack and break at the corners. Any voids under a floor with a thin gauged porcelain tile will cause a hollow sound when walking on it. But it’s more than just an annoyance.

In wet areas, such as a shower wall, inadequate coverage allows water to seep through the grout and get under the tiles. Even on walls in dry areas, tiles with improper coverage can get loose and literally fall out.

Check the First Tile

The only way to make sure you’re getting adequate coverage is to set your first tile, then pull it up and flip it over for an examination. Look closely at the coverage to check for voids. As always, the goal is 100% coverage—particularly along all four edges.

If you see any problems, you can go through a checklist of possible solutions:
• Do you need to back butter the tile with mortar first?
• Is there notable warping in the tiles themselves?
• Do you need to adjust your trowel lines because of the pattern on the back of the tile?
• Did you miss any substrate patches?
• Do you need to re-do the mortar mix?
• Should you try a different size trowel with smaller or wider notches?

Once you’ve resolved any issues, resume troweling and setting the tiles. Remember to continue checking a tile or two as you go along to make sure that your coverage is complete and consistent.

These tips should serve as a useful reminder for best practices that are so important to ensuring a successful tile installation on every job.


This demonstration shows proper mortar coverage.


This image depicts poor mortar coverage in an exterior project. Note the lack of coverage in the center of the tile. This lack of coverage allowed moisture to settle into the voids, breaking the bond and allowing the tile to come loose.

About the Author:
Image: Chris Kain
Chris Kain is the Western Area Technical Manager for H.B. Fuller Construction Products. Chris provides on-site technical advice and education to contractors on proper tile, flooring and surface preparation installation techniques and troubleshooting, and provides technical support and product training for customers. He has held various technical positions over several regions with H.B. Fuller for more than 13 years and has over 30 total years in the construction products industry.

About The Author

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