Laying Large Tile
Large-format tiles have their own set of rules — install them like standard tiles and watch the job fail.
As a second-generation remodeling contractor, I’ve seen a lot of design trends. One that seems to have staying power is large-format tile — that is, any tile with at least one edge more than 15 in. long. Whether they’re 16×16, 18×18, or plank style, open any home-design magazine and chances are you will see large-format tiles.
Most of the techniques used to install larger tile will be familiar to anyone who’s ever set smaller tile. Laying out the tile is similar, cutting it is the same except that you need a bigger saw (which can be rented), and grouting it is no different. However, some normal procedures used for installing smaller tile take on heightened importance when working with large-format tile, and there are several differences that are crucial to both the final look and to the longevity of the floor.
Flaws in the substrate can cause any tile floor to fail, but their effect is greater with large-format tile. Also, regular thinset mortar can’t support the size and weight of large-format tile. Large tile exaggerates lippage (variations in the height of adjoining tiles). While inconsistent mortar application and setting pressure are the leading causes of lippage, tile warpage plays a role. Most tile will be warped to some degree, but the size of large-format tile exaggerates warpage. Especially when laying tile in a running bond pattern, where the end of one row aligns with the centers of neighboring rows, installers need to rely on grout joints wide enough to disguise the problem, and even then it can be impossible to achieve an acceptable level of lippage without using a clamping system.
The good news is that although large-format tiles have their own rulebook, manufacturers and the Tile Council of North America (tcnatile.com) offer helpful solutions. Address the above concerns, and you’re well on your way to creating a durable and stylish floor.
Prep the substrate
The substrate is the most important part of any tile floor, but according to the Tile Council of North America (TCNA), “As tile size increases, the effect of substrate irregularities is compounded.” Larger tiles can be more prone to cracking from subfloor flex, and because of the length of their sides, a substrate that isn’t flat makes it much harder to avoid lippage. Getting to flat may mean sanding high spots or using floor-leveling compound. Installing an uncoupling membrane or backerboard is also a good way to isolate the tile from movement in the framing, which helps prevent cracking. The National Tile Contractors Association and the TCNA each specify deflection, flatness, and subfloor thickness in their manuals, and also provide specifications for mortars and other details.
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Tile isn't flat (duh!) so let's assume it's curved and that a 12" tile and a 24" tile have the same curvature- that is, if you put a 12" ruler against the concave side, you'll get the same lippage, say 1/16". The ruler imitates a flat substrate (yeah, sure.) Now if you put a 24" ruler on the 24" tile, you'll get FOUR times the lippage, 1/4".
It's a bit surprising, but you can prove it to yourself with a garage floor, a string and a pencil. Draw a large arc, say 60" radius. Now, starting from the same point on the arc, strike 2 more arcs, one with 12" of string, the next with a 24" radius. Draw the 2 straight lines from the original point on the arc to the two points at 12" and 24". Now measure the maximum distance from each straight line to the original arc- you'll get about 5/16" on the 12" line, about 1-1/4 on the 24" line. That is, 4 times the lippage.