Most guys skip the leveling compound. They think the underlayment will hide the dip. It won’t. I spent three days grinding concrete on a job last month just so the floor wouldn’t click like a castanet. That job was a wake up call for the homeowner who thought their slab was flat because the house was only five years old. It was not. It was a topographical map of the Rockies in miniature. When you are dealing with tile, especially in showers or across vast expanses of hardwood floors, you are fighting physics. The wall is never straight. The floor is never level. If you do not understand the mechanics of the shim, you are just waiting for a callback. I have spent twenty five years with my knees on the ground and my hands in thin-set. I smell like WD-40 and oak dust most days. I have seen 15,000 dollar installations fail because a guy thought he could eyeball a plumb line. You cannot. You need the right tools, and in the world of tile, the plastic shim is the unsung hero that prevents a total aesthetic and structural collapse.
The concrete lie that breaks every tile layout
Subfloor leveling and wall plumbness are the absolute foundations of a professional tile installation that lasts decades. Using high-density polyethylene shims allows an installer to compensate for bowed studs and uneven slabs without compromising the mechanical bond of the thin-set mortar. Without these tools, grout joints will vary in width and lippage will occur across the surface. I remember a kitchen in a high-rise where the subfloor was out by nearly two inches across a ten-foot run. The client wanted 24×48 large format porcelain. If I had not spent the time grinding the high spots and using heavy-duty shims to set my starting course, that floor would have been a trip hazard before the grout even dried. You have to respect the TCNA standards for substrate preparation. A floor is not just something you walk on. It is a performance surface that must withstand thousands of pounds of pressure per square inch. If there is a void behind a tile because you tried to ‘back-butter’ your way out of a hole, that tile will crack. It is a mathematical certainty. You use the shim to create the space required for the material to sit true, regardless of what the framing is doing behind it.
“A floor is only as good as the subfloor beneath it; deflection is the enemy of every joint.” – Master Flooring Axiom
The molecular reality of high density polyethylene
Plastic shims are manufactured from high-density polyethylene to ensure they do not compress under the weight of heavy masonry or tile. These horseshoe spacers and wedge shims provide a non-porous support system that resists mold growth and chemical degradation within the alkaline environment of cementitious mortar. Unlike wood, they do not swell. When you are working in showers, moisture is the enemy. Old-school guys used to use wood slivers to level their bottom row. That is a crime. Those wood pieces eventually rot or swell when moisture penetrates the grout, and suddenly your bottom row is shifting. Modern plastic shims are color-coded by thickness. You have your 1/16th inch, your 1/8th inch, and your 3/16th inch. This allows for a level of precision that is impossible with improvised tools. You are essentially building a bridge between the reality of a crooked house and the perfection of a flat tile. The chemistry of the plastic is vital because it does not react with the polymers in modified thin-set. It stays inert. It stays stable. It holds the line while the mortar cures and the mechanical bond is established. If you use a material that compresses, you lose your level. If you lose your level, you lose the job.
| Shim Material | Compression Resistance | Moisture Tolerance | Recommended Use |
|---|---|---|---|
| Hardwood Slivers | Low | Very Low | Temporary bracing only |
| Cardboard | Zero | Zero | Trash can |
| High-Density Polyethylene | Extreme | 100% Waterproof | Showers and wet areas |
| Soft Plastic Wedges | Medium | High | Lightweight wall tile |
Why showers demand more than just a level
Waterproofing membranes and shower pan slopes require precise shim integration to maintain the integrity of the drainage system. Every shower installation must account for the capillary action of water and the structural deflection of the wall studs under the weight of cement board and tile. If your wall is leaning out, your grout will eventually fail at the corners. I have seen guys try to fix a leaning wall by just piling on more thin-set. That is a amateur move. Thin-set is not a build-up material. It is a connector. It has a maximum thickness rating for a reason. If you exceed that, the mortar shrinks as it dries, pulling the tile out of alignment. This is where the plastic shim saves your life. You place the shim behind the cement board or under the first course of tile to ensure that every subsequent row is perfectly vertical. This ensures that the water flows down the face of the tile into the drain, rather than pooling in a dip created by a crooked wall. When you are dealing with grout, consistency is everything. A 1/32nd of an inch mistake at the bottom becomes a half-inch gap by the time you hit the ceiling. The math does not lie. You use the shims to force the house to be what it is not: perfect.
The geometric disaster of a sloping corner
Corners are the most frequent point of failure in tile layouts because they reveal the lack of squareness in the framing. Using horseshoe shims at the internal corners allows the installer to maintain a consistent expansion gap while keeping the grout lines aligned across perpendicular planes. If you do not shim your corners, you will end up with ‘slivers’ of tile on one side and full tiles on the other. It looks like garbage. I hate seeing a beautiful piece of stone ruined by a lazy layout. You have to measure the room at three different heights. You have to find the shortest wall. You have to shim out the difference. This is especially true when transitioning to hardwood floors or laminate. The heights must match. A zero-threshold transition is the gold standard of modern flooring, and you do not get there by accident. You get there by calculating the thickness of the subfloor, the underlayment, the thin-set, and the tile itself. If your wall is wonky, your transition will be crooked. I carry a laser level on every job, and I trust it more than I trust my own eyes. But even a laser is useless if you do not have the shims to act on the information the laser gives you.
“Deflection is the enemy of every joint; a rigid substrate is the only path to a lasting finish.” – TCNA Technical Handbook Reference
The structural failure of the cardboard shim
Never use organic materials like cardboard or wood as permanent shims behind tile or within the floor assembly. These materials absorb moisture from the mortar and the environment, leading to compressive failure and the delamination of the tile assembly. I have seen ‘contractors’ use pieces of the tile box to shim up a low spot. It makes my blood boil. Cardboard is a sponge. As soon as that thin-set hits it, the cardboard softens. The tile sinks. The level is gone. Professional plastic shims are notched so they can be easily removed once the mortar has set, or they are designed to be fully encased in the mortar without creating a weak point. When you are installing laminate, you might get away with a bit of foam underlayment to hide a minor dip, but tile has no such mercy. It is rigid. It is heavy. It is unforgiving. If the support fails, the tile cracks. It is that simple. I tell my apprentices that if they touch a piece of wood for shimming a tile job, they can find a new career in framing. We are architects of the surface. We use materials that outlast the house itself.
- Check the substrate for level using a 10-foot straight edge
- Identify high spots and grind them down before applying any thin-set
- Use horseshoe shims to maintain consistent spacing on vertical walls
- Verify plumbness with a laser level every three courses
- Always remove shims before grouting to prevent ‘ghosting’ through the joint
- Ensure the expansion gap at the perimeter is at least 1/4 inch
The ghost in the expansion gap
Expansion gaps are required at every perimeter and change of plane to allow for the natural movement of the building. Without a proper silicone-filled joint at the corners and base, the tile will tent or crack as the house settles and expands with humidity changes. This is a massive issue when you have hardwood floors meeting a tile entry. Wood moves. Tile does not. If you lock them together without a shim-controlled expansion gap, the wood will literally push the tile off the floor. I have seen it happen in the humid summers of the South. A beautiful oak floor expands and buckles the marble foyer because the installer forgot that the house is a living, breathing thing. You use the shims to hold that gap during installation. You ensure that the space is clean of mortar. Then you fill it with a high-quality 100% silicone caulk that matches your grout. This allows the materials to move independently. It is the difference between a floor that lasts five years and one that lasts fifty. You are not just laying tile. You are managing the stresses of the entire structure. Every shim you place is a tiny insurance policy against the inevitable movement of the earth.
The 1/8 inch that ruins everything
Precision in the grout joint is the only way to hide the inherent imperfections of the tile manufacturing process. Most large format tiles have a slight bow or warpage from the kiln firing process, which requires calibrated shimming to minimize lippage. If you try to run a 1/16th inch joint on a 12×24 tile without shimming for levelness, you will have edges sticking up everywhere. It will feel like a cheese grater on your feet. You use the shims to lift the low corners of the tiles until they are flush with the neighbors. Some people call this cheating. I call it craftsmanship. The Janka Hardness Scale might tell you how tough your hardwood floors are, but no scale can measure the frustration of a homeowner who keeps stubbing their toe on a proud tile edge. You have to be a stickler. You have to be obsessed. You have to check every joint with your thumb. If you feel an edge, you pull the tile, add a shim, or adjust the mortar. There is no ‘good enough’ in this business. There is only right and wrong. And wrong is expensive.