Why You Should Use Plastic Shims for Your Laminate Installation

The expansion gap is the heartbeat of your floor

Plastic shims ensure a consistent expansion gap which is the most critical component of a floating floor system. They prevent the laminate from binding against the wall. If the floor cannot move it will eventually buckle. Plastic shims maintain their shape under pressure and won’t compress like wood alternatives. 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 because the subfloor was a disaster. I walked onto that jobsite smelling like oak dust and diesel, knowing the previous installer had tried to use scrap pieces of cedar to shim the perimeter. The wood shims had compressed under the weight of the floor, the expansion gap vanished, and the whole living room had peaked in the center like a mountain range. It was a fifteen thousand dollar heartbreak that could have been avoided with five dollars worth of plastic shims. Most homeowners think laminate is just a cheap alternative to hardwood floors, but it is actually a sophisticated engineered product that requires more precision than solid wood. If you don’t respect the physics of expansion, the floor will destroy itself from the inside out. Laminate is essentially high-density fiberboard (HDF) wrapped in a photographic layer and a wear layer. That HDF core is hygroscopic, meaning it absorbs moisture from the air. When the humidity rises, the floor grows. If it hits a wall because you didn’t leave a 3/8 inch gap, the pressure has nowhere to go but up. Plastic shims are the only way to guarantee that gap stays uniform during the entire installation process. Wood shims split, they shift, and they vary in thickness. Plastic is consistent, rigid, and reliable.

The hidden chemistry of the HDF core

The internal composition of laminate flooring determines how much lateral force it exerts against your walls. High-density fiberboard is comprised of wood fibers bonded with urea-formaldehyde resins. These fibers are incredibly dense but they remain reactive to atmospheric changes. When moisture molecules enter the core, the fibers swell and the board expands. While most people want the thickest underlayment, too much cushion actually causes the locking mechanisms on LVP or laminate to snap under pressure. This is because the floor deflects too much when you walk on it, putting stress on the tongue and groove. You need a stable subfloor and a rigid perimeter. When I’m working near wet areas like showers or kitchens, I see people trying to treat laminate like tile. They want to use grout in the gaps or caulk the expansion joint. This is a death sentence for the floor. A floor needs to breathe. If you lock it down with grout or heavy cabinetry, you are asking for a failure. I always tell my apprentices that a floating floor is like a living organism. It moves. It shifts. It expands and contracts with the seasons. Plastic shims act as the temporary spacers that define the boundaries of that movement. Unlike wooden shims, plastic spacers won’t rot if they come into contact with a damp slab. They provide a predictable stop for your first row of boards, which is the foundation of the entire installation. If your first row isn’t straight and properly spaced, every subsequent row will be crooked. You will end up with gaps in the butt joints and a floor that looks like an amateur did it. I’ve spent twenty-five years on my knees fixing these mistakes. It always comes back to the basics of subfloor prep and perimeter spacing.

“A floor is only as good as the subfloor beneath it; deflection is the enemy of every joint.” – Master Flooring Axiom

Why cedar shims are a recipe for disaster

Softwood shims like cedar or pine are unsuitable for flooring because they lack the compressive strength needed to resist lateral expansion. As the laminate floor expands, it pushes against the perimeter. A cedar shim will simply crush or flatten under this pressure, effectively shrinking your expansion gap. Once the gap is gone, the floor binds. Plastic shims are engineered to withstand hundreds of pounds of pressure without deforming. This ensures that the 3/8 inch gap you measured at 8 AM is still a 3/8 inch gap when you finish the job at 5 PM. I have seen jobs where the installer used cardboard or folded pieces of the laminate box as shims. This is pure laziness. Cardboard absorbs moisture from the subfloor and becomes a breeding ground for mold. It also provides zero structural resistance. When you are hammering the planks together with a tapping block, you are applying significant force. A plastic shim stays put. A cardboard or wood shim will slide or squash, leaving you with an uneven gap. This is especially important when you are transitioning to other surfaces. If you are moving from laminate to a tiled area near showers, you need that gap to be perfect so the T-molding can sit correctly. If the floor has moved because the shims failed, your transition strip won’t line up and you’ll be left with an ugly, uneven mess that catches every toe that walks by. Accuracy in the first row is everything. I don’t care how many years you’ve been doing this, if you aren’t using a level and a moisture meter, you aren’t a pro. You’re just a guy with a saw.

The ghost in the expansion gap

The expansion gap is often invisible once the baseboards are installed but its presence determines the lifespan of your floor. Many installers try to cut corners by eye-balling the gap. They think a quarter inch here and a half inch there won’t matter. They are wrong. A floor is a massive sheet of material that moves as a single unit. If one side of the room is tight against the drywall, the entire floor will shift toward the other side. This causes the joints to pull apart or the floor to buckle. Plastic shims allow you to set a precise, repeatable distance around the entire room. I like to use the stackable plastic shims because they allow for adjustments when the wall isn’t perfectly straight. Let’s be honest, no wall is ever perfectly straight. You might have a bow in the studs that pushes the drywall out by an eighth of an inch. With plastic shims, you can stack them or thin them out to maintain a perfectly straight line for your floor while respecting the expansion requirements. This is where the structural engineering meets the cosmetic finish. You are building a performance surface. It has to handle the weight of furniture, the traffic of kids and pets, and the constant cycle of temperature changes. I’ve seen hardwood floors that cost twenty dollars a square foot fail because the installer didn’t understand moisture migration. Laminate is no different. It is a technical product that requires a technical approach. Don’t be the guy who thinks he knows better than the NWFA standards. Those standards exist because people like me had to go out and fix the messes made by people who didn’t follow them.

• Always use a moisture meter on the subfloor before starting.
• Ensure the subfloor is level within 3/16 inch over a 10-foot radius.
• Place plastic shims every 12 inches along the perimeter.
• Verify that the gap is clear of debris before installing baseboards.
• Never nail baseboards into the flooring planks themselves.

The 1/8 inch that ruins everything

Small errors in spacing at the start of a laminate project compound into massive structural failures across the entire surface. If you start with a 1/8 inch deviation, by the time you are ten rows in, that gap has turned into a crooked floor that won’t lock together. Plastic shims are the guardrails that keep you on track. I remember a job where a guy tried to install a wide-plank laminate in a sunroom. He didn’t use shims and he didn’t account for the heat gain from the windows. The sun hit that floor, the HDF core expanded, and because he had no gap, the floor actually lifted the baseboards off the wall. It was a total loss. He had to rip it all out and start over. That is the price of arrogance. You have to respect the material. Laminate is a fantastic product when installed correctly, but it is unforgiving of mistakes. Plastic shims provide the physical barrier necessary to ensure the floor stays where it belongs while having the room it needs to function. It is not about aesthetics during the install, it is about long-term durability. When you pull those shims out at the end of the day and see that perfect, consistent gap, you know the floor is going to last twenty years. If you see a tight spot where the wood has already touched the wall, you know you’re in trouble. Use the right tools. Use plastic shims. Don’t let a five-dollar mistake ruin a five-thousand-dollar floor. Stick to the chemistry, follow the physics, and treat the subfloor like the foundation of a skyscraper. It might just be a floor to the homeowner, but to us, it is a piece of engineering. Use the shims, keep your lines straight, and for heaven’s sake, check the moisture levels in your slab before you even think about opening a box of planks.