How To Use Floor Shims for Leveling a Wood Subfloor

How To Use Floor Shims for Leveling a Wood Subfloor

Is it time to replace an old floor in your home? Do you have a sloping floor or uneven spots on the floor right now?

When remodeling an older home, it's common to find squeaky floors that are uneven and not level. Ideally, you want a subfloor that's both flat and level. In some homes, it's not possible to get the floor completely level, and that's OK if it is "in-plane" or flat. A floor can be level from one wall to the other, but it's not flat if there are dips or high spots in the middle.

When any rigid hardwood flooring covers a dip or hump, it will most likely come apart and cause gaps. One plank can rise above the next one creating a "lip" and tripping hazard.

Easiklip solid oak floors are the easiest hardwood floor to install. But before you do, you must check if the subfloor is flat and level. A stable and level subfloor is critical to having a beautiful, straight, and squeak-free floor. We will share some builder's tricks to level a subfloor depending on the cause and condition.

Wood shims are the remedy to fix most floors that are out-of-level or out-of-plane.

Two Ways to Determine if the Floor is Uneven, And Why

Have you heard of the "Marble Method" to determine if the floor is out-of-plane? It's a fool-proof method and works by dropping a few marbles around the floor to see where they roll. You can quickly tell if you have a slope or if there are high and low spots (called "heaves" and "dips" in the flooring trade) within the flooring area.

The other method is to use a 6 or 8-foot carpenter's level to check the floor's slope. If you see a gap under the level, that's a dip. If the level rocks slightly, that means there is a heave under part of it. You'll see a slight gap between the level and the floor at the high point.

Possible Causes of the Floor's Unevenness

Before you can fix the level problem, you need to find the root cause of it. Typically, age and settling cause an unlevel floor. Other causes of an uneven floor include:

  • Structural issues and foundation problems
  • A cracked or damaged floor joist
  • A rotted sill plate, the beam that the floor joist sits on
  • Delaminated subfloor wood

To find the exact cause, you may need to get under the floor if there is a crawlspace or basement. Otherwise, you'll need to take up part of the subfloor to examine the structure below. 

If you find termite damage, cracks in the foundation, or rotted joists, call in an expert to repair them. It's critical to fix these issues before attempting to level the subfloor. 

Construction or Foundation Issues

Occasionally, shoddy construction or a weak foundation can be the cause of an unlevel floor. Before there were uniform building codes, some builders saved money using undersized lumber or spaced the joists too far apart when building the floor joist system. These shortcuts resulted in weak, sagging, or bouncy floors. 

The best way to fix the spacing issue is to have a flooring professional "sister" the joists. It's a technique of bolting the new joists onto the old ones and using the same sill plate.

Especially in older homes, the foundation footings can sink, or the basement walls can crack and shift due to soil and water pressure. It's best to have a specialist inspect the foundation if you think there may be a problem. 

Even after repairing the foundation, there's no guarantee that the floors will be level again, but at that point, shimming the joists may be a feasible option.

Methods to Level a Wood Subfloor

Once you've uncovered where the problem areas are on the floor, here are some remedies you can do yourself.

Plane a Bowing Joist

If you performed the marble test and they rolled away from a certain point, it probably means that a floor joist bowed up. Bowing creates a heaved area on the floor. 

Fixing this is not too difficult. It requires removing the subfloor over the bowed joist and then planing or shaving down the high part until it's level across the top. Mark it by popping a chalk line along the top edge of the bowed joist from end to end. The chalk will highlight the high point. Then you can use a plane or sander to remove the excess wood.

Replace the Delaminated Subfloor

Most modern wood subfloors consist of plywood or OSB (Oriented Strand Board). They are sturdy building products unless water has a chance to soak in. Then they expand and delaminate, causing heaves to form. 

The first step is to eliminate any water issues, such as leaky plumbing. Next, you'll need to replace any warped or delaminated subflooring with new material. 

Self-Leveling Underlayment

You can apply a cement-based self-leveling floor compound for sloping or multiple dips in the floor. The liquid slurry settles into low spots or the low side, creating a level surface. 

Because of the weight and the liquid, it's a technique best used on cement floors on the ground floor or basement. Installing hardwood over an unlevel concrete floor requires a leveling compound. If you have a concrete floor, you can learn more in our blog, "How To Level An Unleveled Floor With Floor Leveling Compound & More."

For minor floor leveling, using wood floor shims is the easiest way to level floors.

What is A Flooring Shim, and When Do You Use It?  

If you are not familiar with wood floor shims, they are thin, tapered wedges that look like doorstops. Made of wood or plastic, builders use them to make slight adjustments to doors, windows, and floors to make them level. 

wood floor shims

Flooring contractors use wood floor shims by wedging them between a subfloor and floating floor or the joist and subfloor to make them level and flat.

Can You Shim Hardwood Floors?

Shimming a hardwood floor is a common practice. Anytime a wood floor meets or transitions to another type of floor such as carpeted floor or ceramic tile floors, like tile, there is a difference in the two thicknesses. Installers must shim the hardwood floor to match the height of the tile floor to make it look good and avoid a tripping hazard. Shimming can create a seamless transition and avoid a threshold in the middle of a room. 

For example, Easiklip is a 3/4" solid oak floor. If you plan to install an Easiklip floor next to the tile, it may be lower than the tile, and you may need to add shims. This article will help you, "Floor Transition Strips & All Your Options for Wood Floor Transitions."

How To Shim a Hardwood Floor

Leveling a wood subfloor or hardwood floor depends on the extent of the problem. If the entire floor slants to one side, you'll need to cut long shims from 2 x 4s. They attach to the original floor to support a new subfloor that creates a level surface across the room. This video shows one way to cut long shims for flooring with a circular saw. 

Another method of leveling a wood subfloor with swells or many dips is to make plywood flooring shims under the new subfloor. This video shows you "How to level sagging floor in an old house using shims."

If the subfloor is in reasonably good shape but has dips in several areas, you can use wood veneer or door skins to level the areas thoroughly. This video demonstrates the technique.

Before installing an Easiklip floating floor, it's better to ensure that the sub-floor is flat and level. As you install the oak floorboards, keep checking to ensure they stay level. You can install thin hardwood floor shims under individual boards to make them tight. It's best to staple or glue the shim to the subfloor to keep it from moving or squeaking in the future.

5 Tips and Reminders

  1. Before installing an Easiklip floor, check the subfloor. If nail or screw heads are sticking up, drive them deeper. 
  2. Check that the subfloor or joists are not warped or delaminated anywhere. Repair or replace as necessary.
  3. Check the floor in both directions for levelness and uneven spots, working in 1-foot increments.
  4. Use a minimum of a 6-foot carpenter's level. 
  5. A floor doesn't need to be perfectly level, especially in older homes, but it must flat, so the finished floor looks good and won't squeak.

Installing an Easiklip floor on a flat, level subfloor will make it look like a professional installation. It will last for decades. Learn more about how to install Easiklip solid oak flooring here.


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