How Thick Plywood Under Quartz Countertop

Installing quartz countertops can transform the look of your kitchen or bathroom. Quartz is an engineered stone that is non-porous, stain-resistant, and durable, making it a popular choice for countertops. When installing quartz, proper support underneath is crucial to prevent cracking or sagging. Here is a detailed guide on how thick plywood you need under quartz countertops.

Introduction to Plywood Under Quartz

Quartz countertops need to be installed over a very solid, stable substrate. Plywood is commonly used under quartz to provide this critical support. The thickness of the plywood will depend on the dimensions of your countertop span.

The plywood creates a robust underlying framework to sustain the weight of the quartz without bending or flexing. It levels out the cabinet base and compensates for any unevenness. Proper plywood thickness also prevents any cracks from forming in the quartz.

What Thickness of Plywood is Required?

The thickness of plywood required under quartz countertops depends on the length of the countertop span:

  • For counter spans up to 6 feet: Use 3/4 inch (23/32 inch) thick plywood. This is sufficient for most standard sized countertops.
  • For counter spans 6 – 10 feet: Use 1 inch thick plywood for additional rigidity.
  • For counter spans 10 – 12 feet: Use 1 1/4 inch plywood for maximum strength.

These recommendations are provided by quartz manufacturers and countertop installation experts. The longer the span of the countertop, the thicker the plywood should be to prevent sagging.

Why Proper Plywood Thickness Matters

Choosing the right plywood thickness is important for several reasons:

  • Prevents cracking and damage to the quartz countertop.
  • Provides a stable and level base for installation.
  • Handles the weight load without bending.
  • Reduces vibration and noise.
  • Compensates for any unevenness in the cabinet base.
  • Improves structural integrity for longer-lasting countertops.

Insufficient plywood thickness can lead to cracks, weak seams, and even collapse under the heavy weight of quartz slabs. It is better to err on the side of caution and use thicker plywood for durability.

Plywood Specifications

For best results, choose high-quality plywood to go under your quartz countertops. Avoid particle board or low-grade plywood as they will sag over time.

The plywood should be:

  • Made with Baltic Birch wood or other high-density wood.
  • Moisture resistant to avoid warping.
  • Free of knots or voids on the surface.
  • Sanded smooth on both sides for even installation.
  • At least 3-ply construction for strength.

5-ply plywood is ideal for spans over 8 feet. Follow manufacturer instructions on which direction to place the sheets (with the wood grain).

Preparing and Installing the Plywood

Proper preparation is key to ensure the plywood provides optimal support:

  • Cut plywood sheets to 3-4 inches wider than cabinet base.
  • Secure plywood to cabinets with screws into studs.
  • Make sure edges of plywood are fully supported.
  • Fill any cracks or gaps with wood putty.
  • Sand surface smooth.

Carefully follow all manufacturer guidelines for any additional steps in preparing the plywood sub-base before installing the quartz countertop.

Alternative Options to Plywood

While plywood is the most common substrate, there are some alternative options:

  • MDF boards: Moisture resistant MDF provides rigidity for shorter spans.
  • Reinforced plastic: Does not warp but not as rigid as plywood.
  • Metal frame: Aluminum or steel frames offer sturdy support.
  • Hardwood: More expensive than plywood but lasts longer.

Plywood remains the preferred choice in most cases due to its strength, versatility, and cost effectiveness. Consult a countertop specialist to see if an alternative material will work for your specific project.


Choosing the proper thickness of plywood is vital for providing underlying support for quartz countertops. Follow the recommendations of using:

  • 3/4 inch plywood for up to 6 feet span.
  • 1 inch plywood for 6-10 feet span.
  • 1 1/4 inch plywood for 10-12 feet span.

Thicker plywood prevents cracking and reinforcement. Prepare the plywood properly and install according to manufacturer guidelines. With adequate support, your beautiful quartz countertop will last for many years!

Frequently Asked Questions

What thickness of plywood is best for a standard 25″ depth countertop?

For a typical 25” deep countertop span of 6 feet or less, 3/4” plywood is usually sufficient and recommended by most manufacturers and installers. The 23/32” inch actual thickness provides adequate rigidity.

Can I use a thinner 1/2″ plywood?

You can, but 1/2” plywood may not offer enough support, especially on longer spans, and may ultimately lead to cracking or other damage in the quartz. It is better to be safe and use the recommended 3/4” plywood.

What if my cabinets are uneven?

If the underlying cabinets are uneven, you can use plywood up to 1” thick and tap shims under the plywood as needed to level it out before installing the quartz. The extra plywood thickness helps bridge any gaps.

Is OSB board ok instead of plywood?

Oriented strand board (OSB) is not recommended as it can swell and delaminate when exposed to moisture. Plywood is a superior choice for moisture-resistance and strength in supporting quartz countertops.

How is plywood attached to the cabinets?

The plywood substrate should be securely screwed into solid blocking within the cabinets, not just the cabinet face frames. Gluing the plywood to the cabinets also provides added stability.

Can I install quartz directly over old laminate countertops?

It is not advisable to install quartz over old laminate. The laminate surface is not stable enough. It is better to remove laminate and install quartz over new plywood substrate.


Properly installing plywood under quartz countertops provides the necessary structural framework for durable, long-lasting countertops. Following the thickness guidelines based on span lengths will prevent cracking and other damage. Be sure to use high-quality moisture-resistant plywood secured firmly to the cabinets below. With the right plywood base, you can enjoy beautiful, stable quartz countertops that will serve you well for many years.