Do You Need Plywood Under Quartz Countertop?

Quartz countertops are a popular choice for kitchen and bathroom remodeling projects due to their durability, low maintenance, and stylish appearance. However, proper installation is key to ensuring your quartz countertops last for years to come. One common question that arises is whether you need to install plywood under quartz countertops. The short answer is yes, plywood provides an essential supportive base layer for quartz countertops. Read on to learn why plywood underlayment is recommended for quartz counters, what thickness to use, and how to install it correctly.

Why Plywood is Recommended Under Quartz

There are several important reasons why installing plywood under quartz countertops is advised:

Provides a Stable Base

Plywood creates a stable, solid surface to adhere the quartz slabs to. Without plywood, the quartz could crack or warp over time as the cabinets and walls shift slightly. The plywood underlayment creates a uniform base that will minimize any irregularities or movement in the cabinetry that could otherwise transfer up through the countertop.

Prevents Moisture Damage

Plywood forms a moisture barrier between the quartz and cabinet box. If spills seep through the quartz into the cabinetry, it can cause the substrate to swell and damage the counters. The plywood will protect the cabinet integrity and prevent moisture issues.

Allows Secure Attachment

The plywood gives the quartz installer a solid material to drill and screw into to securely adhere the countertop. Attaching directly to cabinetry is not as sturdy. The plywood provides strength and stability to the attachment points.

Smooths Out Minor Variances

Plywood can bridge any small gaps or unevenness where cabinets meet. This allows the quartz to be installed over a perfectly flat, seamless surface. Without plywood, any imperfections in the cabinetry would telegraph through the finished quartz countertop.

Adds Reinforcement

Because plywood is a rigid, dense material, it reinforces the countertop and prevents flexing or sagging, especially on longer spans or overhangs. This added structural integrity prevents cracking or weak spots in the quartz material.

Resists Impact Better Than Cabinets

Plywood stands up to impacts from falling objects or during installation better than cabinetry alone. Its durability provides a more scratch-resistant surface under the quartz.

So in summary, plywood is highly recommended as it stabilizes the base, resists moisture, secures the attachment, smooths inconsistencies, reinforces the structure, and withstands impacts. Properly installed plywood will extend the life of your quartz countertops for many years.

What Thickness of Plywood is Best?

For most standard quartz countertop installations, 3/4 inch plywood is ideal. Here is why 3/4 inch thickness is recommended:

  • Matches thickness of countertop – 3/4 inch plywood matches the thickness of a standard 1 1/4 inch quartz countertop with buildup. This even surface avoids lippage.
  • Provides adequate strength and rigidity – At 3/4 inch thickness, the plywood has enough density to prevent flexing while not being overly heavy.
  • Smooths over cabinet inconsistencies – The 3/4 inch builds up enough to bridge gaps up to 1/4 inch between cabinet sections.
  • Widely available – 3/4 inch plywood is a standard size sold at most hardware stores and lumber yards.
  • Cost effective – It balances sufficient thickness without the cost premium of 1 inch plywood.
  • Easier to handle and cut – Thinner than 1 inch plywood so requires less effort to install.

In some cases, 1/2 inch plywood may be sufficient, for example on a plateau with no seams or gaps in a small area like a laundry room. However, 3/4 inch plywood remains the recommendation for the majority of installations to provide adequate strength and stability. Only use 1/2 inch if the cabinetry underneath has no inconsistencies and the span is quite short.

How to Install Plywood Under Quartz Countertops

Installing the plywood properly is just as important as using it in the first place. Here are best practices for installing plywood underlayment for quartz counters:

Select Exterior-Grade CDX Plywood

Opt for CDX grade as a minimum, which is exterior plywood with a C-grade front, D-grade back, and X grade inner plys. The C face is smooth and defect-free for countertop installation. Use an 8ft x 4ft panel for easiest handling.

Check for Level and Make Any Adjustments

Ensure the cabinet boxes are level front to back and side to side before installing the plywood. Shim any high spots as needed to create a flat plane.

Cut Plywood to Size

Measure and cut the plywood to match your countertop layout. Leave a 1/8 inch gap between panels and use scarf joints where plywood meets. Seal edges with silicone caulk.

Secure Plywood to Cabinets

Use coarse-threaded drywall screws to secure the plywood to the cabinet every 6 inches along the perimeter and every 12 inches in the field. Avoid nailing.

Fill Any Gaps

Fill any small gaps between plywood panels or open screw holes with silicone caulk to create a seamless surface.

Sand Surface Smooth

Lightly sand the plywood surface to remove any splinters, high spots, or defects so the quartz slabs sit flush.

Clean Thoroughly

Vacuum and wipe down the surface to remove dust before applying adhesive for quartz installation.

Apply Adhesive

Apply a bead of construction adhesive designed for quartz around the perimeter and in a zig-zag pattern across the plywood.

Following these best practices for installing and preparing plywood underlayment will result in a smooth, level foundation for flawless quartz countertops that will last for many years of heavy use. Don’t skip this important step!

Frequently Asked Questions About Plywood Under Quartz

Should I use pressure treated plywood under quartz?

No, pressure treated plywood is not necessary under quartz counters. Standard CDX grade exterior plywood provides adequate moisture protection. Pressure treated plywood is harder to find, more expensive, and can potentially leach chemicals.

What about using MDF instead of plywood?

Plywood is preferable to MDF for underlayment under quartz. MDF (medium density fiberboard) swells easily with any moisture exposure. Plywood is more rigid and durable. MDF sags more easily on longer spans as well. Go with plywood for the best support.

Should I use construction adhesive and screws to attach the plywood?

Yes, use both adhesive and screws to attach the plywood. The adhesive bonds the plywood to the cabinet for stability. Screws provide mechanical fastening and account for any gaps in the adhesive coverage. Use coarse drywall screws and apply adhesive in a zigzag pattern.

Can I install the quartz directly on my existing countertops?

It’s best not to install quartz directly over your old countertop surface. The adhesives adhere best to a clean wood surface. Remove old laminate, tile, solid surface, or other countertops first and install the quartz on new plywood instead for optimal durability.

Is underlayment required for all quartz countertop installations?

In most cases yes, plywood underlayment should be used in quartz installations. The exceptions would be a plateau with no seams on extremely sturdy cabinetry. Using a full plywood layer is advised for strength, durability, and to smooth any imperfections.

How thick should the plywood be for a 10 ft long quartz countertop span?

For longer countertop spans exceeding 8 feet, use 1 inch thick plywood instead of 3/4 inch. The extra thickness prevents sagging or bowing under the weight of the slab. Longer spans require additional reinforcement that 1 inch plywood provides.

Can I get away with just using a strip of plywood where the seams are?

No, it’s best to install full plywood underlayment underneath the entire quartz countertop, not just at the seams. This prevents inconsistencies in height. The full plywood layer also reinforces the entire structure. Small strips can crack under weight.


Installing high-quality plywood underlayment is a crucial best practice when installing quartz countertops. The plywood provides essential support, structure, and moisture protection. A 3/4 inch CDX exterior grade plywood secured with adhesive and screws is ideal for most standard quartz installations. With diligent preparation and proper underlayment installation, your beautiful quartz countertops will last for many years of heavy daily use while maintaining their sleek, flawless appearance. Be sure to follow the recommendations outlined here for a successful quartz countertop installation.