Installing quartz countertops can greatly upgrade the look and feel of your kitchen. With their sleek, modern aesthetic and durability, quartz countertops are a popular choice for many homeowners. However, proper installation is key to ensuring your countertops last for years to come. One important consideration is selecting the right type of plywood substructure to support the weight of the quartz.
Introduction
When installing quartz countertops, you need a solid foundation that can handle the weight. Quartz is a heavy material, weighing approximately 15 pounds per square foot on average. The substructure must be able to bear this weight without sagging or warping over time. Plywood is commonly used to provide this critical support system. However, not all plywood is created equal when it comes to structural integrity. Selecting the proper plywood foundation is an important first step in any quartz countertop installation.
In this article, we will discuss the key factors to consider when selecting plywood for under quartz countertops. We will overview the different grades and types of plywood to help determine the best choice for your specific project. With the right information, you can feel confident you are providing the necessary reinforcement for a successful, long-lasting quartz countertop installation.
Choosing the Correct Plywood Thickness
When selecting plywood for under quartz countertops, thickness is an important consideration. A 3/4-inch plywood substructure is typically recommended to provide adequate strength and durability. While 1/2-inch plywood may seem thick enough, it is more prone to sagging or warping under the weight of quartz over time. The extra 1/4-inch thickness of 3/4-inch plywood makes a significant difference in providing a stable, rigid base.
For standard countertop depths of 25-26 inches, a single sheet of 3/4-inch plywood is usually sufficient. For deeper countertops or overhangs, you may need to double up the plywood to achieve 1 1/2 inches of total thickness. This provides extra reinforcement to prevent flexion. Consult the quartz manufacturer’s recommendations for the required thickness based on your specific project parameters.
It is better to err on the side of a thicker plywood substructure when possible. The minimal additional cost of 3/4-inch over 1/2-inch plywood is well worth it for the enhanced strength and stability it provides as the foundation under quartz.
Evaluating Plywood Grades
Plywood comes in different grades, which indicate the quality of the wood veneers used. Higher grades have fewer knots or defects and provide greater structural integrity. When choosing plywood for under quartz countertops, you generally want to select the highest grade possible.
CDX grade plywood is the minimum recommended for countertops. It has a solid C-grade face with minimal knots, and the inner plies are exterior-grade. While budget-friendly, CDX may be more prone to swelling and warping under moisture.
Marine-grade plywood is a top choice for under quartz countertops. The faces and core plies are B-grade or better, meaning there are few to no knots. It is designed to withstand moisture in boat construction and holds up well indoors too. Although more expensive, marine plywood provides exceptional stability under heavy quartz countertops.
BB or CC plywood grades are other options that offer good quality and performance. They have intermediate knotting that shouldn’t affect strength significantly. Talk to your countertop provider about the ideal plywood grade for your specific application and budget.
Using Exterior Glue Plywood
In addition to grade, the glue used in plywood manufacture also impacts its structural performance. Exterior grade adhesive makes the plywood more water resistant and stable. This prevents the plies from delaminating under the stress of heavy countertops. Exterior glue plywood is strongly recommended for under quartz installations.
With exterior adhesive, the plywood can better handle minor moisture penetration or spills that occur in kitchens. Exposure to some moisture will not immediately cause the plywood to crack or swell. Look for plywood stamped “Exterior” or “Exposure 1” to ensure exterior-grade adhesive was used.
Selecting the Best Plywood for Quartz Countertops
Here are some top plywood choices to consider for under quartz countertops:
- Marine Grade – Optimal performance; highly resistant to warping, twisting, and moisture damage
- BB Grade Exterior Glue – Excellent strength and stability; moderately priced
- CC Grade Exterior Glue – Very good performance for most residential uses
- CDX Exterior Glue – Most economical option that still provides adequate support
A 3/4-inch thickness is recommended for any of these plywood options to give the best reinforcement under heavy quartz countertops. Avoid using particleboard, MDF, or other inferior materials unable to withstand prolonged weight and moisture. Paying a little more for quality plywood now will pay off in the long run with stable, warp-free countertops for decades.
Alternatives to Plywood Substructures
While plywood is most common, there are some alternative substructure options to consider:
- MDF: Moisture-resistant MDF provides good support. MDF edges may swell slightly with moisture over time.
- Reinforced fiberboard: Composed of wood fibers set in resin, this material resists swelling and has good screw-holding ability.
- Aluminum: Extremely rigid and moisture-resistant, but requires metal fabrication expertise.
Plywood remains the first choice for most quartz countertop installations. But MDF, fiberboard, or aluminum may work for some applications, such as lower-use areas. Consult your countertop provider for advice on the best substructure options.
Installing the Plywood Foundation
Once you select the appropriate plywood type and thickness, proper installation is critical:
- Cut plywood 1/8” smaller than the base cabinet to allow for expansion.
- Secure plywood to cabinets tightly with screws every 6 inches along the perimeter and every 12 inches in the field.
- Make sure seams are tightly fitted. Misalignments cause instability.
- Fill any seams with adhesive to prevent flexing and cracking.
- Carefully follow all manufacturer instructions for fastening quartz to plywood.
Taking the time to correctly install the plywood substructure prevents problems down the road after countertops are installed.
Conclusion
Choosing the right plywood is a key first step to successful quartz countertop installation. Select plywood with proper thickness (3/4” minimum), exterior glue, and a high grade such as Marine or B-grade materials. Careful installation provides a solid foundation that keeps quartz counters flat and stable for decades of beauty and performance. Investing in quality plywood gives you the peace of mind that your countertops are built to last.
Frequently Asked Questions
What thickness of plywood is best under quartz?
3/4-inch plywood is the recommended minimum thickness for a quartz substructure. This provides adequate strength and rigidity. Thinner plywood can lead to instability and flexion under the weight of quartz over time.
Is CDX plywood good for under quartz countertops?
CDX plywood provides an economical option for quartz substructures. However, other grades like Marine and B-grade offer better moisture resistance and less risk of warping. CDX meets minimum requirements but higher grades deliver superior performance.
What’s the difference between Marine plywood and regular plywood?
Marine plywood uses higher quality veneers with few to no knots. It is designed to withstand constant moisture when used for boat decking. These attributes make it an excellent choice for the quartz substructure against damage from minor kitchen moisture over time.
Can I use particle board under quartz countertops?
Particle board is not recommended for under quartz. It does not have the structural integrity to support the weight of quartz without sagging or cracking over time. Plywood provides much better screw holding ability and resistance to flexion and moisture.
Is underlayment the same as plywood for quartz countertops?
Underlayment is a generic term for any material installed under flooring or countertops. Good quality 1/2” or 3/4” plywood is a common underlayment used under quartz. But “underlayment” does not define the thickness or grade needed for optimal performance with quartz.