When installing a quartz countertop, one of the key considerations is whether plywood is required underneath. Quartz is an engineered stone made from crushed quartz particles combined with resins and pigments. It is a popular choice for countertops due to its durability, aesthetics, and low maintenance. However, proper installation is crucial to ensure the countertop lasts. So does a quartz countertop need plywood or not?
What is Quartz?
Quartz countertops, also known as engineered stone countertops, are made from crushed quartz crystals combined with resins and pigments. The exact composition varies by brand and product line, but quartz countertops typically contain:
- 90-94% crushed quartz
- 6-10% polymer resins
- Pigments for color
The quartz crystals give the material its strength and rigidity, while the resins bind it together into a durable, non-porous surface.
Compared to natural stone like granite or marble, quartz offers superior durability and stain resistance, while providing an attractive appearance that resembles natural stone. Quartz requires very little maintenance and does not need to be regularly sealed like natural stone.
Brands such as Caesarstone, Cambria, Silestone and DuPont Zodiaq are among the leading quartz countertop manufacturers. There are many color and pattern options to choose from, including designs made to look like marble, concrete, granite and more.
Benefits of Quartz Countertops
There are several reasons why quartz countertops are so popular:
- Durability – Quartz is highly resistant to scratches, chips and cracks, making it ideal for heavy use in kitchens.
- Low Maintenance – Requires no sealing and easily resists stains. Simple cleaning with soap and water is all that is needed.
- Stain Resistance – The non-porous surface makes it highly resistant to stains from spills, oils and juices.
- Heat Resistance – Can withstand exposure to normal cooking temperatures from pans and baking dishes. However, direct high heat should be avoided.
- Aesthetic Appeal – Available in a wide array of colors and realistic stone-like patterns. Maintains a like-new appearance for years.
- Non-Porous – The dense surface does not harbor bacteria or need sealing.
- Customization – Can be fabricated into custom shapes and sizes. Edges can be finished smoothly or with an apron-front.
With this winning combination of practicality and beauty, it’s no wonder quartz has become a go-to choice for today’s kitchens and bathrooms.
Plywood Substrate for Quartz – Is It Required?
When installing quartz countertops, there are differing schools of thought regarding whether plywood should be used underneath or not. So does quartz really need plywood or can it be installed directly over cabinets?
Here are the key factors to consider:
Structural Support
Quartz is an extremely rigid, solid surface. For proper support, the substrate underneath needs to be equally rigid and sturdy.
Plywood is inherently rigid and resists flexing, bowing or distorting when weight and pressure are applied. This makes it well-suited for providing a stable foundation for quartz.
Without plywood, the countertop could potentially crack or separate over time if the underlying base cabinets lack adequate reinforcement. The plywood helps prevent movement and flexing.
Smooth Surface
Plywood creates a flush, even substrate. Quartz should be installed over a substrate with less than a 1/8” variation across 10 feet.
Plywood can be sanded completely smooth. In contrast, cabinet boxes may have uneven joints, staples and imperfections that could potentially telegraph through the quartz. The plywood creates an ideal smooth sub-layer.
Moisture Protection
Plywood forms a moisture barrier between the quartz and underlying wood cabinetry. This prevents any moisture issues leading to warping, cracking or deterioration of the cabinets.
Thickness
A 3/4” plywood deck provides an ideal thickness for securing quartz with a standard adhesive system. Thinner substrates may not provide enough depth for proper adhesion.
Installation Over Different Base Materials
Different materials require different approaches when installing quartz:
Existing Countertops
Quartz can be installed over most existing countertop surfaces with proper preparation. The old countertop must be intact, secure, level and provide adequate support.
For solid surface materials like an existing quartz or granite countertop, mechanically abrading the surface and applying a bonding primer will allow the new quartz to bond securely.
Laminates should be carefully evaluated to determine if they provide enough structural rigidity. A new plywood overlay may be recommended prior to installing quartz.
Tile countertops are generally not suitable for installing quartz over, as the grout lines will likely crack the new quartz slab. Tiles should be fully removed before installation.
Wood Cabinetry
Wood cabinets should always receive a new 3/4” plywood deck prior to quartz installation. This is considered essential by most fabricators and installers.
The plywood must be structurally fastened to the cabinets using screws, construction adhesive and joints that align with cabinet framing underneath. Allow a 1/8” gap between sheets to allow for expansion.
Some laminated cabinet surfaces may also need extra reinforcement underneath to prevent any flexing or movement after the heavy quartz is installed.
Metal Frames
Some modular assembly metal frame systems are designed specifically to accept quartz countertops without plywood. In these systems, the metal provides ample structural support.
The frame must be entirely level, secure and engineered to bear the weight of the quartz without any twisting or flexing. Consult frame manufacturer specifications.
Concrete
For concrete substrates, the surface must be fully cured, level, and prepped by grinding/sanding to ensure adhesion. If the concrete is sealed, it will need to be mechanically abraded. New poured concrete requires a full 28 day cure time before installing quartz.
Is Plywood Absolutely Necessary?
Based on all the factors above, the general consensus is that plywood provides critical benefits for proper quartz installation and long-term performance. However, there are some exceptions:
- If the existing substrate is completely level, secure, rigid and able to properly support the quartz, plywood may not be mandatory. This might apply to some metal or existing quartz countertops.
- Some proprietary cabinet systems are engineered to directly receive quartz without plywood. However, the cabinet box construction must provide ample reinforcement against sagging or distortion when weight is applied.
- Very short runs of quartz may be able to omit plywood on some cabinet installations, especially areas that will see limited use or impact. However, this is somewhat controversial and some fabricators maintain plywood should always be used regardless of length.
Unless the existing substrate can meet the criteria above, plywood remains highly recommended by most quartz fabricators and manufacturers. The small added cost and effort of adding plywood helps prevent future problems and ensures the longest possible lifespan for your investment.
Plywood Specifications
If using plywood, what specific type should be used? Here are the recommended specifications:
- Minimum Thickness – 3/4 inch nominal
- Plywood Grade – Exterior or CDX. Avoid softwood plywood.
- Surface – PS1-rated for smooth appearance
- Voids – No voids on surface or edges
- Veneer Repairs – Limit repairs to dark wood plugs only
- Joints – 4-6 inches between sheets, aligned with cabinet framing
- Fasteners – Coarse thread screws every 6-8 inches along all joints and internal framing
- Construction Adhesive – Applied generously before screwing and clamping
- Gap – 1/8 inch gap maintained between plywood sheets
Following these specifications helps provide the most stable and flush substrate for quartz installation. Anything less may compromise performance. Going above and beyond 3/4” thickness is not considered necessary however.
Installation Process Over Plywood
Once suitable plywood has been fastened securely to the cabinets, the general installation process includes:
- Ensuring the plywood surface is completely clean. Sanding may be needed to smooth any rough areas.
- Making precise measurements and cuts for the quartz slab(s) that will cover the surface. Leaving a 1/8” gap between the wall and quartz edge.
- Dry fitting the quartz pieces and verifying all seams are tight.
- Cleaning the plywood and applying primer.
- Applying adhesive evenly across the surface, strictly following adhesive specifications. Not allowing any gaps in adhesive coverage.
- Carefully lowering the quartz slab into place, aligning correctly and pressing firmly to set. Rolling entire surface to ensure optimal adhesion and squeeze out any trapped air bubbles.
- Allowing adhesive to fully cure according to manufacturer instructions before use and cleaning. Typically 24-48 hours.
- Cleaning any residual adhesives and performing final finishing around edges and seams.
With proper plywood preparation and installation, you can enjoy durable, flawless quartz countertops that will last for decades in your home.
Frequently Asked Questions
Does quartz need 3⁄4” plywood or can I use 1⁄2”?
3⁄4” plywood is highly recommended as the minimum thickness needed to provide adequate structural support and prevent any flexing or movement that could crack quartz slabs. 1⁄2” plywood is generally considered too thin and more prone to warping or telegraphing imperfections from below. The small added cost of 3⁄4” plywood helps ensure proper installation and lifespan.
What screws should be used to install the plywood?
Use coarse thread wood screws that are at least 1” to 1-1/4” long. Make sure they bite into the cabinet framing securely. Space screws every 6-8” around the perimeter and interior framing. Avoid fine thread screws which have less holding power.
Can I use OSB, particle board or MDF instead of plywood?
No, traditional plywood is far superior for underlayment under quartz. Other wood products lack the proper structural rigidity, moisture resistance, and fastener holding ability. Always use PS1-rated plywood minimum.
Should there be a front overhang when installing quartz over plywood?
It’s recommended to have a 1/8” overhang in front for quartz installation. This prevents any chance that the plywood edge could otherwise be visible. The quartz can then fully cover the plywood edge.
Do I need to use adhesive between the plywood and cabinets too?
Yes, use construction adhesive generously when fastening the plywood to the cabinets. This creates a stronger bond and minimizes any potential for movement or squeaking down the road. Screws alone may loosen over time without adhesive.
How long does the adhesive under the quartz need to cure?
Adhesive cure times vary by specific product used, but usually need 24-48 hours before stressing the countertop or cleaning. Consult adhesive instructions. Some take 72 hours for full cure strength. Insufficient curing can undermine bond strength.
Conclusion
In summary, quality plywood should be used under quartz countertops in most situations, with very few exceptions. Plywood provides essential structural reinforcement, moisture protection, and a flush surface needed for proper quartz installation and performance. While not absolutely mandatory in all cases, the benefits of adding 3⁄4” exterior-grade plywood far outweigh any marginal extra costs or effort involved. Following the recommendations above helps ensure your beautiful quartz countertops will last for the long run. With proper installation, you can enjoy a durable, low-maintenance countertop surface that maintains its beauty for decades.