Quartz countertops are a popular choice for many homeowners due to their durability, aesthetics, and low maintenance. However, proper installation is crucial to ensure quartz countertops last and perform well in your home. A common question that arises is whether quartz countertops need plywood underneath or if they can be installed directly over existing countertops or cabinets.
What is Quartz?
Quartz countertops, sometimes called engineered stone, are made from ground natural quartz crystals combined with polyresin, pigments, and other compounds. The result is a man-made material that is non-porous, stain resistant, and durable.
Quartz is an extremely hard mineral, rating 7 on the Mohs hardness scale. Only diamonds rate higher on the scale. This hardness makes quartz very resilient to scratches, chips, and cracks. Quartz countertops are about twice as strong as granite countertops.
Key properties of quartz:
- Extremely hard and durable surface
- Non-porous so resists staining
- Mold, mildew, and bacteria resistant
- Low maintenance
- Wide variety of colors and patterns
- Glossy smooth finish
Do Quartz Countertops Require Plywood?
Installing quartz over existing countertops is generally not recommended. Quartz countertops require a very solid base for support. Plywood is the preferred substrate in most cases when installing new quartz countertops.
There are a few reasons why plywood is recommended under quartz:
Prevents Cracks and Damage
Quartz is rigid and inflexible. Without a solid foundation, normal shifts in the home can cause stress that leads to cracking. The flexibility of plywood allows it to absorb subtle movements without affecting the quartz.
Plywood is also excellent at distributing weight evenly. Too much weight focused in one spot can crack quartz slabs. With plywood underneath, the weight disperses to avoid damage.
Provides a Smooth Base
Minor imperfections in the underlying cabinets can telegraph through quartz slabs resulting in uneven surfaces. Plywood creates an even, smooth layer to prevent this.
Adds Support
Quartz slabs are heavy, weighing approximately 15 pounds per square foot on average. A sturdy base is required to support the weight without sagging over time. Plywood sufficiently supports the quartz.
Allows Adjustments
Plywood can be built up, planed, or sanded to create a perfectly level surface for quartz installation. This allows adjustments even with slightly uneven walls or floors.
What Type of Plywood is Best?
High quality plywood is crucial for the best quartz countertop installation. Avoid using low-grade plywood, particle board, or MDF which can sag or deteriorate over time.
Marine Grade Plywood
Marine grade plywood is constructed with waterproof structural adhesives. It is more moisture resistant than other grades, which is beneficial for kitchens. Goes on smoothly and resists warping.
Hardwood Plywood
Plywood made from layers of hardwood, like birch or oak, provides the most durability and strength under quartz. It has a smooth surface and resists swelling from moisture.
Void-Free Plywood
Look for void-free plywood, which has no gaps within its layers. This creates an incredibly solid surface that won’t compress or move under the weight of quartz slabs.
In most cases, 3⁄4 inch thick plywood is recommended for quartz installations. Go with the highest grade, void-free plywood you can find and afford. Saving money on cheap plywood often leads to problems and regrets down the road.
Can You Install Quartz Over Existing Countertops?
Installing quartz over existing laminate, tile, wood, or other countertops often leads to problems or failed installations. Here are the main issues with installing over old countertops:
- Uneven support leading to cracks
- Telegraphed bumps and lines affecting appearance
- Insufficient strength for the weight of quartz
- Moisture issues leading to deterioration
In some cases, quartz may be installed over existing stone countertops like granite or quartzite. This is only possible if the existing stone is in perfect condition and completely level. The slab being installed must also be kept very thin.
Many installers caution against this practice as well, noting that any unevenness in the underlying stone can still cause issues with the new quartz. Small lippage where countertops meet or uneven settling may not be noticeable under the old stone but affects the new slab.
Can You Install Directly Over Cabinets?
Installing quartz directly over cabinet boxes is also not advised. Here are some of the potential problems with forgoing plywood:
- Cabinet boxes often have vertical seams between pieces that could transfer to the surface.
- Most cabinet wood is too thin and flexible.
- Natural wood movement will occur over time.
- No adjustments possible for unlevel cabinet fronts.
- Insufficient support for heavy slabs.
- Eventual compression of cabinet tops under weight.
While plywood is preferred, some alternative options to installing directly over cabinets include:
- Use thick hardwood boards instead of plywood. Hard maple works well.
- Add metal brackets for support under quartz near the edges and seams.
- Double up 3⁄4 inch plywood for 1 1⁄2 inches of substrate.
Keep in mind that skipping the plywood still carries risks. For best results, take the time to add a plywood sublayer before installing quartz.
Can You Use Other Substrates Besides Plywood?
While plywood is typically recommended, there are a few other substrate options that provide adequate support for quartz countertops:
MDF (medium density fiberboard)
MDF is smoother and less prone to swelling than regular particle board. Look for MDF thicker than 1 inch for quarts substrate. Avoid thin, cheap MDF.
Reinforced concrete
Concrete needs to cure fully and be reinforced internally or with rebar. This creates an incredibly solid subsurface for quartz.
Metal
Stainless steel in at least 16 gauge can support quartz. Aluminum is not recommended as it can compress under weight.
Tile backerboard
Cement boards like Durock can work as an underlayment for quartz installation. Not quite as solid as wood but offers moisture resistance.
Regardless of material, the key is selecting a substrate that is highly rigid, level, and smooth. Any flexing or unevenness in the underlayer will transfer to the countertop. Stick with 3⁄4” void-free plywood whenever possible.
Installation Process With Plywood
Installing quartz countertops over plywood is a fairly straightforward process:
- Remove existing countertops and clean surfaces thoroughly.
- Inspect cabinets for level and plumb. Make any adjustments needed to ensure cabinets are true.
- Cut 3⁄4” plywood to size and lay over cabinets, with edges fully supported.
- Secure plywood to cabinets through the backside of the cabinet boxes using construction adhesive and screws.
- Fill any seams or gaps in the plywood and sand smooth.
- Run a bead of silicone along the wall/cabinet connection.
- Set the quartz slab and check for level. Make any necessary adjustments.
- Apply adhesive on the plywood and set quartz into place fully seated.
- Clamp securely until adhesive cures fully. Wipe away any excess.
- Once cured, clean quartz and seal edges to complete installation.
Following this process with high quality plywood minimizes the risk of cracks or other damage occurring in the quartz. Taking the time to properly prepare the cabinets and install over plywood results in a beautiful and problem-free countertop that will last for many years.
FAQ About Plywood Under Quartz
Here are answers to some frequently asked questions about using plywood under quartz countertops:
Should I use pre-made plywood or make my own?
Pre-made plywood is easier to install and offers optimal smoothness and durability. Making your own risks uneven seams.
Can I use plywood under just part of the slab?
Plywood should support the entire slab for even support. Don’t use it in only part.
What thickness should the plywood be?
3⁄4” plywood is the standard quartz substrate. 1/1⁄2” can be used for extra strength.
Do I need to secure plywood to the cabinets?
Yes, plywood must be screwed into cabinets every 6 inches around the perimeter and every 12 inches inside.
Can I use construction adhesive instead of screws?
Adhesive alone is not enough. You need mechanical fasteners like screws too.
Is it okay if my plywood overhangs the cabinets a bit?
Yes, having plywood overlap the cabinet sides an inch or two is perfectly fine and provides more support.
Can I install my farmhouse sink before the quartz?
Best practice is to install sinks after quartz so the slab can be fully supported without cutouts.
How smooth should the plywood be for quartz?
It should be sanded until perfectly smooth to prevent any bumps telegraphing through the quartz.
Is pressure treated plywood okay?
Avoid pressure treated plywood as it can warp. Use marine or void-free plywood instead.
How long should I let plywood cure before installing quartz?
Let plywood cure fully for 72 hours minimum to prevent any shrinkage or movement under the quartz.
Conclusion
Quartz countertops are a smart investment that pay off for years in beauty, durability, and easy maintenance. Paying close attention to the substrate and installing over high-quality plywood prevents many problems and disappointments.
While plywood adds some cost up front, it is worthwhile to provide full support and prevent future cracking or maintenance issues. Follow best practices for preparing and securing the plywood, and your quartz countertops will shine for decades.
With a stunning quartz surface installed over proper plywood, you can enjoy cooking and entertaining with confidence your countertops will perform as desired. Take the time to do it right, use recommended materials, and consult an experienced installer if needed. Your beautiful new quartz countertops will be a lasting feature to enjoy.