Do we have to put HardieBoard under the quartz countertops?

What is HardieBacker Cement Board?

HardieBacker cement board is a premium tile backing board made from Portland cement and coated fiberglass mesh on both sides. It is designed to provide a stable, water-resistant layer for tiling projects. Key features of HardieBacker include:

  • Waterproof and moisture-resistant
  • Will not deteriorate or delaminate under prolonged water exposure
  • Highly durable and impact resistant
  • Provides excellent support for tiles and stones
  • Allows for thin-set mortar application directly onto the board

HardieBacker is often used as a subsurface for tile installations in wet areas like bathrooms and kitchens. It prevents moisture from seeping into the subfloor below and provides a solid foundation for the tile.

Do You Need HardieBacker Under Quartz Countertops?

In most cases, HardieBacker cement board is not required under quartz counters. Here are some points to consider:

  • Quartz strength – Quartz slabs are engineered to be extremely strong and durable. At 3000 psi, quartz has nearly triple the flexural strength of granite. The quartz material itself does not require additional support.
  • Level substrate – More important than cement board is ensuring the substrate underneath is level. Small imperfections can be accounted for with the mortar bed.
  • Manufacturer guidelines – Most quartz manufacturers do not specifically require cement board under their material. Always check the installation guide for the brand you choose.
  • Thin application – Quartz counters are attached with a thin layer of adhesive rather than mortar. The thinner bond line does not necessitate the added support.
  • Moisture resistance – Since quartz resists water and moisture, the waterproof properties of HardieBacker become less important.

When to Use HardieBacker under Quartz

There are some instances where HardieBacker can be beneficial under quartz:

  • If the substrate has large imperfections, the cement board can fill gaps and create a more level surface.
  • For a flooring application like a bathroom, the waterproof properties help protect the subfloor.
  • If installing quartz on an alternative substrate like plywood, HardieBacker prevents moisture damage.
  • In a kitchen or other high-traffic area, it adds an extra layer of strength and durability.
  • For thin or tiled quartz, the cement board provides stability if the slabs will be cut into pieces.

Ultimately, the decision depends on the specifics of your installation and substrate. Many pros opt to skip the HardieBacker since quartz can be installed directly on cabinet boxes, plywood, and other materials.

Installation Tips if Using HardieBacker

If you do decide to use HardieBacker, follow these best practices:

  • Carefully follow all manufacturer instructions for proper selection and installation.
  • Cut boards to size using a straight edge and carbide-tipped scoring knife. Avoid ragged edges.
  • Make sure the substrate is cleaned, prepped and level before laying the HardieBacker.
  • Use corrosion-resistant screws designed for cement board when fastening to framing.
  • Overlap seams at least 1/4 inch and cover with mesh tape.
  • Fill any gaps, holes, and seams with thin-set mortar.
  • Consider applying a waterproofing sealer like RedGard to all seams and junctions.

With the right precautions, HardieBacker cement board will provide a stable foundation for your new quartz countertops. But in many cases, you can install quartz directly over plywood, existing counters, and other substrates without the added layer. As always, consult the manufacturers’ guidelines before beginning the project.

FAQs About HardieBacker Under Quartz

Q: Is HardieBacker better for quartz than plywood?

A: Not necessarily. Plywood provides sufficient strength and stability for most quartz installs. The main advantage of HardieBacker is its waterproof properties in wet areas.

Q: Can I install quartz directly on particle board?

A: Particle board alone is not recommended – it can swell from moisture over time. Add a layer of 3/4″ exterior grade plywood on top first before installing the quartz.

Q: What thickness of HardieBacker should I use under quartz?

A: The most common thickness is 1/4″ or 1/2″, which provides adequate support. Go with 1/2″ for flooring or larger slab installs.

Q: Should I use Pre-Merged Thin-Set Mortar with HardieBacker and quartz?

A: Yes, apply a layer of thin-set mortar to the surface of the HardieBacker before laying the quartz to help bond the materials together.

Q: Can I tile directly over HardieBacker under the quartz?

A: Yes, as long as the tile is adhered with a quality polymer-modified mortar and proper tiling techniques are followed. The HardieBacker provides a stable surface.

Q: How is adhering quartz different than adhering tile?

A: Quartz uses a thinner bead of flexible adhesive, while tile uses a thicker layer of mortar. With quartz, precise seams are less important since you install large uniform slabs.


Installing HardieBacker cement board beneath your new quartz countertops is an option but not always required. For most standard installs over plywood or existing counters, the engineered quartz material does not need additional support. HardieBacker is more beneficial for wet areas like bathrooms or tiled quartz to prevent moisture damage. Follow manufacturer guidelines, prep your substrate appropriately, and your beautiful new quartz counters will provide many years of service.