Backer board is a rigid panel—typically cement, fiber-cement, foam, or glass-mat gypsum—installed behind tile or stone to create a flat, stable, moisture-resistant surface that resists movement and prevents damage to finishes, making it a better choice than drywall or plywood in wet or high‑traffic areas such as showers, tub surrounds, floors, and backsplashes.
What is a backer board?
Backer board is a purpose-made panel installed under or behind tile and stone to provide a flat, strong, and moisture-resistant base. Unlike drywall or plywood, it doesn’t swell or break down when exposed to moisture, and it helps spread loads so tiles and grout don’t crack. It’s commonly made from cement or fiber-cement, foam with cementitious skins, or glass-mat gypsum designed for tile.
Why it matters in home projects
Tile and stone rely on the surface beneath them. Movement, flex, or moisture in that surface leads to cracked grout, loose tiles, and hidden water damage. Backer board reduces those risks by:
- Creating a solid, level substrate for tile and stone.
- Resisting moisture and mold better than drywall or wood-based products.
- Adding stiffness over floors and walls so finishes last longer.
Backer board is not a structural panel; it supplements a proper subfloor or wall framing and supports the finish materials that people see and use every day.
Common uses and applications
- Showers and tub surrounds
- Bathroom and laundry room floors
- Kitchen backsplashes and countertops
- Fireplace surrounds (check product heat ratings)
- Entryways and mudrooms
- Exterior facades for thin stone or stucco (only products rated for exterior use)
Types of backer board
- Cement backer board (CBU): Portland-cement and sand with fiberglass reinforcement. Strong, dimensionally stable, and highly water resistant, but heavy and dusty to cut. It is not waterproof by itself; use a waterproofing membrane in wet zones.
- Fiber-cement board: Similar performance to cement board but slightly lighter and a bit easier to cut. Also water resistant, not waterproof.
- Foam tile backer board: Extruded or expanded polystyrene core with cementitious or fiberglass skins. Very light, easy to cut with a utility knife, and many products are waterproof when seams are sealed. Often used in showers to save weight and time.
- Glass-mat gypsum tile backer: Moisture-resistant gypsum core with fiberglass mats. Suitable for damp areas like bathrooms and backsplashes; typically requires a waterproof membrane in wet areas such as showers. Lighter and easier to handle than cement boards.
- Magnesium oxide (MgO) board: Dense, moisture-resistant, and fire-resistant panel used in some regions. Always follow the specific manufacturer’s instructions, as performance and fasteners vary.
Typical thicknesses: 1/4 inch for floors and countertops, 1/2 inch for walls and ceilings to match drywall thickness and provide better stiffness. Some foam boards come in thicker options for building shower benches, niches, and curbs.
How to choose the right product
- Location and moisture exposure: For showers and tub surrounds, choose cement, fiber-cement, or foam boards and plan for a waterproofing membrane. For backsplashes in dry kitchens, glass-mat gypsum can be fine.
- Weight and handling: Foam boards are light and fast to cut. Cement boards are rugged but heavy and dusty.
- Waterproof vs. water resistant: Cement and fiber-cement resist water but absorb it; they need a sheet or liquid membrane in wet zones. Many foam boards are waterproof once seams and penetrations are sealed with the manufacturer’s system.
- Compatibility: Make sure your thin-set mortar, fasteners, and waterproofing products are approved for the backer board you select.
- Thickness and transitions: Use 1/2 inch on walls to align with drywall, or plan a trim transition. Use 1/4 inch on floors over a sound subfloor.
Installation tips that lead to long-lasting tile
- Substrate first: Ensure the framing or subfloor meets deflection requirements for tile. Tighten subfloor screws and add blocking where needed.
- Bed floor panels in thin-set: On floors, spread thin-set mortar with a 1/4 inch trowel under the backer board before fastening. This fills voids; it’s not glue. Follow with corrosion-resistant screws at the spacing your board specifies (often 6–8 inches on center). Do not skip the mortar bed.
- Use the right fasteners: Use manufacturer-approved backer board screws with corrosion resistance; avoid drywall screws. Keep fasteners slightly recessed but do not break the surface.
- Stagger joints: Offset backer board seams from the seams in the subfloor and from each other. Leave 1/8 inch gaps between sheets and at walls.
- Tape and mud seams: Embed alkali-resistant fiberglass mesh tape in thin-set mortar over all joints and corners. This ties panels together and reduces cracking.
- Waterproof where needed: In showers and wet areas, apply a sheet membrane or liquid-applied waterproofing over the board, including carefully treating seams, corners, and fastener holes per manufacturer guidelines.
- Cut safely: Score-and-snap cement or fiber-cement boards with a carbide scoring tool; use a jigsaw with a carbide blade for curves or holes. Wear eye protection and a dust mask. Foam boards cut cleanly with a utility knife.
- Plan penetrations: Seal plumbing penetrations and niches with the approved sealant or membrane accessories for your system.
Maintenance
Backer board is buried behind finishes, so it doesn’t need routine care. The best “maintenance” is preventing water from getting to it:
- Keep grout and caulk in good condition, especially at corners and changes of plane.
- Address leaks quickly.
- In showers, ensure the waterproofing layer remains intact if you remodel or drill holes for accessories.
Common mistakes to avoid
- Using drywall (even greenboard) in shower wet zones without a waterproof membrane.
- Skipping the thin-set bedding under floor backer board.
- Using drywall screws that rust or snap.
- Failing to tape and mud seams with alkali-resistant mesh tape.
- Not waterproofing foam or cement boards in showers.
- Aligning backer board seams directly over subfloor seams.
- Setting tile immediately over flexible plywood without an approved underlayment.
- Forgetting movement joints at room perimeters and across large tiled areas.
Related terms
- Subfloor and underlayment: Layers below flooring; backer board is one kind of underlayment for tile.
- Thin-set mortar: Cement-based adhesive used to bond tile and to bed floor backer boards.
- Mastic (pre-mixed adhesive): For dry areas only; avoid in showers and wet zones.
- Uncoupling membrane: A plastic membrane (e.g., dimpled sheets) that separates tile from subfloor movement; an alternative to backer board on floors.
- Waterproofing membrane: Liquid or sheet barrier applied over backer board in wet areas.
Practical examples
- Tiling a shower: Use 1/2 inch cement or foam backer board on studs. Tape seams with alkali-resistant mesh and thin-set. Apply a sheet or liquid waterproofing membrane over the entire shower, seal penetrations, then set tile with thin-set mortar.
- Bathroom floor upgrade: Over a sturdy plywood subfloor, spread thin-set, lay 1/4 inch cement board, fasten at 6–8 inches on center, tape and mud seams, then set tile. Leave a 1/4 inch gap at walls for movement and cover with base trim.
- Kitchen backsplash: For a dry backsplash, install glass-mat gypsum or use existing drywall if sound and flat; for added durability, a thin foam backer board works well. Prime as required, then set tile with thin-set. Caulk at the countertop.
- Fireplace surround: Choose a cement or MgO board rated for the temperature range and follow clearances. Do not use foam boards near high heat unless specifically approved.
A well-chosen and properly installed backer board gives your tile or stone a stable, moisture-smart foundation—one of the key differences between a project that looks good on day one and one that still looks good years later.