Do You Need to Waterproof Cement Board in a Shower

Cement board has long been a trusted substrate for tiled showers and wet rooms because of its strength and dimensional stability in damp environments. But one question still causes confusion for homeowners and contractors alike: Do you need to waterproof cement board in a shower?

The answer is yes. Cement board is moisture resistant, but it is not waterproof. Without a dedicated waterproofing layer, moisture can pass through seams, fasteners, and the board itself, eventually reaching the structure behind it.

At ProLux Materials, we supply genuine PROVA waterproofing systems designed to create a continuous barrier against liquid water and vapor, helping ensure long-term durability and code-compliant shower installations.

Why Cement Board Alone Is Not Waterproof

Cement board is manufactured from cementitious materials reinforced with fibers, giving it excellent strength and stability even when wet. However, the material remains porous. When exposed to continuous water, as in a shower environment, it can absorb and transmit moisture through capillary action.

Over time, that moisture may migrate into wood framing, insulation, or other adjacent materials, increasing the risk of swelling, decay, or mold growth.

Even when tile and grout appear intact, water can still pass through microscopic pores, grout joints, and movement cracks. Cement board itself does not break down easily, but persistent moisture behind the tile layer can weaken adhesive bonds and create hidden moisture accumulation within the wall assembly.

To prevent this, the shower wall system must be sealed with a continuous waterproofing layer before tiling. This layer should be properly installed, vapor resistant when required, and compliant with ANSI A118.10 and ASTM E96 standards. PROVA-based shower assemblies are designed to meet these requirements when installed according to manufacturer guidelines.

How Moisture Moves Through Shower Walls

Water in a shower does not move only downward. Moisture travels laterally and as vapor. During hot showers, steam diffuses through grout joints and microscopic openings, where it can condense behind the tile layer if the assembly is not properly sealed.

Even small discontinuities such as unsealed seams, corners, or penetrations can allow moisture to enter the wall system. While each individual exposure may be minor, the cumulative effect over time can become significant. Proper waterproofing protects both the visible tile surface and the concealed structural components behind it.

Waterproofing Options for Cement Board Showers

There are two reliable and code-compliant ways to turn cement board into a fully waterproof shower substrate.

1. Surface-Applied Waterproofing Membranes

Surface-applied membranes include both sheet membranes and liquid-applied membranes. When installed correctly, both approaches are widely accepted in the tile industry.

Liquid-applied membranes rely heavily on achieving the correct wet and dry film thickness and allowing proper curing time. When applied consistently and according to manufacturer specifications, they can perform very well. However, installation variability can affect performance.

Sheet membranes reduce that variability by providing a factory-controlled thickness. In a PROVA system, PROVA Mat Waterproofing Membrane is bonded directly to the cement board using polymer-modified thin-set mortar, forming a continuous waterproof layer across the entire surface.

Best practice is to first treat all joints, seams, and corners. Preformed PROVA Waterproofing Corners are used at inside and outside corners, while PROVA Joint Waterproofing Membrane Band is used to reinforce seams and transitions. Once these details are completed, PROVA Mat is installed over the field areas, creating a uniform, flat, and fully sealed surface ready for tile.

2. Using a Waterproof Backer Board Instead of Cement Board

An alternative approach is to replace cement board entirely with a waterproof backer board such as PROVA Board Plus+ Waterproof Tile Backer Board. This board features an extruded polystyrene (XPS) core with reinforced facings that are inherently waterproof and vapor resistant.

Because the board itself does not absorb moisture, only seams, fasteners, and penetrations require detailing. PROVA Board Plus+ is significantly lighter than cement board, easier to cut, and does not wick water, making it especially attractive for new builds and full renovations.

Many professionals prefer foam-core boards for their reduced weight, faster installation, and simplified moisture management, while cement board remains a widely used and accepted substrate when properly waterproofed.

Do You Need to Waterproof Cement Board in a Shower?

Yes. If cement board is used in a shower, it must be protected with a waterproofing system. While the board itself is moisture resistant, it is porous and will absorb and transmit moisture when exposed to continuous water.

Alternatively, a waterproof backer board such as PROVA Board Plus+ provides built-in waterproofing and vapor resistance, eliminating the need for a full surface membrane over the field areas. Only joints, fasteners, and penetrations require sealing, reducing installation steps and long-term moisture risk.

In practice, you either waterproof the cement board or choose a substrate that is already waterproof. Both approaches can produce durable, long-lasting showers when executed correctly, but integrated PROVA systems reduce variability and potential failure points.

Table: Comparison of Shower Substrates and Waterproofing Requirements

Common Waterproofing Mistakes to Avoid

• Skipping waterproofing entirely: Cement board alone does not provide a waterproof barrier.
• Inconsistent liquid membrane application: Uneven thickness can create weak points.
• Improper corner detailing: Corners should use preformed waterproofing corners, not improvised patches.
• Mixing incompatible systems: Components from different manufacturers may not bond correctly.
• Ignoring vapor control: Steam showers require vapor-rated membranes to limit condensation within walls.

Code Compliance and Industry Standards

Most building codes require waterproofing in wet areas. Cement board alone does not meet ANSI A118.10 requirements for bonded waterproof membranes.

PROVA Mat Waterproofing Membrane and PROVA Board Plus+ are tested for waterproof performance, vapor transmission, and bond strength. Using approved systems helps maintain compliance with tile, grout, and adhesive manufacturer warranties and simplifies inspections for permitted projects.

The ProLux Advantage

ProLux Materials is more than a supplier. As a verified PROVA distributor, we provide system-based guidance, product compatibility support, and access to genuine materials stored and shipped under controlled conditions.

Whether you are waterproofing a residential shower or a commercial wet area, our team helps you select the correct combination of PROVA boards, membranes, and accessories for durable, code-compliant results.

Frequently Asked Questions

Is cement board waterproof by itself?

No. Cement board is moisture resistant but porous. It must be protected with a waterproofing system in wet areas.

Are liquid waterproofing membranes acceptable?

Yes, when installed correctly and at the proper thickness. Sheet membranes reduce installation variability, but both approaches are industry accepted.

Do I need to waterproof screw holes and joints?

Yes. Every seam, corner, and fastener should be sealed to maintain a continuous barrier.

Can tile be installed directly over PROVA Board?

Yes. PROVA Board Plus+ is inherently waterproof and tile ready. Only seams and penetrations require sealing.