Cement Board Over Drywall: DIY Install Guide You NEED

Cement Board Over Drywall: A DIY Installation Guide

This guide will walk you through the process of installing cement board over drywall. While it might seem unusual to install cement board on top of drywall, there are situations where it’s a viable option, especially in areas prone to moisture like bathrooms or kitchens. Remember to always consult your local building codes before starting any construction project.

Understanding When to Use Cement Board Over Drywall

Why Choose This Method?

Before diving into the how-to, let’s understand why you might consider this approach. Typically, you’d remove the drywall entirely. However, going over existing drywall can be appropriate in certain scenarios:

• Reinforcement: Existing drywall can provide a level surface to which you can attach the cement board, preventing sagging or unevenness, particularly on walls with studs spaced further apart than code typically allows.
• Sound Dampening: The added layer of drywall and cement board can provide some additional sound insulation.
• Less Demolition: You avoid the mess and labor associated with tearing down existing drywall.

When Not to Use This Method

It’s crucial to recognize when not to proceed.

• Mold or Water Damage: If there’s any evidence of mold or water damage behind the drywall, you MUST remove it completely and address the source of the moisture before proceeding with any installation. Covering it will only exacerbate the problem.
• Structural Issues: If the existing drywall is severely damaged or the studs behind it are compromised, adding another layer will not fix the underlying problem. You need to address the structural issues first.
• Increased Thickness: Consider how much thicker the wall will become and if this interferes with door and window trim, outlets, or other fixtures. You may need to adjust these elements.
• Significant Weight: Drywall already adds weight. Cement board is significantly heavier. Be certain your wall studs can handle the combined load. Consulting a structural engineer is recommended if you have any doubts.

Preparation and Tools

Proper preparation is key to a successful installation.

Gathering Your Materials

You’ll need the following:

• Cement Board: Choose a cement board suitable for your application (e.g., bathroom, shower, floor). Common thicknesses are 1/4" and 1/2".
• Cement Board Screws: These are specifically designed for attaching cement board and won’t corrode.
• Alkali-Resistant Mesh Tape: Used to tape the seams between cement board sheets.
• Thin-Set Mortar: Used to embed the mesh tape and create a smooth, waterproof surface. Choose a high-quality modified thin-set.
• Primer: A waterproof primer designed for cement board.

Essential Tools

• Measuring Tape: Accurate measurements are vital.
• Utility Knife: For scoring and cutting cement board.
• T-Square or Straight Edge: Ensures straight cuts.
• Drill/Impact Driver: For installing cement board screws.
• Cement Board Scoring Tool (Optional): Makes scoring the cement board easier.
• Mixing Bucket and Paddle: For mixing thin-set mortar.
• Notched Trowel: For applying thin-set mortar. Choose the correct notch size for the cement board you are using. Check the manufacturer’s recommendations.
• Grouting Sponge: For cleaning up excess thin-set.
• Safety Glasses: Protect your eyes from dust and debris.
• Dust Mask: Essential when cutting cement board.
• Gloves: Protect your hands from the thin-set mortar.

Preparing the Drywall Surface

1. Clean the Drywall: Remove any dust, dirt, or debris from the existing drywall using a damp cloth.
2. Repair any Damage: Patch any small holes or cracks in the drywall with joint compound and let it dry completely before sanding smooth. Make sure you use a quality drywall compound and let it dry and cure fully as per the manufacturer’s recommendation.
3. Locate Studs: Mark the location of the wall studs using a stud finder. These will be your primary anchoring points.

Installing the Cement Board

Cutting Cement Board

Cement board is usually scored and snapped rather than sawed, which reduces dust.

1. Measure and Mark: Measure the area where you need to install the cement board and mark the dimensions on the board.
2. Score the Board: Using a utility knife and a T-square, score the surface of the cement board along the marked line. Score deeply, making several passes. A cement board scoring tool will make this task easier.
3. Snap the Board: Place the scored board on a firm surface with the scored line facing up. Apply pressure on either side of the line until the board snaps cleanly.
4. Smooth the Edges: Use a rasp or utility knife to smooth any rough edges.

Attaching the Cement Board

1. Position the Cement Board: Align the cut cement board with the area you’re covering. Ensure it fits snugly. Leave a small gap (about 1/8 inch) between the cement board and any adjacent surfaces (walls, floors, ceilings). This allows for expansion and contraction.
2. Fasten with Screws: Using cement board screws, attach the cement board to the drywall and into the studs. Space the screws every 6-8 inches along the edges and every 8-12 inches in the field (middle of the board). Drive the screws flush with the surface of the cement board, but avoid over-tightening, which can strip the screws or damage the board.

Taping and Mudding the Seams

This step creates a waterproof barrier at the seams.

1. Apply Thin-Set: Using a notched trowel, apply a thin, even layer of thin-set mortar over all the seams between the cement board sheets. The notch size will be determined by the manufacturer’s recommendations for your cement board.
2. Embed the Mesh Tape: Press alkali-resistant mesh tape into the wet thin-set mortar, covering the entire seam. Use a putty knife to smooth the tape and remove any air bubbles.
3. Apply a Second Coat: Once the first coat of thin-set has dried (according to the manufacturer’s instructions), apply a second, thin coat of thin-set over the mesh tape, feathering it out to blend with the surrounding cement board surface.
4. Allow to Cure: Allow the thin-set to cure completely according to the manufacturer’s instructions. This may take 24-48 hours.

Priming and Finishing

1. Clean the Surface: Remove any dust or debris from the cement board surface.
2. Apply Primer: Apply a waterproof primer specifically designed for cement board to the entire surface. This will help improve adhesion of your chosen finishing material (tile, paint, etc.).
3. Finishing: Once the primer has dried, you can proceed with your chosen finishing material, following the manufacturer’s instructions for installation.

Example Table of Screw Spacing

Location	Screw Spacing
Edges (perimeter)	6-8 inches
Field (center area)	8-12 inches

Alright, you’re all set to tackle that cement board over drywall project! Get out there, get your hands dirty, and transform that space. Happy tiling!