How to Install Floor Cement Board Underlayment for Tile

Installing tile flooring can transform the look and feel of any room. However, proper tile installation requires a solid, stable substrate. Cement board provides an ideal underlayment for tile floors, offering durability, stability, and water resistance. Learning how to install cement board properly ensures your tile flooring will last for years to come.

Selecting the Right Cement Board

Cement board, also known as backerboard, is made of cement and fiberglass mesh. It is designed not to expand or contract making it the perfect underlayment for tile. There are a few types of cement board to choose from:

Fiber cement board – The most commonly used cement board. It is waterproof and mold/mildew resistant. Brands include HardieBacker and PermaBase.

Fiber-reinforced cement board – Offering extra strength and increased resistance to damage. Brands include HardieBacker 500 and Permabase Plus.

Mold-resistant cement board – Has additives to further prevent mold and mildew growth. Brand name is DenShield.

Waterproof cement board – 100% waterproof throughout the entire board. Brand names include Schluter Kerdi-Board and James Hardie HardieBacker Cement Board HydroDefense.

Consider your needs and choose the proper cement board accordingly. For most tile floor installations, standard 1/2″ fiber cement board is sufficient.

Gather the Right Installation Supplies

Installing cement board requires some special tools and materials. Be sure to have the following on hand:

  • Cement board – Have at least 10% more than you think you need to account for mistakes and trimming.
  • galvanized roofing nails or cement board screws
  • SERF strap or fiberglass mesh tape – For sealing seams
  • Thin-set mortar or mastic – Check if manufacturer recommends a specific type
  • Cement board saw or utility knife – For cutting
  • Drill with countersink bit – For pre-drilling holes
  • Safety gear – Gloves, eye protection, knee pads
  • Straight edge/T-square
  • Level
  • Tape measure
  • Pencil
  • Chalk line

Having these supplies ready makes the installation process go quicker and smoother.

Prep the Subfloor

Cement board must be installed over a clean, dry, and structurally sound subfloor. Here are some tips for proper subfloor prep:

  • Remove existing flooring if necessary – Tile, vinyl, carpet etc. must be taken up prior to cement board installation.
  • Clean thoroughly – Sweep and mop subfloor to remove all dirt, debris, grease etc.
  • Check for level – Use a straightedge or level to identify any low or uneven spots. Grind or fill areas as needed to achieve a flat surface.
  • Repair issues – Address any loose areas by renailing or gluing down. Fill cracks and holes with a floor leveler.
  • Moisture test – Use a meter to check for excess moisture. Take steps like sealing concrete or improving ventilation if moisture is too high.
  • Let dry – Allow any patching compounds or sealers to fully cure before installing cement board.

Prepping the subfloor takes time but it’s a critical first step. Ensure the subfloor is clean, dry, flat and structurally sound before moving on.

Plan the Layout

Taking a few minutes to map out your cement board layout saves a lot of frustration down the road. Here are some tips:

  • Draw a diagram – Sketch the shape and dimensions of the floor space. Indicate where seams and edges will fall.
  • Mark reference points – Identify permanent structures like walls, cabinets, pipes etc. Mark their locations on your diagram.
  • Determine seam placement– Seams should not align with tile joints. Place them in the center of tiles whenever possible.
  • Account for borders – If using perimeter border tiles, adjust seam placement accordingly.
  • Calculate materials – Use your layout diagram to determine how many boards you need. Have at least 10% extra to allow for errors and cutting.

Planning and marking your cement board layout keeps the boards properly aligned and makes tiling go smoother. Take the time to carefully measure and map it out.

Set the Starting Point

Once your subfloor is prepped and layout planned, choose the starting point. Here are some tips:

  • Start in the center of the room – This ensures you don’t end up with narrow boards around the perimeter.
  • Begin on the longest wall – Helps minimize cross seams.
  • Use edges and walls – Align factory edges with walls whenever possible.
  • Mark reference lines – Snap perpendicular chalk lines to follow when fastening the boards.
  • Dry fit boards – Place and test fit the boards before actually fastening them.
  • Check dimensions – Ensure the board lengths and layout lines maintain proper space from walls.

Establishing the right starting point keeps the rows straight and prevents mistakes. Take time to dry fit boards and check measurements before fastening them down.

Fasten the Cement Board Panels

With your starting point set and materials ready, it’s time to fasten the cement board panels to the subfloor. Follow these best practices:

  • Fasten every 8 inches – Use cement board screws or galvanized roofing nails placed every 8 inches across the panels.
  • Pre-drill holes – Prevent breakage by pre-drilling screw holes with a countersink bit.
  • Space from edges – Keep fasteners at least 3/8” from panel edges and 2” from seams.
  • Drive fasteners flush – Set screws or nails flush with the board surface without over-driving.
  • Check straightness – Use straight edges and levels to ensure evenness as you progress. Shim any low spots.
  • Stagger seams – Offset all cement board seams by at least two rows from subfloor seams beneath.
  • Account for movement – Leave 1/8” gaps at walls and cabinetry to allow for expansion.

Properly fastening the panels prevents cracks, lifts and other issues. Use the recommended pattern and spacing for your fasteners.

Cut the Cement Board

For custom sizing around walls and fixtures, the cement board must be cut. Here are some tips:

  • Use straight edges – Clamp guides in place to ensure straight cuts.
  • Cut with score and snap – Use a utility knife to score boards then snap upwards. Make multiple passes until it snaps.
  • Use shears for curves – For rounded cuts, use cement board shears. Go slowly on the curves.
  • Cut backerboard outside – Produces less mess indoors.
  • Wear safety gear – Gloves, goggles and masks are a must when cutting cement board.
  • Allow space at walls – Leave 1/8” gaps between board and walls unless using perimeter fastening.

Measure carefully and take it slow on the cuts. Change utility knife blades frequently for best results. Practice on scrap pieces first if needed.

Tape and Mud the Seams

Once the cement board panels are fastened down, the seams must be sealed. Here’s how:

  • Vacuum up dust – Remove all dust and debris from surface of boards before applying tape/mud.
  • Apply SERF tape – Embed special fiberglass mesh tapes into thinset mortar over each seam.
  • Apply mortar over seams – Use a trowel to spread additional thinset over the fiberglass tapes.
  • Feather out edges – Taper the mortar edges so there is a smooth transition over the seams.
  • Fill corners – Spread mortar over all inside and outside corners. Embed corner bead reinforcing if desired.
  • Let cure fully – Allow mortar to cure per manufacturer instructions before tiling or walking on it.

Taping and mudding the seams prevents cracks by reinforcing and leveling the cement board joints. Take your time to do it right.

Seal Exposed Areas

For moisture protection, any cut edges or fastener penetrations in the cement board should be sealed:

  • Vacuum and wipe down – Remove dust and debris from the board surface before sealing.
  • Apply silicone or waterproofing – Brush or roll on silicone, waterproofing membranes or acrylic sealants.
  • Coat exposed edges – Seal all cut board edges, especially around fixtures.
  • Seal holes – Brush sealant into any gaps around pipes, drains, posts etc.
  • Let cure fully – Allow sealant to fully dry before moving to next steps.

Properly sealing exposed cement board prevents moisture getting in that could compromise adhesion or lead to mold growth under the tile.

Install Tile and Grout

Once the cement board underlayment is fully prepped, you’re ready to start tiling:

  • Dry fit tiles – Do a test layout with spacers to ensure desired placement and fit.
  • Apply mortar – Use a notched trowel to spread thinset mortar over the cement board. Work in sections.
  • Set tile – Press tiles into the mortar, twisting for full coverage. Use spacers between. Check alignment often.
  • Grout tiles – After tile mortar cures fully, apply grout between tiles. Spread diagonally across the joints.
  • Clean haze – Wipe grout haze from the tile surface with a sponge and clean water.
  • Seal grout – Apply grout sealer once fully cured to prevent staining and increase water resistance.

With proper prep and installation of the cement board underlayment, you can expect your beautiful new tile flooring to last for decades. Enjoy!

Tips for Success

Installing cement backerboard like a pro takes practice and patience. Keep these tips in mind:

  • Carefully follow all manufacturer’s instructions for preparation and installation.
  • Wear proper safety gear at all times when handling cement board. The dust and fibers can be harmful.
  • Take time to accurately measure and lay out boards to minimize waste and seams.
  • Use cement board screws or galvanized roofing nails specifically designed for fastening cement board.
  • Pre-drill all screw holes with a countersink bit to prevent breakage.
  • Make sure joints are properly taped and mudded to prevent cracks.
  • Allow thinset mortar and grout full cure times before walking on or grouting tiles.
  • Arrange tile layout to avoid placing tile edges over cement board seams wherever possible.

With the right supplies, careful preparation, and good installation technique, you can install cement backerboard like a professional. Soon you’ll be enjoying an incredibly durable and water-resistant underlayment for your beautiful new tile flooring.

Frequently Asked Questions

Can I use drywall instead of cement board for tile floors?

No, drywall should never be used as an underlayment for tile floors. It lacks the durability and water resistance needed. Cement board is specially engineered for use under tiles.

What thinset mortar should I use to install cement board?

Standard dry-set mortar or a latex-modified thinset is recommended. Check manufacturer instructions as some require specific mortar types.

How thick should cement board underlayment be for floor tile?

A thickness of 1/2″ is standard and suitable for most residential floor tile installations. Use 5/8” for high traffic commercial applications.

Should the seams in cement board align with tile grout lines?

No, tile and cement board seams should be offset from one another by at least a few inches whenever possible. Aligning them can lead to cracking.

Can I use cement board outside for a patio or balcony?

Yes, exterior-grade cement board is designed to withstand weather and can be used for outdoor tile installations. Be sure to use galvanized fasteners.

What is the maximum span for floor joists under cement board?

With 1/2″ cement board, floor joists for new construction should be spaced no more than 16″ on-center according to manufacturer recommendations.

How soon can I walk on the installed cement board?

Allow the thinset mortar used to install the cement board to fully cure per manufacturer specs before walking on it, typically 24 – 48 hours.


Installing cement backerboard properly requires careful prep work, the right tools, and good technique. But the end result is a durable, water-resistant underlayment that will provide many years of service beneath your beautiful new tile flooring. With some planning and patience, you can install tile on cement board successfully. Just focus on good subfloor preparation, proper layout, careful seam taping, and allowing materials full cure times. Your tile installation over cement board will be strong, stable and built to last.