Project Guide

How to Install a Cement Board

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1
What is Backerboard?
A man installs cement backerboard.
  • Cement board, often referred to as backerboard, is a thin layer of concrete with fiberglass mesh on both sides. It’s available in sheets of many sizes, with 3-foot by 5-foot sheets being the most common for most projects. 
  • Whenever you’re laying tile on a wood subfloor, you need to first install cement backerboard to prevent leaks and water damage that could harm your flooring and the structure of your home. Unlike wood or drywall sub-surfaces, cement backerboard will not rot, warp or grow mold and mildew when exposed to water. That’s the main reason they’re a go-to choice for permanent installations like this.
  • Another reason you’ll need to work with cement backerboard is that it’s never safe to install new tiles over existing tiles. You also can’t safely install tile flooring over plywood subfloor or onto an unprotected wood floor without serious risk of rot, water damage, warping and mold.
2
Plan and Mark the Backerboard Placement
A man measures a sheet of cement backerboard.

You have your needed materials and tools handy and now you’re ready to begin installing backerboard like a pro. The first step is proper planning and placement. Here’s how you can tackle step one correctly: 

  • Plan the placement of the cement backerboard sheets.  
  • Plan so the joints in the backerboard won’t line up with the joints in the subfloor. The backerboard should completely cover the subfloor joints. 
  • Do a dry run and lay the backerboard down to fully cover the area you plan to tile. 
  • If needed, cut the backerboard to fit in small spaces or around obstructions. Score the board with a utility knife, then cut it using a jig saw with a carbide blade. 
  • Snap a grid to the subfloor using a chalk line to mark your cement board placement.  
3
Spread Mortar
Mortar spread out on a floor before cement backerboard is installed.
  • Use thin-set mortar when installing cement backerboard. Thin-set dries slowly, allowing you to fix and readjust backerboard sheets as needed. (Thin-set mortar ingredients are cement, fine sand and a water retaining agent. It's used to bond ceramic or porcelain tile and cement backerboard.)
  • With the smooth side of a 1/4-inch notched trowel, spread enough mortar for one sheet. 
  • Ridge the mortar with the notched side of the trowel.  
4
Lay the Backerboard Sheets
A man lays cement backerboard sheets over mortar.
  • While the mortar is still wet, place a sheet of backerboard onto it. 
  • Repeat this process, spreading mortar and laying backerboard one sheet at a time. 
  • Leave a 1/4-inch gap between the backerboard and the wall. 
  • Leave a 1/8-inch gap between each backerboard sheet. The corners and edges of the sheets should not touch one another. 
5
Secure Backerboard with Screws
Man secures cement backerboards with screws.
  • Drill screws through the backerboard sheets every 6 to 8 inches. 
  • Position the screws at least 1/2-inch, but no more than 2 inches, from the edge of the sheet. 
  • Drive the screws in so that the heads are slightly below the surface of the backerboard sheet. 
6
Fill the Joints with Mortar
 A person fills backerboard joints with mortar.
  • Use the flat side of your notched trowel to fill the joints between the backerboard sheets. 
  • Smooth the mortar so it extends about 1 1/2 inches on each side of the joint. The extra mortar will make it easier to bed the fiberglass tape. 
7
Cover the Joints with Fiberglass Tape
A person covers the backerboard joints with fiberglass tape.
  • Cut the fiberglass seam tape to length with the thin side of your trowel. 
  • Push the tape into the joint mortar. 
  • Once the tape is embedded, scrape off any excess mortar.
8
Cover Fiberglass Tape with Mortar
A person covers fiberglass tape with mortar.
  • Spread a thin layer of mortar over the fiberglass tape with the flat side of your trowel.  
  • Feather the edges of the mortar across each board so the surface is as smooth as possible. 
9
Allow Mortar to Set
A bathroom with tile floors and walls.

Let the mortar fully set and dry for 24 to 48 hours before tiling.

10
Cement Board Tips
A man installing cement board on a shower wall.

Here are a few things to keep in mind when learning how to install cement board. 

  • Cement board can be used just as effectively on walls, such as showers and bathtub surrounds, as it is on floors. Doing this will require that you remove the existing surface material before installing backerboard over the studs. 
  • Cement board can also be used on countertops when laying tile; however, the process will involve using an extra sheet of backerboard.  
  • Never use standard screws when installing cement board. It will corrode the screws. Always use cement board screws. 

Learning how to install cement board on floors is an important step when you’re preparing to lay tile. Once installed, the backerboard will protect wood and plywood subfloors from water damage and create a smooth, solid foundation for your tile. Ready to get the tools and materials you need? The Home Depot delivers online orders when and where you need them.