If you are about to tile a floor, a shower, or a backsplash and you are wondering what goes under the tile, the answer in most cases is cement backer board for tile. This rigid, cement-based panel gives your tile a stable, water-durable surface to bond to, and it is the single most important reason a tile job lasts twenty years instead of cracking and failing in two. I have torn out too many jobs where someone tiled straight over drywall or particleboard to skip this step, so let me walk you through doing it right.
What Cement Backer Board Is
Cement backer board, often called cement board or by brand names like Durock, is a sheet of portland cement reinforced with fiberglass mesh or wood fibers, typically sold in 3×5-foot panels. Unlike drywall, it does not dissolve or lose strength when it gets wet, and it will not grow mold as a food source. That water tolerance is why it became the standard substrate in bathrooms and kitchens.
Just as important, it is dimensionally stable and rigid. Tile is brittle and unforgiving; if the surface underneath flexes, the grout cracks and tiles pop loose. A properly installed cement board panel over a solid subfloor or framing gives tile the stiff, consistent base it needs to stay bonded. It is not itself waterproof, an important distinction I will come back to, but it is water-durable, meaning it can get wet repeatedly without degrading.
The Major Brands Compared
Three names dominate the aisle. Durock, made by USG, is a true cement board with a slightly gritty surface and mesh scrim on both faces. It is heavy, extremely tough, and my go-to for shower pans and floors where durability matters most. WonderBoard, from Custom Building Products, is similar, another traditional aggregated cement board that is rugged and takes abuse well.
HardieBacker, from James Hardie, is a fiber-cement product rather than a pure aggregated cement board. It is a bit lighter, cleaner to cut, and has a smoother surface that many installers prefer for floors and walls. All three are proven substrates. My general rule is that any of them beats the alternatives, so pick based on availability and whether you value the extra toughness of Durock or the easier handling of HardieBacker.
Thicknesses: 1/4 Inch Versus 1/2 Inch
Backer board comes in two common thicknesses, and choosing correctly matters. The 1/4-inch panels are meant for floors and countertops, where the board is fully supported by a solid subfloor beneath it. You are not relying on the board for strength there; you are relying on it to provide a bondable, stable surface, so thin is fine and it keeps the finished floor height down.
The 1/2-inch panels are for walls, tub surrounds, and shower enclosures, where the board spans between studs 16 inches on center and needs the extra rigidity to resist flexing. Using 1/4-inch on a wall between studs is a mistake because it can flex and crack the tile. There is also a 5/8-inch product for specific applications, but for the vast majority of home projects you will use 1/4-inch on floors and 1/2-inch on walls.
Where You Use It and Why It Beats Greenboard
Cement backer board belongs anywhere tile meets moisture or foot traffic: shower walls, tub surrounds, bathroom and kitchen floors, mudrooms, and backsplashes. In dry areas you can technically tile over regular drywall, but for anything that gets wet, backer board is the correct call.
This is where the comparison to green board comes in. Green board is just moisture-resistant drywall, gypsum with a treated paper facing. People used it in showers for years and it failed constantly, because once water gets past the tile and grout, the gypsum core turns to mush and the tile falls off the wall. Cement backer board for tile has no gypsum to soften and no paper face to feed mold, so it survives the wet-dry cycling of a shower where greenboard rots. Modern codes reflect this, and greenboard is no longer accepted as a tile backer in wet areas in most jurisdictions.
How to Install Cement Backer Board
Installation follows a consistent sequence whether you are doing a floor or a wall. On a floor, start by troweling a bed of thinset mortar onto the subfloor with a 1/4-inch notched trowel. This layer is not glue so much as a void-filling bed that eliminates gaps and prevents the board from rocking. Set the panel into the wet thinset, then fasten it with backer-board screws or roofing nails every 8 inches, keeping fasteners about 2 inches from panel edges and 3/8 inch from the edge minimum.
Leave a 1/8-inch gap between panels and at the walls to allow for movement. Once the boards are down, tape every seam with 2-inch alkali-resistant fiberglass mesh tape and skim it with thinset so the joints become continuous and rigid. On walls, you skip the thinset bed and screw the panels directly to the studs, then tape the seams the same way. In a shower, you then add a waterproofing membrane or liquid such as RedGard over the board, because the board itself is not waterproof.
Cutting and Fastening Tips
Cutting cement board is easier than people fear. For straight cuts, score the panel deeply with a carbide-tipped scoring knife along a straightedge, then snap it over the line and cut the back mesh, exactly like scoring drywall but with more passes. For openings and curves, a carbide hole saw or a jigsaw with a carbide-grit blade works, though it is dusty, so wear a respirator and cut outside if you can.
Use the correct fasteners. Backer-board screws have a specially designed head that sits flush without tearing the surface, and hot-dipped galvanized roofing nails also work. Do not use ordinary drywall screws, because they can corrode and their heads are the wrong shape. Drive fasteners flush, not countersunk deep enough to crush the panel. Getting the fastening right is what keeps the board flat and rigid, which is the entire point of installing it.
What It Costs
Cement backer board is inexpensive relative to the tile job it protects. A 3×5-foot sheet of 1/4-inch board runs roughly $11 to $16, and a 1/2-inch sheet runs about $13 to $20, depending on brand and region. A typical bathroom floor might need two or three sheets, and a tub surround three to five, so the board for most projects totals well under $150.
Add a bag of thinset, a roll of mesh tape, a box of backer-board screws, and a waterproofing membrane for wet areas, and your substrate materials still usually land under $250 for a bathroom. That is cheap insurance. Spending a little more time and money on cement backer board for tile up front is what separates a tile installation that lasts decades from one you will be tearing out and doing over far too soon.
Cement Board Versus Other Substrates
Backer board is not the only option, and it helps to know where it fits among the alternatives. Foam tile backers like Wedi and the Schluter Kerdi-Board are lightweight, waterproof out of the box, and cut with a utility knife, which is why some pros have switched to them for showers. They cost considerably more per sheet, often two to four times the price of cement board, and they are less rigid, so they are not my first pick for a floor that takes heavy point loads.
Uncoupling membranes such as Ditra serve a different purpose entirely. They go over a solid subfloor to isolate the tile from subfloor movement and are excellent on floors, but they do not add the rigidity you need on a wall between studs, so they complement backer board rather than replace it everywhere. For most homeowners doing a bathroom on a normal budget, cement board remains the best balance of price, rigidity, and durability, with foam boards worth the premium only when weight or install speed is the priority.
Common Installation Mistakes to Avoid
The failures I get called to fix almost always trace back to a handful of shortcuts. The most common is skipping the thinset bed under a floor panel and just screwing the board down dry, which leaves voids that let the board flex and crack the tile. The second is forgetting to tape and mud the seams, so each panel moves independently and the grout lines telegraph and crack right along the joints.
Another frequent error is treating the board as if it were waterproof. Cement board absorbs and passes water; it simply does not degrade doing so. In a shower you must add a membrane over it or behind it, or you will get moisture in the wall cavity even though the board itself survives. Finally, people crowd the fasteners too close to the edges and crack the panel corners, or they use plain drywall screws that rust and fail. Respect the edge spacing, use the right screws, tape every seam, and bed your floor panels, and cement backer board will do exactly the job it is famous for.
Prefer to explore on your own?
See how new flooring looks in your actual room in seconds — free, no signup.
Try the free Floor Studio →
