Tile is only as good as what it’s stuck to, and in wet areas that substrate is almost never drywall — it’s concrete board, better known as cement backer board. Unlike gypsum, cement board doesn’t dissolve, swell, or feed mold when water reaches it, which is why every tub surround, shower wall, and tiled floor over wood framing calls for it (or an approved equivalent) by code and by every tile industry standard. This guide explains what cement board is, compares Durock, HardieBacker, WonderBoard, and the newer foam boards, and walks through thickness selection and installation the way tile setters actually do it.
What Cement Board Is
Cement backer board is a sheet good — typically 3×5 feet to keep bathroom handling manageable — made of Portland cement and aggregate, reinforced with fiberglass mesh faces (Durock, WonderBoard) or formed as a compressed fiber-cement composite (HardieBacker). Two properties define it: it is dimensionally stable when wet, and it provides the rigid, high-bond surface thinset mortar wants. One misconception to kill early: cement board is water-durable, not waterproof. Water passes through it without harming it. In showers, you still need a waterproofing layer — either a membrane behind the board (poly sheeting or felt lapped into the tub flange) or a topical membrane over it (RedGard, Kerdi, Hydro Ban). Skipping both is how “we used cement board” showers still rot studs.
The Big Three Brands, Compared
USG Durock Cement Board: The classic mesh-faced true cement board. Extremely tough, completely inert, the traditional choice for floors and heavy service. It’s the heaviest and roughest of the three — cutting releases more dust and the edges shred utility-knife blades — and its abrasive face is harder on hands. About $11 to $15 per 3×5 half-inch sheet.
James Hardie HardieBacker: Fiber-cement construction makes it lighter, smoother-faced, and far cleaner to score and snap, which is why DIYers and many pros default to it — it cuts almost like drywall. The 500-series (0.42 inch) suits walls and counters; 1/4-inch suits floors. It shouldn’t be used in areas subject to prolonged full immersion (pool interiors), and older non-“HydroDefense” versions want the same waterproofing layer as any board. About $12 to $17 per sheet. The HydroDefense variant ($20 to $25) builds waterproofing into the sheet itself — seams still need treatment.
Custom Building Products WonderBoard: Mesh-faced cement board similar to Durock, often the cheapest on the shelf ($10 to $13), widely used under floor tile. Same handling character as Durock: heavy, dusty to cut, indestructible in service.
| Board | Type | Weight (3×5, 1/2″) | Cutting | Best use |
|---|---|---|---|---|
| Durock | Mesh-faced cement | ~36 lbs | Score/snap, dusty | Floors, high-abuse walls |
| HardieBacker | Fiber-cement | ~30 lbs | Cleanest score/snap | Walls, counters, DIY floors |
| WonderBoard | Mesh-faced cement | ~36 lbs | Score/snap, dusty | Budget floors |
| Foam board (Kerdi/GoBoard/Wedi) | XPS/polyiso composite | 7–12 lbs | Utility knife, clean | Showers, waterproof-in-one |
Foam backer boards (Schluter Kerdi-Board, Johns Manville GoBoard, Wedi) deserve their line in the table: waterproof through the core, one-tenth the weight, knife-cuttable — and two to four times the price ($40 to $70 per equivalent sheet, plus sealant for seams). For a shower renovation where speed and guaranteed waterproofing matter, they’re often worth it; for a 200-square-foot floor, cement board wins on cost.
Thickness and Where Each Belongs
- 1/2-inch: Walls and ceilings — it matches drywall thickness for flush transitions and spans studs 16 inches on center. Use it for tub surrounds, showers, and fireplace surrounds.
- 1/4-inch: Floors and countertops only — over a subfloor, the board isn’t structural; it’s a stable bonding layer. Using 1/4-inch on studs is a code and warranty violation; it will flex and crack tile.
- Floor stackup rule: Tile floors want a minimum 1-1/8-inch total subfloor (typically 3/4-inch plywood or OSB plus 1/4-inch or 1/2-inch backer) with deflection under L/360. Bouncy floors crack grout no matter what board you buy.
Where does regular or moisture-resistant drywall remain correct? Everywhere tile isn’t getting wet — backsplashes and powder-room walls tile fine over drywall. The dividing line is direct, repeated water exposure. For the gypsum side of that boundary, see our drywall guide.
Installation: The Details Inspectors Check
- Cutting: Score with a carbide scoring tool (2–3 passes) and snap, mesh-face up; cut holes with a jab saw or carbide hole saw. Power saws work but generate silica dust — cut outside with a respirator.
- Walls: Vapor/waterproof layer per your chosen system, then board fastened with 1-1/4-inch or 1-5/8-inch cement board screws (corrosion-resistant, ribbed heads) every 8 inches into studs. Leave 1/4 inch off the tub deck; don’t rely on board touching the tub for support.
- Floors: Trowel a bed of thinset mortar onto the subfloor with a 1/4-inch square-notch trowel, embed the board into it, then screw every 8 inches on the marked grid. The thinset bed isn’t adhesive — it eliminates voids so the board can’t flex and drum. Skipping it is the number-one DIY floor-tile failure. Gap sheets 1/8 inch and stagger joints; never align backer seams with subfloor seams.
- Seams: Alkali-resistant fiberglass mesh tape bedded in thinset (not drywall mud, not standard mesh) across every joint and corner, feathered flat.
- Waterproofing: In showers, apply your topical membrane over board and seams per the manufacturer’s coverage rate, or confirm your behind-board membrane laps correctly into the pan or flange.
Costs and Quantities
A standard tub surround (three walls to ceiling height) takes six to eight 3×5 sheets — $80 to $130 in board, plus $30 to $50 for screws, tape, and thinset, plus membrane. A 100-square-foot floor takes seven 3×5 sheets of 1/4-inch ($75 to $110) plus a bag of thinset for bedding. Labor to install backer runs $2 to $4 per square foot when hired separately, though most tile bids fold it in. As a sanity check on any tile quote: substrate prep should be a visible line item — bids that skip straight from “demo” to “tile” are telling you where they’ll cut the corner. Curbs, benches, and niches in showers are also commonly formed from backer over framing, or bought as foam prefabs. And note the naming collision: “concrete board” here means tile backer — structural cement panels and siding are different products, as is the fiber cement used in concrete wall cladding systems.
Working Safely: Silica, Weight, and Disposal
Cement board’s one genuine hazard is dust. The boards are heavy with crystalline silica, and power-cutting them releases respirable particles regulated under OSHA’s silica rule for good reason — repeated exposure causes irreversible lung disease. The homeowner protocol is simple: prefer score-and-snap (which produces little dust), do any saw or grinder cuts outdoors with the wind at your back, wear an N95 minimum (P100 for repeated cutting), and never dry-sweep the powder — vacuum with a HEPA-equipped shop vac or wet-wipe it. Carbide scoring tools cost $10 to $15 and eliminate most of the problem; a $60 pair of electric shears is the dust-free upgrade for whole-bathroom jobs.
Weight deserves respect too: a 3×5 half-inch cement sheet runs 36 pounds and carries all of it awkwardly, with edges that abrade knuckles through the mesh. Carry sheets vertically with a partner or a panel-carry hook, and wear gloves — the faces are essentially fixed sandpaper. When demoing old backer, expect it to come out in screwed-down fragments rather than clean sheets; a flat bar and patience beat a sledge, which just multiplies the dust. Disposal is straightforward — cement board is inert construction debris accepted by any landfill or dumpster service — but the density adds up fast, so quote dumpster weight allowances accordingly: a gutted tub surround’s backer alone can push 250 pounds. Handled with these basics, the material itself is about as benign as building products get.
Concrete Board FAQ
Do I need cement board behind shower tile? Yes — cement board or an approved foam board, plus a waterproofing layer. Greenboard drywall alone is a 1980s failure waiting to repeat.
Durock or HardieBacker? Performance is comparable when installed right. HardieBacker cuts cleaner and weighs less; Durock is the traditionalist’s floor board. Buy whichever your store stocks at the better price.
Can I screw backer board with drywall screws? No — they corrode and their heads snap in cement board. Use rated backer screws.
Does cement board go under a vanity or tub? No need — it’s required only where tile is bonded. Run it to the fixtures, not beneath them.
Cement board is cheap, unglamorous, and the single biggest determinant of whether your tile job survives its second decade. Bed the floors in thinset, screw on the grid, tape seams with the right materials, and waterproof the wet zones — the tile above will never let you down.
