How to Install Cement Board for Tile
To install cement board for tile, first confirm the floor is stiff, sound, and flat. On a wood subfloor, spread the specified mortar bed, set and fasten the boards to the manufacturer’s pattern, and leave required gaps. Tape the joints, add compatible waterproofing where needed, then set tile with proper coverage and movement joints.
I once saw a careful tile job begin to fail because the installer focused on the cement board but never checked the floor beneath it. The boards were fastened neatly, yet the floor flexed underfoot. That is a costly lesson: cement board creates a suitable tile surface when installed correctly, but it does not strengthen a weak floor or fix an uneven one.
The safest order is to check the structure, prepare the surface, install the board, and only then set tile. Follow the instructions for your specific board, mortar, membrane, and tile. Their requirements can differ, so do not treat a common measurement as a universal rule.
Inspect the Framing, Subfloor, and Concrete Substrate
The substrate is the material beneath the cement board or tile. It must be sound, suitable for the selected installation system, and flat enough for tile. Check framing, subfloor construction, moisture conditions, and surface defects before covering anything. Cement board is an underlayment, not a structural repair or leveling product.
Check the wood floor before installing board
Deflection means movement or bending under load. The complete floor assembly must meet the required stiffness: at least L/360 for ceramic or porcelain tile, and L/720 for natural stone. These are limits for the whole assembly, not just the cement board. If the floor feels springy or the framing does not meet the requirement, stop and correct the structure before tiling.
Check joist spacing, subfloor thickness, grade, and fastening against the board manufacturer’s instructions. Requirements vary by product. Repair loose panels and squeaks where appropriate, and remove dust, paint, oil, or other material that could interfere with mortar. Do not assume cement board will prevent cracking caused by excess movement.
Check flatness and concrete conditions
Flatness is how much the surface varies beneath a straightedge. For tile with all edges under 15 inches, the maximum variation is 1/4 inch in 10 feet. If any tile edge is 15 inches or longer, the limit is 1/8 inch in 10 feet and 1/16 inch in 2 feet. Correct the surface before setting tile; cement board is not a leveling layer.
A concrete slab should be sound, clean, and within the limits of the selected setting materials. Check moisture when the product instructions or project conditions call for it. There is no single moisture-vapor emission rate (MVER) or in-situ relative-humidity limit that applies to every cement-board system. Follow the limits published for your mortar, membrane, and other components. Do not cover active water intrusion or hydrostatic pressure unless an approved system is designed for that condition.
| Substrate | What to confirm | Next step |
|---|---|---|
| Wood framing and subfloor | Deflection, joist spacing, panel thickness, board requirements | Repair or strengthen before board installation if requirements are not met |
| Concrete slab | Soundness, cleanliness, moisture limits, product compatibility | Use cement board only if its manufacturer approves that application |
| Either surface | Flatness for the tile size | Correct uneven areas with a compatible method before tile work |
Next step: Confirm the structure and substrate meet the requirements of every product you plan to use.
Install Cement Board and Set Tile to Manufacturer Specifications
Cement board, also called a cementitious backer unit (CBU), is a stable underlayment for tile. It is not waterproof and does not stiffen the floor. Install it with the specified mortar and corrosion-resistant fasteners, then treat joints and set tile according to the product instructions and applicable installation standards.
Lay out, cut, and bed the panels
Plan the board layout before mixing mortar. Stagger board joints so they do not line up with subfloor joints, and avoid four corners meeting at one point. Follow the manufacturer’s panel direction and layout rules. Dry-fit panels first, allowing about 1/8 inch between board edges and 1/4 inch at walls, pipes, or other fixed objects unless the product instructions say otherwise.
On a wood subfloor, spread a continuous bed of the specified mortar with the required trowel. Set each panel into the fresh mortar and fasten it promptly, following the manufacturer’s fastener pattern, edge distance, and spacing. Mortar fills small voids beneath the board; it does not make the floor structural. Do not spot-bond or dot-mount panels.
Use only approved corrosion-resistant cement-board screws or nails. Fasteners should finish flush with the board surface, not stand proud or break through it. Eight inches on center is common spacing, but it is not universal. Check the instructions for your exact board.
Cement-board cutting can create fine dust. Use the cutting method recommended for the product, work with suitable ventilation, and wear eye and respiratory protection. Keep cut edges clean and make sure each board is fully supported as required.
Tape joints and prepare for tile
Embed alkali-resistant mesh tape in the specified mortar at board joints. Ordinary drywall tape is not a substitute. Keep the joint treatment smooth; a raised ridge can telegraph through the tile work. Allow the mortar and other products to cure as directed before covering them.
For tile, use the mortar recommended for the tile and substrate. Key mortar into the surface with the flat side of the trowel, then comb consistent ridges in one direction. Press the tile into the ridges and lift a tile from time to time to inspect coverage. ANSI A108.5 calls for at least 80% coverage in dry interior areas and 95% in wet areas and exterior applications, with edges and corners well supported.
| Installation detail | Common reference | What controls the work |
|---|---|---|
| Board-edge gap | About 1/8 inch | Cement-board instructions |
| Gap at walls and obstructions | About 1/4 inch | Cement-board instructions |
| Fastener spacing | 8 inches on center is common | Board pattern and fastener requirements |
| Tile mortar coverage | 80% dry interior; 95% wet or exterior | ANSI A108.5 and the tile-system instructions |
Next step: Check the board instructions before mixing mortar, then verify tile coverage by lifting tiles during installation.
Detail Movement Joints, Perimeters, and Floor Transitions
Movement joints allow tile assemblies to respond to changes in temperature, moisture, and building movement. Keep the required gaps open at perimeters and other specified locations. Do not fill movement joints with grout or bridge them rigidly with tile, mortar, or cement-board joint treatment.
Preserve movement joints
A movement joint is a planned break in a tile assembly. Follow the project design and the Tile Council of North America’s EJ171 guidance for movement joints, including at perimeters and other required locations. Existing structural or expansion joints must be carried through the tile installation rather than covered over as if they were ordinary board seams.
At walls and fixed objects, preserve the required clearance beneath baseboards or trim. At doorways, plan the tile edge and transition before setting the final row. Use the specified sealant or joint system where movement joints occur, and keep grout out of those joints.
Waterproof only with a compatible system
Cement board is not waterproof. Water can pass through it and reach the framing or subfloor. If the design requires waterproofing, use a compatible surface-applied membrane or another approved system. Follow its instructions for seams, corners, drains, fasteners, and curing.
Do not combine products simply because each is intended for tile. Confirm that the board, mortar, membrane, and tile system manufacturers allow the combination. On concrete, use cement board only where the selected manufacturer approves the method; a sound slab may call for a different approved tile-underlayment system.
Next step: Mark movement-joint locations and decide how wet-area waterproofing will be handled before tile covers the board.
Verify Deflection, Protect the Assembly, and Maintain the Finished Floor
A finished tile floor depends on the whole assembly, from framing or slab to grout and movement joints. Inspect the work before it is covered, protect fresh mortar from traffic, and follow stated cure times. Good board installation cannot compensate for a flexible structure, poor coverage, or unresolved moisture.
Learn from common installation problems
Field example: a flexible wood floor. In a typical failure pattern, an installer adds cement board over a floor that still moves underfoot. The board may be fastened correctly, but movement can crack tile or grout and loosen the bond. The fix is to verify and correct the complete floor structure before installing the underlayment.
Field example: a lippage-prone large tile. Large-format tile placed over an uneven surface can leave noticeable height differences between neighboring edges. Cement board does not flatten that surface. Check the stated flatness limits and correct the substrate first, then lift tiles during setting to confirm mortar coverage.
Field example: a damp area without a waterproofing plan. Cement board alone does not stop water from reaching the structure. Choose a compatible waterproofing system and detail it as instructed before setting tile. If water is actively entering the area, resolve the source or use a specifically approved system before proceeding.
Protect the work and finish the floor
Keep traffic off newly installed board, membrane, and tile until the relevant product instructions allow it. Grout only after the tile mortar has reached its stated cure time. Keep movement joints free of grout, and fill them with the specified sealant or joint system.
At a transition to another floor covering, leave the tile assembly room to move and use a suitable transition detail. Do not wedge tile tightly against the adjoining floor. Before grouting, check that tile edges are supported, joints are clean, and movement gaps remain open.
Next step: Recheck the floor for movement, board gaps, joint treatment, and transition clearances before setting tile.
Frequently Asked Questions
Can cement board make a weak floor strong enough for tile?
No. It does not correct structural deflection. Verify the complete floor assembly meets the required stiffness before installing tile.
Do I need mortar beneath cement board on a wood floor?
Most cement-board instructions require a continuous mortar bed over wood subfloors. Use the mortar type and trowel specified for your product.
Can I install cement board directly on concrete?
Only if the board manufacturer approves that method and the slab meets the system’s requirements. A sound slab may be better suited to another approved underlayment method.
Is cement board waterproof?
No. Add a compatible waterproofing system where the design requires one, and follow its instructions for seams, corners, and other details.
Should I leave gaps between cement-board sheets?
Usually, leave about 1/8 inch between board edges and 1/4 inch at perimeters or obstructions. Follow the instructions for your specific board.
Can I use drywall tape on cement-board seams?
No. Use alkali-resistant mesh tape embedded in the specified mortar.
How far apart should cement-board screws be?
Eight inches on center is common, but spacing and edge distance vary. Follow the board manufacturer’s fastener pattern.
Can cement board level an uneven floor?
No. Correct substrate flatness before setting tile. The allowable variation depends on tile size.
How do I know if tile has enough mortar beneath it?
Lift a tile periodically and inspect coverage. Aim for at least 80% in dry interior areas and 95% in wet or exterior areas, with edges and corners supported.
When should I grout the tile?
Wait until the tile mortar has cured for the time stated by its manufacturer. Keep movement joints free of grout.
(This article was written by one of our staff writers, David Chen. Visit our Meet the Team page.)
