Quick answer
Tile backer board is a rigid panel, most often made of portland cement reinforced with glass-fiber mesh, that is fastened to studs or a subfloor to give ceramic and stone tile a stable surface that will not swell, soften or rot when it gets wet.
Cement board is the best-known type. Fiber-cement, glass-mat gypsum and foam boards are other backers. Most backers resist water but are not waterproof without a membrane.
Quick facts
The key facts about Tile Backer Board, with sources:
- Product standard (cement backer units)
- ANSI A118.9, current edition 2023 2
- Wall stud spacing (one maker)
- Maximum 16 in on center 1
- Wall fastener spacing (one maker)
- 8 in on center; 6 in on ceilings 1
- Joint treatment
- Alkali-resistant glass-mesh tape in mortar or adhesive 1
Key takeaways
- Backer board is the hard, water-tolerant layer between the framing and the tile.
- Cement board does not rot when wet, but water can still pass through it, so showers need a waterproofing layer too.
- Plain drywall and old “green board” are not accepted as tile backers inside tub and shower walls under current model codes.
- Joints get alkali-resistant mesh tape set in mortar, not paper tape and drywall mud.
- Cutting cement board makes silica dust; score and snap or use a saw with dust collection.
What tile backer board is and what it is not
Tile is heavy and rigid. It needs a base that does not flex, swell or lose strength when wet. Backer board is that base.
Cement board, the classic type, is a sheet of portland cement with glass-fiber mesh on both faces.
USG says its Durock cement board will not swell, soften, rot, warp, delaminate or disintegrate if exposed to water 1.
Tile installers call cement panels CBUs, short for cementitious backer units. The American National Standard that sets their test methods and specifications is ANSI A118.9, published through the Tile Council of North America (TCNA); its current edition is dated 2023 2.
Backer board is not the same thing as waterproofing. Water that soaks through grout can wet the board and pass to the studs behind it.
That is why tiled showers add a waterproof membrane, such as a sheet membrane bonded with thinset 4, or a liquid membrane over the board. The membrane standard is ANSI A118.10 2.
How backer board works inside a tiled wall
- Framing carries the load. Studs must be stiff; USG lists a maximum deflection of L/360 for its cement board walls 1.
- The board spreads it. Fasteners every 8 in on walls hold the panel tight to each stud 1.
- Joints are locked. Seams are filled with mortar or adhesive and covered with alkali-resistant mesh tape, so panels act as one surface 1.
- Waterproofing goes on (in wet areas). A membrane covers the board where water will hit 4.
- Tile is set in thinset. The rough face of cement board helps mortar grip 1.
- Tile and groutThe finished surface
- Thinset mortarBonds the tile
- Waterproofing membraneSheet or liquid, per ANSI A118.10
- Tile backer boardCement board or other approved backer
- Wood or steel studsMax 16 in on center for cement board walls in one maker’s guide
Some systems use a vapor retarder behind the board instead of a surface membrane; never use both unless the maker allows it. Source: [1] [2] [4]
Types of tile backer board
| Backer | Make-up | Standard named | Good to know |
|---|---|---|---|
| Cement board (CBU) | Portland cement with glass-fiber mesh faces 1 | ANSI A118.9 2 | Heavy; will not rot; not waterproof by itself |
| Fiber-reinforced gypsum panel | Gypsum with fibers, no paper face | ASTM C1278, as listed in a code memo 3 | Only products meeting the standard are accepted in showers 3 |
| Glass-mat gypsum backer | Gypsum core with coated glass-mat faces | ASTM C1178, as listed in a code memo 3 | Lighter than cement board; check maker’s wet-area rules |
| Foam board backers | Extruded foam core with cement or fabric facing | Maker’s evaluation report | Very light; many are waterproof as a system |
| Regular drywall or “green board” | Paper-faced gypsum 5 | ASTM C1396 | Not a tile backer inside tub and shower walls |
Example: fastening schedule for a 5 ft tub surround
A bathroom remodel replaces a tub surround with tile on three walls. This walk-through applies USG’s Durock installation guide 1. A different backer has its own schedule.
| Item | Figure from the guide | What it means on this job |
|---|---|---|
| Board thickness for walls | 1/2 in or 5/8 in only | 1/4 in board is for floors and countertops, not walls |
| Stud spacing | Max 16 in on center | Studs at 24 in need blocking or reframing first |
| Fastener spacing | 8 in on center on walls | Screws or nails every 8 in along each stud |
| Fasteners on wood studs | 1 1/4 in cement board screws or 1 1/2 in galvanized roofing nails | Drywall screws are not on the list |
| Gap at the tub | 1/4 in spacer above the tub lip | Board stops short of the tub so it does not wick water |
| Joints | Mortar or adhesive plus alkali-resistant mesh tape | Paper tape and joint compound are not used |
The figures above all come from one maker’s guide; follow the product you buy. Waterproofing is added on top in a shower, using a membrane system the board maker approves 14. For the bathroom budget as a whole, try the bathroom remodel cost calculator.
Where backer board shows up in a remodel
On the quote. A good tile bid names the backer (brand and thickness), the waterproofing system and how joints are treated. If a quote says “drywall” behind shower tile, ask why.
In showers and tub surrounds. This is the main use. It pairs with a shower pan on the floor and a waterproofing membrane on the walls.
Under floor tile. Thin (1/4 in) cement board goes over a plywood or OSB subfloor of at least 5/8 in, set in mortar or adhesive and fastened every 8 in 1.
On countertops and backsplashes. Tiled counters use a board over a 3/4 in plywood or OSB base with a moisture barrier between them 1.
At inspection. Some jurisdictions check backer and waterproofing before tile covers them. A Morro Bay, California memo shows how a building department can reject backers that do not meet the listed ASTM standards 3.
Pros and cons of cement backer board
Strengths
Limits: framing stiffness, water and fastening
The board is only as stiff as the framing. Tile cracks when walls or floors bend. USG’s limits for its cement board walls are studs no farther apart than 16 in and a stud deflection of L/360 1. Floors need a solid subfloor, at least 5/8 in in USG’s guide 1.
Water passes through cement board. The board survives, but studs and insulation behind it do not, and mold can grow on hidden surfaces 6. That is the reason for a waterproofing membrane in wet areas.
Wrong fasteners fail. Cement is alkaline and wet areas are damp, so makers name corrosion-resistant screws or hot-dipped galvanized nails, not drywall screws 1.
What drives the cost of a tile backer, and how long it lasts
We do not publish backer board prices. Cost depends on the backer type (cement, foam or glass-mat), the waterproofing system on top, the condition of the framing (24 in studs need blocking or reframing), the number of niches and benches, and how much old tile and backer has to come out.
Waterproof foam board systems cost more as material but can cut labor steps.
Installed correctly and kept behind a working waterproof layer, backer board lasts as long as the tile. It fails early when water reaches the framing behind it. Our bathroom remodeling guide explains where backer and waterproofing sit in a full project.
How a tile installer puts up backer board
A tile setter or general contractor usually does this work. Shower rebuilds often need a permit when plumbing is changed.
- Remove old tile and backer; inspect studs for rot or mold, and fix any leaks.
- Check that studs are straight and no more than 16 in apart (add blocking where needed) 1.
- Install any vapor retarder or membrane the chosen system calls for behind the board.
- Cut panels by scoring and snapping, or with a dust-collecting saw 1.
- Hang panels with a 1/4 in gap above the tub or pan lip 1.
- Fasten every 8 in on walls with the approved screws or nails 1.
- Fill joints with mortar and embed alkali-resistant mesh tape 1.
- Apply the waterproofing membrane over the board where the system requires it 4.
- Set tile in thinset once the board and membrane are ready.
Protecting the backer after the tile is up
- Keep grout and caulk joints intact. Cracked grout lets water reach the board.
- Re-caulk changes of plane where walls meet the tub or shower pan.
- Run the bath fan during and after showers to dry the room 6.
- Fix leaks fast. EPA advises drying wet materials within 24 to 48 hours 6.
- Avoid harsh acid cleaners on grout that can erode joints over time.
Warning signs behind the tile, and when to call a pro
- Loose or hollow-sounding tiles: the bond to the backer may have failed.
- Cracked grout lines that keep returning: framing movement or wet board.
- Stains on the ceiling below or on the other side of the wall: water is getting past the backer.
- Soft spots when you press on the tile: possible rot in the framing.
- Musty smell from the shower that does not clear.
Cement board dust contains silica
USG’s guide warns that cement board dust can contain crystalline silica and that prolonged breathing can cause lung damage and cancer. It says to cut with a knife and straightedge, use a power saw only with dust collection, and wear a NIOSH-approved respirator and eye protection 1.
Codes and standards for tile backers
Model code. The International Residential Code has a table of approved tile backers for tub and shower walls (Table R702.4.2 in recent editions).
A 2024 memo from Morro Bay, California, applying the 2022 California Residential Code version of that table, says glass-mat gypsum products must meet ASTM C1178 and fiber-reinforced gypsum products ASTM C1278, and that products without that data are not permitted in showers 3. Check which edition your city adopted.
ANSI tile standards. TCNA lists ANSI A118.9 (cementitious backer units, 2023) and A118.10 (load-bearing, bonded waterproof membranes for thin-set tile, 2023) as current 2.
Maker instructions. Fastener type and spacing, stud spacing, joint treatment and approved membranes come from the board maker’s guide, which the warranty depends on 1.
Backer board vs drywall vs waterproofing membrane
| Layer | What it does | Waterproof? | Where it belongs |
|---|---|---|---|
| Cement backer board | Rigid, rot-proof base for tile 1 | No, water-resistant only | Behind tile in showers and on floors |
| Drywall | Paper-faced wall surface for paint 5 | No; damaged by sustained wetting 6 | Dry walls; outside the shower |
| Sheet waterproofing membrane | Bonded waterproof layer under tile 4 | Yes, as a system | Over backer board in wet areas |
| Foam backer system | Base and waterproofing in one board | Yes, when seams are sealed per maker | Showers, steam rooms, curbless designs |
Backer board myths
- Myth Cement board is waterproof.
- Reality It will not rot, but water passes through; showers need a membrane 14.
- Myth Green board is fine for a shower.
- Reality Current code tables list only backers that meet named standards for tub and shower walls 3.
- Myth Drywall screws work in cement board.
- Reality Makers name corrosion-resistant backer screws or galvanized roofing nails 1.
- Myth Seams can be taped with joint compound.
- Reality Use mortar or adhesive with alkali-resistant mesh tape 1.
- Myth Board can sit on the tub lip.
- Reality Leave a 1/4 in gap so the edge does not wick water 1.
Local code adoption decides which backers pass
The backer rules sit in the residential code each state adopts, and building departments can be strict about which products meet the listed standards.
The Morro Bay memo is an example: a named product line was refused in showers because its data did not show compliance 3.
Ask your building department or check our permits guide before buying, especially for newer foam or gypsum backers.
Picking a backer for each tiled surface
For shower and tub walls, choose a backer your code accepts and pair it with a waterproofing method the maker approves.
Cement board plus a bonded membrane is the familiar choice; a foam board system can combine both. For floors, thin cement board set in mortar over a sound subfloor is common. For walls outside the splash zone, ordinary drywall is fine.
When you compare bids, make sure each names the backer and the waterproofing. The quote comparison tool helps line them up.
- If Tile floor over wood subfloor
- Then 1/4 in cement board in mortar over at least 5/8 in subfloor 1
- If Studs at 24 in on center
- Then Add framing to 16 in before cement board 1
- If Tile backsplash on a dry wall
- Then Drywall is usually acceptable; follow the tile setter
- If Steam shower
- Then Use a membrane rated for steam and vapor control 4
Questions about Tile Backer Board
What is the difference between cement board and backer board?
Cement board is one type of backer board. “Backer board” covers any panel made to go behind tile, including cement board, fiber-reinforced gypsum, glass-mat gypsum and foam boards. Cement board is the most common type and is made of portland cement with glass-fiber mesh. Each type has its own install rules.
Is cement board waterproof?
No. Cement board is water-resistant: it will not swell, soften or rot when wet. But water can pass through it to the studs behind. Tiled showers add a waterproofing membrane over the board or use a backer system that the maker rates as waterproof when seams are sealed.
Can you use drywall instead of cement board in a shower?
Not inside the shower or tub area. Current model codes list specific backers for tub and shower walls, and standard paper-faced drywall is not one of them. Drywall is fine for bathroom walls outside the wet zone, as long as the room is ventilated.
What thickness of cement board should be used on walls?
Walls typically use 1/2 inch board. USG’s Durock guide limits walls and ceilings to the 1/2 inch and 5/8 inch thicknesses. The thinner 1/4 inch board is meant for floors and countertops over a solid wood base. Check the maker’s guide for the board you buy.
What screws do you use for cement board?
Use the corrosion-resistant screws or nails the board maker lists. USG names 1 1/4 inch cement board screws or 1 1/2 inch hot-dipped galvanized roofing nails for wood framing. Standard drywall screws are not on the list and can corrode in a wet, alkaline setting.
How do you tape cement board seams?
Fill the joint with thinset mortar or the adhesive the maker names, then press alkali-resistant glass-mesh tape into it. Do not use paper tape or drywall joint compound. If a waterproofing membrane is going on top, follow the membrane maker’s seam steps too.
Do I need a vapor barrier behind cement board?
It depends on the system. Some designs use a vapor retarder behind the board, while others put a bonded membrane on the face that also acts as a vapor retarder. Using both can trap moisture between them. Follow one maker’s complete system rather than mixing parts.
Should cement board touch the tub?
No. USG’s guide says to place a 1/4 inch spacer around the lip of the tub or shower pan to hold the bottom edge of the board off the fixture. The gap keeps the board from wicking water and is later filled with sealant per the tile system.
Can you put cement board over drywall?
Makers generally want backer board fastened directly to framing in wet areas, not over existing drywall. In a remodel, the old drywall inside the shower area usually comes out. Outside the wet area, follow the backer maker’s guide for the substrate it accepts.
Sources
- USG, Durock Brand Cement Board with EdgeGuard Installation Guide, retrieved .
- Tile Council of North America, ANSI Standards status list, retrieved .
- City of Morro Bay Building Division, Memo 010: Shower Backer Board Material (2024), retrieved .
- Schluter Systems, Schluter-KERDI bonded waterproofing membrane product page, retrieved .
- ASTM International, C1396/C1396M Standard Specification for Gypsum Board (summary page), retrieved .
- U.S. EPA, A Brief Guide to Mold, Moisture and Your Home, retrieved .
Expert review
Written by the HyreRemodel Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Install figures (thickness, stud and fastener spacing, fasteners, joints, tub gap, silica warning) are from one maker’s guide, USG Durock with EdgeGuard, and labelled as such.
ANSI A118.9 and A118.10 editions are from TCNA’s standards status page. The IRC table is described through a 2024 Morro Bay building memo applying the 2022 California Residential Code, because the ICC code viewer blocked automated access on the retrieval date; the IRC section number should be checked against the edition your jurisdiction adopted.
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