Tile Backer Board: Cement or Foam, Which Thickness, Fixings

A tile backer board is a rigid, water-resistant sheet you fix to studs, a timber floor or an uneven wall so the tiles have something flat, strong and stable to grip. Plasterboard will take tiles too, but it has a lower weight limit and it softens if water gets behind the grout. Backer board removes both problems, which is why it is the standard base for shower walls and tiled floors over timber.

This page covers the two families of board you will find at UK merchants, which thickness goes where, how each one is fixed, and the details that decide whether the tiles stay on for twenty years or crack in two.

Cement board or foam board: the two types

Almost every board sold in the UK falls into one of two groups.

Cement and fibre cement boards

These are heavy, dense sheets made from cement reinforced with fibres or mesh. HardieBacker from James Hardie is the one most tilers know. It is sold in 1200 x 800mm sheets, with the 6mm board aimed at overlaying timber floors, masonry walls and old plasterboard, and the 12mm board aimed at stud walls. No More Ply sells fibre cement in 6mm to 22mm thicknesses, including tongue-and-groove floor boards that can span joists. Cement boards are water-resistant rather than fully waterproof: water will not rot them, but it can pass through, so a shower still needs tanking on top.

XPS foam boards

These are extruded polystyrene cores with a cement-polymer coating and a mesh on each face. Marmox Multiboard is the best-known example; Marmox lists thicknesses of 4, 6, 10, 12.5, 20, 30, 40, 50 and 60mm and sheet sizes of 600 x 1200mm, 600 x 2400mm and 1200 x 2400mm. Wedi and No More Ply make similar boards. Foam boards are light enough to carry up a loft ladder with one hand, cut with a knife, add insulation (useful under electric underfloor heating) and are sold as waterproof, with the joints sealed by a waterproof tape.

Which to choose

Shower walls and wet rooms: foam board with taped joints, or cement board plus a tanking membrane.
Timber floors: 6mm cement board or 10mm foam board, bedded in adhesive and screwed, over a sound subfloor.
Floors with underfloor heating: foam board, because it stops the heat going down into the joists.
Heavy stone or large porcelain on walls: board fixed to masonry with a solid adhesive bed, or the thicker cement board on close studs.
Fireplace surrounds: cement board. Foam board is plastic and has no place near a fire.

Why not just tile onto plasterboard?

Two reasons: weight and water. Instarmac, which makes UltraTile adhesives, gives a maximum of 32kg per square metre for unskimmed gypsum plasterboard and 40kg per square metre for tile backer board. Both figures include the adhesive and grout, which typically add 2 to 4kg per square metre, so the tile itself has to come in lighter. Thick porcelain and natural stone can get close to the plasterboard limit once adhesive is added, so check the weight per square metre on the box.

The water risk matters more in a shower. Grout is not waterproof. Once water finds a cracked joint, the paper face of plasterboard soaks it up, the board softens and the tiles start to sound hollow. We cover that comparison in more detail in our piece on tile backer board vs plasterboard.

Which thickness for walls and floors

Thickness is about stiffness and the job the board is doing, not about how strong you want it to feel.

Overboarding a sound wall (old plaster, old plasterboard, masonry): 6mm is usually enough, because the wall behind does the structural work. The board gives a clean, consistent surface and better water resistance.
Fixing direct to timber or metal studs: use the thicker board. James Hardie specifies its 12mm board for stud walls. On foam boards, most tilers use 10mm or 12.5mm on studs at 400mm centres or closer; thin foam flexes between studs.
Overlaying a timber floor: 6mm cement board or 10mm foam is the usual choice. These are overlays, not structural floors. HardieBacker is not designed to go straight over joists without a structural deck such as 22mm boarding underneath.
Levelling out a wall or boxing in pipes: the thicker foam boards (20mm and up) are rigid enough to be self-supporting over short spans.

Check the floor height before you choose. Board, adhesive and tile can add 20mm or more, which changes how the door closes and how the new floor meets the next room.

How to fix tile backer board

Every brand publishes its own fixing guide, and that guide is what a warranty is judged against, so read it for the board you actually buy. The general method looks like this.

Cement backer board screwed over a fireplace surround with the screw pattern visible — Cement backer board fixed around a fireplace; the dots are the screw heads, set in a regular grid | Backer Board by Chris Breeze (CC BY 2.0), via Flickr, https://www.flickr.com/photos/92833011@N00/393016287

On stud walls

Check the studs are straight and add noggins where board edges would otherwise float.
Offer the board up and screw it to the studs. Foam boards need their large washers under each screw head: Marmox describes washers as essential, and its fitting guide asks for screws at least 20mm longer than the board is thick, set in three rows of three on a short board and three rows of five on a long one.
Stagger the vertical joints so four corners never meet at one point.
Tape every joint with the maker’s tape: alkali-resistant mesh tape for cement board, waterproof tape for foam board in wet areas.

On masonry or old plaster

Bed the board onto a solid layer of cement-based tile adhesive rather than dabs, then add a few mechanical fixings while it sets. A full bed leaves no hollow spots for a tile to crack over. Marmox says a board fixed to masonry this way can take loads above 100kg per square metre, far more than any wall tile needs.

On timber floors

Screw down any loose floorboards first and prime the timber if the adhesive maker asks for it. Spread adhesive with a notched trowel, bed the board into it, then screw through at the spacing the board maker gives. Stagger the joints like brickwork, keep joints off the joint lines of the boards below where you can, leave a small gap at the walls, and tape the joints before tiling.

Preparing the board for tiles

Once the board is up, the hard part is done. Prime the surface if the adhesive maker requires it; Instarmac notes that priming may not be needed with a ready-mixed adhesive, but powdered cement-based adhesives often call for it. Choose the adhesive for the tile, not for the board: a flexible adhesive (S1 rated) is the usual pick for porcelain, large formats and any floor over timber. Fill screw-head dimples with adhesive as you go so the bed stays flat.

In a shower, the board is not the waterproofing on its own unless it is a foam board with every joint, screw head and pipe hole sealed as the maker describes. With cement board, add a liquid or sheet tanking system over the board in the wet zone. Our guide to tanking before tiling shows where the membrane has to run.

Common mistakes we see on Bromley jobs

Foam board fixed without washers. The screw head pulls through the coating and the board moves under the tiles.
Thin board straight onto studs. A 6mm sheet on 600mm stud centres flexes when you press it, and grout lines crack along the stud lines.
Untaped joints. Joints are where boards move. Tape and adhesive lock them together.
Silicone left out of the corners. Internal corners and the tray edge need a flexible sealant, not grout, whatever the board.
Cement board treated as waterproof. It will not rot, but water can still pass through to the studs.

If you would rather hand the boarding and tiling to someone who does it every week, Bromley Tilers covers Bromley and south east London, and we price the preparation separately so you can see where the money goes.

Frequently asked questions

Do I need tile backer board in a bathroom?

Not on every wall. Sound plaster or plasterboard can be tiled outside wet zones within its weight limit. Inside a shower enclosure or wet room, backer board (or plasterboard with a full tanking system) is the safer choice because grout is not waterproof.

Is tile backer board waterproof?

Foam boards such as Marmox Multiboard are sold as waterproof, and the joints are sealed with waterproof tape. Cement boards are water-resistant: they will not rot or soften, but they need a tanking membrane in a shower.

What thickness of backer board should I use on a floor?

For overlaying a sound timber floor, 6mm cement board or 10mm foam board is the usual choice, bedded in adhesive and screwed. Neither is meant to span joists on its own; that needs a structural deck or a purpose-made tongue-and-groove board.

Can you put backer board over old tiles?

Often yes, if the old tiles are firmly bonded. Bed the board in adhesive and fix through to the wall behind with screws long enough to pass the tile and grip the substrate. Hollow-sounding tiles should come off first.

How heavy can tiles be on backer board?

Instarmac’s figure for tile backer board is 40kg per square metre, including about 2 to 4kg for adhesive and grout. Boards fixed to masonry on a full adhesive bed can carry more; the board maker’s data sheet gives the figure for your build-up.

Do you screw or glue tile backer board?

Usually both. On masonry and floors the board goes onto a bed of adhesive and is then screwed. On studs it is screwed, with washers for foam boards, at the spacing the maker gives.

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contact@bromleytilers.co.uk