Technical guide
How wedi boards actually work, and what that means when you specify one
What the board actually is, where the thickness rules come from, why the joints decide the outcome, and how the other systems compare. From wedi's own documentation, and from installing it.
By Mihail Cheptene, certified wedi installer · 25 July 2026
The board came from another industry
XPS was never invented for bathrooms. Dow developed extruded polystyrene in 1941, and for decades it was used as insulation. It already had the two properties that would later matter: closed cells that take next to no water, and a rigid body that stays flat.
In 1983, master tiler Helmut Wedi faced that foam on both sides with glass-fibre mesh and polymer cement, and insulation became a substrate. The market made it popular: showers moved down to floor level, and the tradesmen who could form falls in screed on site were getting fewer. A waterproof board, and later a shower element with the falls already formed, solved both problems.
Standards came later, recognising what had already become practice. The 2018 revision of BS 5385-1 recommended waterproofing every substrate in a wet area and called moisture-sensitive boards unsuitable behind the tiles unless tanked. The backer board stopped being a preference and became the shortest route to a specification that passes.
Not waterproof plasterboard. A different object.
A wedi board is a core of extruded polystyrene (XPS), faced on both sides with glass-fibre mesh and a polymer-modified cement coating. The cement face is there for adhesion: it is the surface that tile adhesive and plaster bond to.
The waterproofing is the core itself. Closed-cell XPS is waterproof through its full thickness, and wedi's own documentation confirms that a board with a damaged cement face is still waterproof. A liquid membrane is the opposite case. Large-format tiles get slid and tapped into position during fixing, and a tile edge easily scratches the membrane — the breach ends up under the tile before anyone sees it. On a wedi board, the same scratch changes nothing.
One factory-made element replaces both the substrate and the tanking layer. The board weighs a fraction of plasterboard or cement board, and the foam core insulates as well as it waterproofs.
Thickness to suit the job, but thin boards have their own rules
wedi boards are made in thicknesses of 4, 6, 10, 12.5, 20, 30, 40, 50, 60, 80 and 100 mm, to meet every need on site: saving space, or building a substrate up to the thickness required. In principle, any board can go anywhere — but for boards from 4 to 12.5 mm wedi sets specific requirements, and they depend on what the board goes onto. The options grow with thickness: from 20 mm the board can go onto studs at centres up to 600 mm and onto adhesive dabs, and it is built into boxings, bath panels and benches; from 50 mm it makes free-standing partitions.
Solid substrate. On an existing wall, boards from 4 to 12.5 mm are bonded on the same logic as large-format tiles: full-coverage tile adhesive, no dot-and-dab. The board only follows the substrate — if its geometry is not acceptable, it is corrected before the board goes on. On a sound wall wedi doesn't require dowels; where the surface can't be trusted to hold the bond — separating layers, old paint, blocking surfaces — they are mandatory: at least 5 per square metre, each reaching at least 35 mm into the structural wall, not just the plaster. We add them anyway once the adhesive has cured, using wedi dowels 50 to 140 mm long.
1. Checking the wall with a level
2. Back-buttering the board, as with large-format tiles
3. Tile adhesive combed full-coverage onto the primed wall
4. wedi dowels once the adhesive has cured
5. Done: 6 mm wedi board on a block wall, full-bed adhesive and dowels
Stud frames. Of the thin boards, only 10 and 12.5 mm go directly onto studs: 10 mm at stud centres up to 300 mm, 12.5 mm up to 400 mm. At 400 to 600 mm centres, the board is 20 mm or thicker. Where the stud centres don't suit the thickness you need, our route is to sheathe the frame in plywood first and fix the wedi board over it with screws and washers — which is also the only way we put 4 or 6 mm boards onto a stud frame.
Boards from 20 mm. From this thickness the board has more ways to be fixed. It can go onto a stud frame at centres up to 600 mm, with screws and wedi washers. It can be fixed on adhesive dabs to uneven masonry where full-surface bonding isn't possible: adhesive is applied in spots at the marked fixing points, the board is set true, and once the dabs have dried the wall is drilled through them and wedi dowels are set — at least 5 per square metre, at least 35 mm into the structural substrate. From 20 mm the tiled board also takes light accessories up to 25 kg — mirrors, dispensers, towel holders — on wedi cavity fixings. Heavier items — heated towel rails, wall cabinets, glass doors — go on the wedi Tools fixings set: mounting plates bonded into a recess in the board with wedi 610 before tiling, so nothing needs to reach the structural wall. Thicker boards are also built into self-supporting pieces: bath panels, boxings, benches. From 50 mm wedi allows the board as a free-standing partition, bonded to the wall and floor with wedi 610 alone, with no special fixings — with 80 mm boards, up to 1.8 m long and 2.6 m high. A 100 mm board is a wall in its own right: it can split a bathroom into zones.
The installer carries full responsibility for the system. So if a drawing shows 12.5 mm board on 600 mm stud centres, or 12.5 mm dabbed onto brickwork, the installer is not obliged to build to the drawing and break the system: the fix is a board of 20 mm or more, closer stud centres, or a solid substrate.
wedi follows the substrate, it doesn't strengthen it
A wedi board adds no strength or stiffness to the substrate — it only follows the floor or wall it is laid on. If the substrate isn't up to the chosen tile, large format for example, it is reinforced before the wedi goes on, or the tile format is changed to one the substrate can carry.
Timber floors. wedi requires the structure to deflect no more than span/300, over a minimum deck of 16 mm sheet material or 21 mm floorboards. The boards are first bonded in a full bed of tile adhesive, and only once it has cured are they fixed with screws and wedi washers — at least 5 per square metre, driven at least 20 mm into the timber. That is the minimum for the board itself, not for the tile on top of it: ceramic is normally specified at span/360, natural stone at span/720.
The joints are the system
Once the boards are fitted to the walls, the work moves to the joints: in dry areas they are reinforced with tape, in wet areas they are sealed.
Dry and wet areas on one wall. Above, reinforcement tape on the joints and the washers left unsealed. Below, in the splash zone, every washer is sealed.
In a dry area, reinforcement tape can be used instead of sealing tape: it is easier to apply and costs less. Sealing tape is also fine there — either will do.
In a wet area, every washer is sealed with wedi 610. Board-to-board joints take sealing tape, and corners take wedi internal and external corner pieces. The tape and corners can be bedded in a flexible tile adhesive, wedi 520 or wedi 610 — all three seal. With tile adhesive, wedi's test certificates are issued for named products — wedi 320 and Mapei Adesilex P9 among them — not any S1 off the shelf. In practice wedi 520 is the easiest to work with, because of its higher workability.
1. Tile adhesive combed along the board joint; washers already sealed
2. Sealing tape bedded into the adhesive over the joint
wedi also recommends bonding the board edges to each other with wedi 610. It is a recommendation, not a requirement, and it can't always be done: a thin board bedded in tile adhesive is pressed down onto the substrate, and the adhesive often squeezes up through the joints. If 610 in the joints is wanted for peace of mind, it is easy to apply on boards of 12.5 mm and up. A joint bonded with 610 already counts as waterproof and needs no sealing tape — only reinforcement, as in a dry area. In practice the tape often goes on anyway.
Baths and shower trays. For the junction with a bath or a shower tray, wedi makes a dedicated tape, so nothing has to be improvised on site. It is much like sealing tape, but with a butyl strip: it is stuck to the rim of the bath or tray before they are pushed against the wall. The part of the tape standing above the rim is then bedded into the wall on the same logic as sealing tape on the joints — pressed into wedi 610 with a roller, or into wedi 520, which has to cover the tape on both faces.
1. Tape stuck along the tray rim before the tray goes in
2. The butyl strip bonds the tape to the rim
3. Tray in place; sealant on the wall, washers already sealed
4. Tape pressed into the sealant with a spatula
5. Done: tape bedded into the wall along the whole rim
wedi doesn't replace the plywood
The board carries tile, not sanitaryware. Where plywood has to hold a basin or a grab rail, or a frame has to carry a wall-hung WC, it goes in before the wedi, and the board is laid over it. In those zones wedi also rules out dot-and-dab and free-standing board: it is bonded full-surface. Where the plywood and frames go is decided at design stage: once the wall is tiled, they can't be added.
The system beyond flat board
The same XPS construction extends into pre-formed elements — and this is the part that changes what a designer can draw.
Niches, benches, bath panels, pipe boxings and tileable washbasins also come from wedi as prefabricated waterproof elements, so they don't have to be built from boards on site.
Steam is a separate case. The standard board resists vapour but is not a vapour barrier. For steam rooms and permanently humid spaces, wedi makes a Vapor board with a factory-applied epoxy barrier on one face. Its joints are sealed with a separate range of materials, not the ones used on the standard board. That is a different system to specify, not an installation adjustment.
What most often goes onto these boards is large-format porcelain — and that material has its own rules. See our specification guide to large-format tiles and porcelain slabs.
Similar boards, same logic
JACKOBOARD is the closest equivalent: an XPS core with a cement coating on both faces, a comparable thickness range and comparable applications. It is frequently the commercial alternative in the UK.
Schlüter KERDI-BOARD uses the same XPS core with a cement-free facing, and belongs to Schlüter's own sealing ecosystem of tapes and membranes.
Cement-based backer boards are dimensionally stable and robust, but not inherently waterproof: they still need a membrane, and they weigh several times more.
The logic of this guide transfers across all of these systems: thin boards need the right substrate, the board follows the substrate rather than strengthening it, waterproofing is decided at the joints, and sanitaryware hangs on the structure behind the board. The certificates and sealing materials don't transfer: each system is certified as a system, and components are not interchangeable between brands.
Common questions, straight answers
Can I tile straight onto a wedi board?
Yes. Once the joints are reinforced, or sealed in a wet area, and the sealant has cured, the cement face takes tile adhesive directly, with no priming. wedi's minimum for tiles under point loads is 100 x 100 mm and 7 mm thick.
Does it still need tanking?
Not across the face: the board itself is the waterproof layer, so a liquid membrane over the field is redundant. In a wet area the waterproofing is done at the lines and points instead: sealing tape over every board joint, wedi corner pieces in internal and external corners, every washer sealed with wedi 610, and sealing tape at every junction to a non-wedi material.
Can it go over underfloor heating?
Not directly over a heated screed: the board is an insulator and would sit between the heat and the room. Electric heating mats go above the board instead, and any floor with mats, or larger than 20 square metres, is reinforced with mesh across the whole surface, not just the joints.
Can I fix boards straight to floor joists?
No — boards do not span open joists. They need a continuous rigid deck first: at least 16 mm sheet material or 21 mm floorboards, deflecting no more than span/300. The boards are bonded to the deck in a full bed of tile adhesive and, once it has cured, screwed down with wedi washers — at least five per square metre. That is wedi's minimum for the board; the tile on top may need a stiffer floor.
What thickness of wedi board do I need on a stud wall?
It depends on the stud centres: 10 mm at up to 300 mm, 12.5 mm at up to 400 mm, and 20 mm or more at up to 600 mm, fixed with screws and wedi washers. 4 and 6 mm boards don't go directly onto studs. Our route for them — and whenever the stud centres don't suit the board you want — is plywood sheathing first, with the wedi board fixed over it.
Can I hang a basin or a wall-hung WC on it?
Not on the board alone. Basins and grab rails hang on plywood, and wall-hung WCs on an installation frame, in the structure behind — both go in before the wedi, with the board laid over them. On boards of 20 mm and up, light accessories up to 25 kg can go on wedi cavity fixings, and heavier items such as heated towel rails, cabinets or glass doors on the wedi Tools fixings set bonded into the board.
Does it comply with BS 5385-1?
BS 5385-1:2018 is a code of practice for tiling, not a product standard, so no board complies by itself. What the 2018 revision asks for — a waterproofed substrate in every wet area, and no unprotected moisture-sensitive board behind the tiles — is what a wedi build-up delivers by construction. For UK specification the board also carries BBA certification.
What certification do wedi boards carry, and how do they behave in a fire?
wedi boards carry BBA certification and a European Technical Assessment (ETA), a document you can hand to building control or an insurer. For reaction to fire, the board is a foam-core material classed Euroclass E. Where the fire performance of the lining is a controlling requirement, resolve it with wedi's technical team at design stage, not on site.
Can I mix wedi boards with another brand's tape, sealant or adhesive?
Not for the waterproofing. Sealing tape, corner pieces and sealants are certified as part of wedi's own system, so a generic silicone or another brand's tape takes the build-up outside it — and if a failure is ever argued over, that substitution is where the argument starts. Tile adhesive is less strict: wedi's guidelines allow a commercially available tile adhesive, but where a certificate names the adhesive it was tested with, use that one.
How do you achieve a level-access shower with it?
With a Fundo element rather than a site-formed screed: the falls are factory-formed, a matching wedi drain — point or linear — goes into it, and the element seals into the surrounding board field. On timber floors the Ligno versions are recessed into the structure so the finished surface runs flush.
Are all backer board brands basically the same?
The cores are close to identical: extruded polystyrene is a commodity, and an XPS core from wedi, JACKOBOARD or Marmox behaves much the same way. What varies is everything around it. The facing dictates which adhesives will bond. The system — tapes, sealants, tested combinations — is where the warranty lives. And the certification each brand holds covers the assembled build-up, not the sheet. The material is interchangeable, the systems are not. Pick a system and stay inside it.