Tile backer board or plasterboard — which do I need in a bathroom?

The question is really three questions stacked together: what happens to this board if water reaches it, how much tile weight will it carry, and what is stopping water reaching it in the first place. People usually only ask the first, and the third is the one that decides whether the wall survives.

Safety

Step by step

1. Separate the shower enclosure from the rest of the room.Inside a shower or over a bath with a shower is a wet area with a different specification from the wall behind the towel rail. Most of a bathroom is not a wet area.
2. Understand standard plasterboard's behaviour when wet.The gypsum core softens and the paper facing delaminates. It does not recover. A tiled wall on it can appear fine for years while the board behind quietly turns to paste, then release in sheets.
3. Understand what moisture-resistant board is and is not.It has additives that slow water absorption. It resists humidity and occasional splashes; it is not waterproof and it is not a substitute for a membrane in a shower.
4. Understand cement and foam backer boards.Cement board is dimensionally stable and unaffected by water; it is heavy, hard on tools and dusty to cut. Foam-cored board with a cement facing is much lighter, easy to cut, and often has a built-in waterproof face — but it is softer, so check its tile weight and fixing requirements.
5. Check the tile weight limit for the board and the stud spacing.Every board has a maximum tile weight per square metre, and it depends on board thickness and how far apart the studs are. Heavy porcelain or stone is where limits get exceeded.
6. Fix boards to the manufacturer's spacing with the right fixings.Screw type, screw spacing and edge distance are stated by the board maker. Cement boards need their own screws; ordinary drywall screws can pull through.
7. Treat the joints and internal corners.Joints and corners are where water gets through even a good board. The board system will specify jointing tape and sealant or a membrane detail — that detail is the weak point and it is worth doing exactly as stated.
8. Tank the wet area regardless of the board.Primer where required, then a liquid or sheet membrane with reinforcing tape at every corner and joint, taken above the highest splash line. Tiles and grout are water resistant; the membrane is the waterproof layer.
9. Seal every penetration as you make it.A screw for a shower screen, a soap dish or a rail punctures the membrane. Seal it as it is fitted rather than hoping.

Tips

• Dot-and-dab is not the way to fix boards in a wet area. The voids behind become a route for water and there is no continuous support behind the joints.
• Foam backer board is the pragmatic domestic choice for most people: light enough to handle alone, cuts with a knife, waterproof, and no silica dust from cutting.
• If you find yourself asking whether an area needs tanking, tank it. It is a fraction of the cost of finding out later.
• Existing walls being retiled in a shower are worth opening up rather than overtiling — you cannot inspect what the last person did behind them.

Common mistakes

• Standard plasterboard in a shower with grout relied on as the waterproofing — Grout is porous by nature and joints are numerous. Water reaches the board over months, the gypsum softens, the fixings lose their grip and the tiled area lets go — usually all at once, and usually after the warranty on everything has expired.
• Assuming moisture-resistant board is waterproof — The green colour is widely misread as a waterproofing claim. It resists humidity and splash; it does not survive standing water or continuous wetting, which is exactly what happens behind a shower once grout lets water through.
• Tanking only the lower half of the shower wall — Water reaches the top of a shower enclosure as spray and steam. A membrane that stops at chest height leaves a band above it where water can get into the board, and that band is where failures often start.

Questions people ask