"What thickness backer board?" sounds like a detail. It decides whether your tile stays put, whether your doors still close, and whether the wall is actually waterproof. The answer is never "the thick one" — it is "the one matched to the substrate and the load."
Why thickness matters
A tile backer board does three jobs: gives tile a rigid, waterproof base, adds a little insulation, and sets the finished build-up. Too thin and the wall flexes; too thick and you lose door clearance and corner room.
The quick reference
| Thickness | Use it for |
|---|---|
| 6 mm | Overboarding a sound wall, tight clearance, tiling straight onto a stable base |
| 10 mm | Standard stud walls, backsplashes, showers over a frame |
| 12.5 mm | Floors, large-format porcelain, extra rigidity |
| 20 mm+ | Decoupling over a poor floor, step build-up, heavy stone |
By substrate
- Stud wall (dry, internal): 10 mm is the everyday spec; screw to every stud on the grid.
- Wet wall / shower: 10–12.5 mm backer board, then tank the whole plane.
- Floor: 12.5 mm for rigidity under foot; pair with a decoupling layer over a suspect slab.
- Over existing tile: 6 mm overboarding saves height while giving a fresh, flat plane.
- Underfloor heating: lay board, then dry UFH panels or embed cable in a grooved board.
Large-format and heavy stone
Big porcelain and natural stone punish any movement. Step up to 12.5 mm boards, fix on a tight grid, and keep the adhesive fully buttered. A flexible, decoupling adhesive behind the tile adds a final safety margin.
Don't forget build-up
- Add board + adhesive + tile to the existing wall before you commit — a 20 mm board can close a door gap.
- In a curbless shower, the tray sets the floor level; the wall board thickness must meet it cleanly at the threshold.
- Where height is tight, 6 mm overboarding beats ripping out the old surface.
Fixing rules that protect the spec
- Use the manufacturer's corrosion-resistant screws on the marked grid — never drywall screws.
- Leave a 2–3 mm gap between sheets; fill with flexible sealant, then tape and tank the joint.
- Overlap board waterproofing into the tray and wall membrane as one plane.
Frequently asked questions
What thickness for a shower wall?
10–12.5 mm backer board, then tank the whole wet plane. The board gives rigidity; the membrane gives the seal.
Can I use 6 mm on a floor?
Only as overboarding over a sound, rigid floor to save height. For a fresh floor build, use 12.5 mm.
Does thickness affect waterproofing?
No — the board is waterproof by manufacture; thickness is about rigidity and build-up, not the seal.
How thick for large-format porcelain?
12.5 mm with a tight fix grid and fully buttered tile; add a decoupling adhesive for safety on a moving base.
Will a thick board hit my door?
Possibly. Always add board + adhesive + tile to the existing surface and check door and threshold levels first.
Size the board to the wall, not the habit
TANYTE supplies XPS tile backer boards from 6 mm overboarding to 12.5 mm floors, with fixing kits and waterproofing in one system.