Warpage is distortion of a sanitaryware piece during drying or firing. A WC base may rock, a rim may not be level, or a wall-hung WC may leave a gap against the wall.
Some warpage is normal, because clay shrinks a lot as it dries and fires. What is acceptable depends on where the warpage is and what the product standard and your contract allow.
Why sanitaryware warps
A WC is a large, hollow casting with walls of different thickness. As it dries and then fires, it shrinks. Thick and thin sections shrink at different rates, and the piece can sag under its own weight in the kiln.
Common causes include uneven casting thickness, poor support during drying, uneven heat in the kiln and wrong placement on kiln cars. Good factories design moulds to allow for shrinkage and check every piece after firing.
Where warpage matters
- The base of a floor-standing WC, which must sit flat without rocking.
- The back face of a wall-hung WC or basin, which must sit flush to the wall.
- The rim, which should be level so the seat sits evenly.
- The joint between cistern and bowl on a two-piece WC, which must seal.
- The top of a counter-top basin and the edge that sits on the vanity.
- Fixing holes, which must line up with standard fixings.
How much is acceptable
Product standards such as EN 997 and ASME A112.19.2/CSA B45.1 set dimensional tolerances, and the factory should have its own internal limits. Ask the factory for both in writing, and name the standard in your contract.
Also ask the factory what limits it uses for each surface. A small curve on a visible side wall may be fine. The same curve on the base or a mounting face can cause installation problems.
If your market has no fixed limit, agree one in the contract. Write the limit for each critical surface and how it will be measured.
How to check it
- Place floor-standing WCs on a flat steel plate or glass sheet and check for rocking.
- Use a straightedge and feeler gauges on the base, the back face and the rim.
- Fit a sample seat and check that it sits evenly.
- Test-fit a cistern on two-piece models to check the seal.
- Check fixing hole positions against a template.
Related reading
The same principles apply to tiles. Our article on floor tile warpage and flatness explains the tile method. For writing limits into an order, see writing a specification for a one-piece WC.
Next step
We check warpage on critical surfaces during inspection and make sure your contract sets clear limits. For a factory's grading process, see our guide on auditing a sanitaryware factory. To discuss your range, book a free sourcing review.