Our builders are putting up cement board in the bathroom, and we noticed the pieces on the left have some stains, they look very different to the rest that looks the same as the ones on the right of the picture. Is it still ok to use them? Thank youenter image description here


As long as the cement board is solid with no large cracks it is useable. The stains may have occurred for any number of reasons. Also the cement board should be flat and even with no ridges. And do not worry if they feel or look wet, moisture won't effect it.


The pattern on the CBU (cementitious backer unit, which is what that stuff is apparently called in the industry) look like one of two things:

  1. It has structural problems where the lines are. Press on it. If it's solid, then it's fine. If it's soft or spongy it is broken and will be a poor tile substrate.
  2. It had something laying atop it that encouraged mould (like maybe wood or paper). It's fine, but clean it well and spray it with tri-sodium phosphate or something equally mould-killing before laying tile over it. Otherwise, you'll get mould behind the tile.

Keep in mind that CBUs are not waterproof. They take water. You don't want that mould behind the tile. (If it were me, I'd put a membrane over the whole thing, like the Kerdi stuff or else that rubbery paint-on membrane. But I'm a little uptight about "waterproof installation" with tile.)

Note that this could just be my impression from the pic, and it could be colour variation. I'm bad at photos.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.