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 you
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:
- 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.
- 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.