I am tiling over OSB board using hardiboard and thinset. I have some water damage from a leaking toilet. The rotted area is about 3 inches square. Once I scraped about a half inch of rot off the top, the floor is still very solid. I don't feel it is necessary to remove the flooring. However, I need to fill it in now so the floor is level before I apply the hardiboard.
What is the best way to do this? Can I just build up the thinset in that spot before I lay the backerboard? The depression is 1/4"-1/2" deep at most.
Best Answer
A 3x3 repair using the technique you describe is just fine, assuming that it's not in a critical location, such as immediately adjacent to the toilet drain opening. If it is, you might find that the stress of the toilet on that spot results in too much flex. In that case, cut the area out and float some lumber under the patch edges and replace the piece.
Otherwise you're unlikely to have a problem. Thinset mortar won't provide much stiffness, but it's certainly hard enough to do the job.