Wet shimming

QUESTION

I have seen installers use thinset against the studs when installing concrete board in order to shim out the studs that are recessed, and install the boards with no dips or humps in the shower walls. Is this a proper method and an okay way to fix walls with questionable framing? Or would it be better to install the board to the wood studs and fill the low areas with thinset after the board is installed?

ANSWER

Some manufacturers may allow that certain of their A118.1/.4/.11/.15 thinset mortars may be used for minor rendering of walls in a wet area so long as the maximum bond coat thickness is not exceeded and that proper cure times are allowed for, and minimum required coverage is achieved.

In regards to “wet shimming” behind backer board to framing or substructure: If a board manufacturer recommends doing this with their board, ask what mortar they recommend using. Then ask whether the mortar manufacturer stands behind that application or use of their product that may not have been designed for that particular application. If this is not recommended by the board and thinset manufacturer, the risk for long-term system performance is on you.

Shims installed behind the board, and cementitious patch material (designed for this purpose) installed over the board are two alternative methods for ensuring substrate flatness for you to consider. 

– NTCA Technical Team