Extra layers of plasterboard will make a partition system stiffer which in turn will increase the maximum cold state (mechanical) height. 

However, if the partition system is required to be fire rated, any additional plasterboard (of a different type) layers can only be added to the outer face of an existing tested Siniat system to ensure that the original performance can still be confirmed, not between board layers or between the board and stud. For example over-boarding a Siniat Fire board with a Siniat Fire MR Board then the cold state height will increase, however the fire state hight will remain unchanged. In other words, the fire state height will remain the same based on the original tested build-up with any additional layers considered as not contributing to the fire performance.

If the additional board layer is the same as that of the tested system, for example over-boarding Siniat dB board with an additional layer of Siniat dB board, then the cold state maximum height can be increased along with the fire state maximum height, but only if the testing allows.