Your shower surround keeps water from coming into contact with the wall behind it.
Tiling a shower wall over drywall.
While it probably does a decent job the drywall above the surround may suffer from water that sprays up or from moisture that accumulates on the drywall.
The wedi board wedi shower system includes special bonding agents referred to as wedi caulk though is much more than caulk that waterproofs the seams in the corners as well as the joints where the 3 5 board pieces join together.
Traditional cement backer boards and fiber boards require some type of moisture barrier to be.
You can rough up the painted surface which allows the mastic or thin set mortar especially those designed for painted drywall to adhere.
Compare this to the common practice of tiling over drywall and you get the idea that you have a totally different shower by using a product that is meant to function in the shower.
Can i put backer board over drywall for shower tiling.