Here goes,builder doing a job at the moment where he dismantling a flat roof and replacing with an "A" roof. He has made the new gable from 4x2 timber studs fixed onto old wall which is cavity block. He then covered the exterior of the studs with waterproof membrane and fixed cement board directly onto membrane/studs. Eml will be fixed onto cement board which is when I come in and scratch coat it and top coat with normal sand cement. He wanted to use marine ply instead of cement board but his foreman persuaded him to use cement board. My question is will this survive and will the area where the cement boards are joined in time crack. He wont use thin render system as basecoat will take too long to dry in these conditions and the scaffold has to be dropped as we invading in the neighbours lane way and already we have overstayed our welcome.