8mm (5/16") standard hexagon washer head fastener.
These fasteners have a minimum 500hr SST coating. Link below.
Complete with 28mm EPDM BAZ washer with stainless steel support washer.
Wings just above the thread to ream out a slightly larger hole in the fibre cement sheet to allow for expansion and movement.
Use a purpose made screwgun for installation. Cordless drivers and drill drivers are ideal with torque set fairly high on a high speed and in driver mode. Please note: Impact drivers should not be used under any circumstance and automatically voids any warranties.