Replacement fascias are so called because they need no backing board. They are a straight replacement for old timber fascias as they are nailed directly onto the rafter feet.
The return leg on the board is slightly rebated to accept a soffit board. The return leg measures 36mm overall from front to back as can be seen in the technical drawing tab.
Available in Different sizes from 150mm to 404mm, replacement fascias should be fixed using 2 x 65mm polytop nails at each rafter.