On deck stairs where PVC fascia is used and railing is not mounted outside the stringers, fascia for the stair sides generally covers the triangular gaps between the bottom of the rail and the stair tread/riser surface.
If these gaps allow a 6″ sphere to pass underneath the railing, the stair will likely not pass code.
To cover this gap as well as the stringer, generally a 15″ piece of fascia must be used.
Azek® and other 12″ PVC fascia boards can be modified to fix this. A 12″ board is installed with a 4″ rip underneath it to cover the rest of the stringer.
This then leaves you with an 8″ wide portion of fascia that can theoretically be cut into risers for your stair, like shown in the diagram below
With this board rip, you can then install your stair fascia in two pieces as seen below
