Now that we have enough information to determine some key information about a given roof structure. If we continue to use the 8-12 slope roof and the 75' by 32' roof deck as our example, we could at this point calculate the length of all common, valley (if any) and hip rafters.

If we say that the ridge thickness (rt) is made up of a double 2x10 then;

1. The value of rt is 3".
2. We also know that in a conventional roof the  (rpt) [rafter plate thickness]  is 0".
3. We also know that the proposed top of the ridge (rz) is 10'-8" (or 128")  for an 8-12 roof with a run of 16', measured from the bottom of the ceiling rafters.

So for a conventionally framed roof the top of the ridge would be located at;

1. arz = rz - rpt + H.A.P. or
2. arz = 128" - 0" + 6" .
3. The arz for this roof is 134".