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 determine 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 2x8, then the value of rt is 3". If we also say the rafter plates (rpt) are make up of a double 2x4, then the value of rtp is also 3". If we also know that the proposed top of the ridge (rz) is 10'-8" (or 128") off the roof deck which is specified on the drawing we now have all the information needed to continue.

So arz = rz - rpt - H.A.P. or arz = 128" - 3" - 6". The arz for this roof is 119".