The H.A.P. is
a term used to describe the amount of rafter stock that should be
left on after the birdsmouth is cut, this amount is then expressed
as a value in inches. The H.A.P. can
be determined by drawing a line plumb with the proposed roof
slope on the rafter stock and measuring the length of the line, then
taking approximately 2/3 of that number. This value will now be the
value of the H.A.P..
What is meant by the term H.A.P.?
 One other important measurement is the length of the "Building Plumb Line". This is the line that the building intersect the rafter at. You always take 2/3's of this measurement to calculate the H.A.P. The H.A.P. value can be easily calculated by using the following formula: Roof Pitch Angle = Tan-1(Roof rise/12) Building Plumb Line = Framing member width / Sin(90 - (roof pitch angle)) H.A.P = (Framing member width / Sin(90 - (roof pitch angle))) * .666 The final aspect of the H.A.P. would be plate bearing. This is to say that if you wished you could "adjust " the H.A.P. in relation to the plate size. This just means that you can cut your birds mouth so it bears fully on the plate. H.A.P. (adjusted) = Building Plumb Line - (plate width * tan(roof pitch angle)) |
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