sbebuilders

10-11-2007, 10:52 AM

Our next commercial building has 6x12 hips and valleys on a 10:12 pitch. I was calculating the height of the hip backing angle rise and was puzzled by the difference in the backing angle rise versus the hip drop adjustment. I checked Joe Bartok's web site and the calculations were correct. I then proceed write a script that printed out all of the hip backing angle, hip backing angle rise and hip drop adjustment height. However, I'm still a little puzzled by the height I came up with for the plan angle height on top of the hip rafter after it is backed out.

Here are the formulas I used to calculate the backing angle rise.

Evae Angle = 90°

Plan Angle = 45°

Pitch Angle = arctan ( Pitch ÷ 12)

Hip Pitch Angle = arctan( tan (Pitch Angle) * sin ( Plan Angle))

Hip Backing Angle = arctan ( sin ( Hip Pitch Angle) ÷ tan ( Plan Angle) )

Hip Backing Angle Rise = ( Material Width * .5 ) * tan ( Hip Backing Angle) ) or Valley Trough Depth

Plan Angle Unit Rise = tan ( Plan Angle )

Hip Unit Rise Run = 12 ÷ sin ( Plan Angle )

Unit Hip Drop = Pitch ÷ ( Hip Unit Rise Run * (Plan Angle Unit Rise * 2 )

Hip Drop Adjustment = ( Material Width * .5 ) * Unit Hip Drop

-----------------------------------------------------------

Plan angle rise on top of the hip backed out hip formula.

A = ( Material Width * .5 ) / cos(Hip Backing Angle)

C = sqrt (A² + A²)

Hip Plan Angle Rise = C * tan ( Hip Pitch Angle)

On a 10:12 pitch with 5 ½ x 11 ½ hips:

A = 2.75 / cos(26.92) = 3.08

C = sqrt ( 3.08² + 3.08²) = 4.36

Hip Plan Angle Rise = 4.36 * tan ( 30.51) = 2.57

This 2.57 inches seems to be incorrect.

Got any suggestions for the correct formula?

http://www.sbebuilders.com/cgi-bin/rafters/hip-backing.cgi Hip Backing Angle Script

http://www.sbebuilders.com/rfi/images/backing_angle.html Bigger image of the Hip Backing Angle Rise

Sim

Here are the formulas I used to calculate the backing angle rise.

Evae Angle = 90°

Plan Angle = 45°

Pitch Angle = arctan ( Pitch ÷ 12)

Hip Pitch Angle = arctan( tan (Pitch Angle) * sin ( Plan Angle))

Hip Backing Angle = arctan ( sin ( Hip Pitch Angle) ÷ tan ( Plan Angle) )

Hip Backing Angle Rise = ( Material Width * .5 ) * tan ( Hip Backing Angle) ) or Valley Trough Depth

Plan Angle Unit Rise = tan ( Plan Angle )

Hip Unit Rise Run = 12 ÷ sin ( Plan Angle )

Unit Hip Drop = Pitch ÷ ( Hip Unit Rise Run * (Plan Angle Unit Rise * 2 )

Hip Drop Adjustment = ( Material Width * .5 ) * Unit Hip Drop

-----------------------------------------------------------

Plan angle rise on top of the hip backed out hip formula.

A = ( Material Width * .5 ) / cos(Hip Backing Angle)

C = sqrt (A² + A²)

Hip Plan Angle Rise = C * tan ( Hip Pitch Angle)

On a 10:12 pitch with 5 ½ x 11 ½ hips:

A = 2.75 / cos(26.92) = 3.08

C = sqrt ( 3.08² + 3.08²) = 4.36

Hip Plan Angle Rise = 4.36 * tan ( 30.51) = 2.57

This 2.57 inches seems to be incorrect.

Got any suggestions for the correct formula?

http://www.sbebuilders.com/cgi-bin/rafters/hip-backing.cgi Hip Backing Angle Script

http://www.sbebuilders.com/rfi/images/backing_angle.html Bigger image of the Hip Backing Angle Rise

Sim