Correct hip rafter length can easily be determined with the use of a Construction Master Calculator. If you enter the right information there is no need to shorten the rafter half the thickness of the ridge board.
If you deduct the thickness of the ridge board when you calculate the common rafter length then the hip length given by the calculator will be correct with no need to further shorten the hip.
Let us use as an example a building that is 24 feet by 28 feet. With a standard hip roof and a 6/12 pitch. Below is a snapshot of the paperless tape feature found on the calculator.
24. FEET -
1 1/2 INCH =
23-10 1/2 FEET INCH ÷
2. =
11-11 1/4 FEET INCH RUN
6. INCH PTCH
13- 4 3/16 FEET INCH DIAG
17-10 7/8 FEET INCH H/V
5-11 5/8 FEET INCH RISE
16. INCH JKOC
1- 5 7/8 FEET INCH INCR
11-10 1/4 FEET INCH JK 1
10- 4 3/8 FEET INCH JK 2
8-10 1/2 FEET INCH JK 3
7- 4 5/8 FEET INCH JK 4
5-10 11/16 FEET INCH JK 5
4- 4 13/16 FEET INCH JK 6
2-10 15/16 FEET INCH JK 7
1- 5 1/16 FEET INCH JK 8
Building Width
Thickness of Ridge Board
Width Minus Ridge Board
Divided by 2
Run of Common Rafter
Pitch of Roof
Common Rafter Length
Hip Rafter Length
Peak of Roof Above Wall Plates
Jack Rafter Spacing
Difference in Jack Length
Longest Jack Rafter
The rest of the Jack Rafters
Hip rafters should be measured along the top from the long point of the plumb cut.
I will often go ahead and make the plumb cuts on the hip rafter so its easier to measure and mark the seat cut and tail cut.
To find the amount to add for the tail just add the run of the overhang minus the thickness of the sub fascia. For a one foot eave with 2 x 6 fascia this would be 10 1/2 inches.
So you would add 15 3/4 inches for the tail cut.
6. INCH PTCH
10 1/2 INCH RUN
15 3/4 INCH H/V
