Jack rafter layout – Johnson Level & Tool Mfg. RAS-1 User Manual

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COMMON RAFTER LENGTH

= 15

' 7-1/2"

SUBTRACT (W – P) FROM TABLE = 1

' 2-3/8"

FIRST HIP JACK RAFTER LENGTH = 14

' 5-1/8"

The tail length must now be added to this length. For all

remaining jacks, subtract the full spacing (W). Example:

finding in the tables the amount to subtract for a spacing

of 24", we get:

FIRST HIP JACK RAFTER LENGTH

= 14

' 5-1/8"

SUBTRACT (W) FROM TABLE

= 2

' 4-7/8"

SECOND HIP JACK RAFTER LENGTH = 12

' 0 1/4"

Continue with this process until you get to the last jack

rafter (the process is identical for valley rafters). Always

remember to measure the jack rafter length on the long

side of the rafter because of the bevel.
Cripple jacks (see Figure 8) use the same method as

above, but must be beveled at both ends. Therefore, the

subtraction for both ends must be made from a common

rafter length as if it went from the plate to the ridge. The

cripple jack rafters are actually being measured from long

point to long point diagonally along the top edge. By mea-

suring diagonally, you will compensate for 1/2 of the hip

thickness and 1/2 of the valley thickness.
TAIL OR EAVE LENGTH: The tail or eave length for all hip

jack rafters is the same as for common rafters. If desired,

use a common rafter for a pattern. Cripple and valley jacks

have no tail (See Figures 14 and 15).
LUMBER LENGTH: The lumber length is found by using the

same procedure outlined for common rafters, but sub-

tracting for the rafters’ position (see above).

JACK RAFTER LAYOUT

JACK RAFTER LAYOUT CUTS: Seat notches and bottom

plumb cuts for hip jack rafters are the same as for com-

mon rafters.