Jerkin Head Calc's (teachers).pub - Mike's Trade Wiki

(TEACHERS COPY)
Jerkin Head Roof
Calculations

Examples of Jerkin Head
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JERKIN HEAD
The shape of the Jerkin head is produced at the span ends by combining hips with a gable roof.
The hips are formed by extending the ridge board past the standard apex position for a hip roof
and creating a soldier wall on top of the end wall to form the part gable.
Fig. 136 Typical Jerkin head details
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Determine Position and Length of Soldier Wall
Similar to the Gambrel roof, the ridge is extended at both ends by standard rafter spacings.
The length of the ridge extension will be equal to the distance in from the outside walls to the
soldier wall. Therefore, the length of the soldier wall will be:
Span - (length of the ridge extension) x 2
The height of the soldier wall will be a direct ratio to the roof pitch. This means that the height
of the wall is equal to the ridge extension by the rise per metre run:
Ridge extension x Rise per m run
Side Elev. Jerkinhead roof
Soldier wall support to
Jerkinhead
Height of Soldier Wall
Fig. 137 Soldier wall proportions
Ridge extension
Jerkinhead & Soldier Wall
1. The length of the wall in this
example will be:
= 4.800 - (1.350 x 2)
= 4.800 - 2.700
= 2.100 m
2. Say, the roof is pitched at 22°
then the height of the wall will
be:
= 1.350 x 0.404
= 0.545 m
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Forming the Jerkin Head
The set out and construction of the Jerkin head is similar to the process required for a standard
hip roof.
The main difference being the new span will be the length of the formed soldier wall.
STEP 1 Extend the ridge by rafter
spacings, similar to the
Gambrel roof.
In this example, the ridge is
extended by 3 rafter spacings,
i.e. 3 x 0.450 = 1.350 m
STEP 2 Construct the soldier wall,
based on the measurements in
the previous figure 137.,
i.e.
2.100m long x 0.545m high
STEP 3 Set out the position of the
crown end and creeper rafters
at 0.450 m spacings, as for a
standard hip roof.
STEP 4 Set out, cut and fix crown end,
hips and creepers to sit on the
soldier wall, based on the same
pitch as the main roof.
SUMMARY
Therefore, once the ridge extension has
been determined, the soldier wall sizes
determined, the soldier wall
constructed and placed in position, the
construction of the Jerkin head end is
exactly the same as for a hip roof.
Fig. 138 Determine new apex position
Fig. 139 Set out as for a hip roof
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BASIC CALCULATIONS for GAMBREL and JERKIN HEAD ROOFS
The following example outlines the procedure for calculation of the members listed:
1. Set out and ordering length of the common rafter for the main roof;
2. Set out and ordering length of the common rafter for the Gambrel roof;
3. Calculation of the height of the soldier wall; and
4. Set out and ordering length of the common rafter for the Jerkin head roof.
SPECIFICATION Pitch Ratio 1: 1.680
Span 6.300 m
Rafter spacings 0.600 m
Brick veneer construction
Eaves width 0.450 m
Ridge extended by 2 rafter spacings to form Gambrel end
Ridge extended by 3 rafter spacings to form the Jerkin head
Common Rafter
for Main Roof
1. Rise per metre run of Common Rafter:
= 1.000
1.680
= 0.595
2. True length of Common Rafter per metre run of Common Rafter:
= 1.0² + 0.595²
= 1.0 + 0.354025
= 1.354025
= 1.164 m
3. Set out length for Common Rafter for the Main Roof:
= 6.300 x 1.164
2
= 3.150 x 1.164
= 3.667 m
4. Ordering length of Common Rafter for Main Roof:
= (Half span + eaves width + brick and cavity) x True length per metre run
= (3.150 + 0.450 + 0.150) x 1.164
= 4.365 m, Order 4.5 m
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Common Rafter
for Gambrel Roof
Height and
Length of Soldier
Wall
1. Set out length for Common Rafter for the Gambrel Roof:
= [Half span main roof - (2 x Rafter spacings)] x True length per m run
= [3.150 - 1.200] x 1.164
= 1.950 x 1.164
= 2.270 m
2. Ordering length of Common Rafter for the Gambrel Roof:
= (Gambrel half span + eaves width + brick and cavity) x True length per m
= (1.950 + 0.450 + 0.150) x 1.164
Not Applicable to Jerkin Head
= 2.550 x 1.164
= 2.968 m, Order 3.0 m
1. Deduction from span of Main Roof (each side):
= 3 x Rafter spacings
= 3 x 0.600
= 1.800 m
2. Height of Soldier Wall:
= Deduction x Rise per metre run
= 1.800 x 0.595
= 1.071 m high
3. Length of Soldier Wall:
= Span of main roof - (2 x deduction)
= 6.300 - (2 x 1.800)
= 6.300 - 3.600
= 2.700 m long
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Crown end Rafter
for Jerkin Head
1. Set out length for Crown end Rafter for the Jerkin head Roof:
= Half span Jerkin Head x True length per m run
= 2.700 x 1.164
2
= 1.350 x 1.164
= 1.571 m
2. Ordering length of Crown end Rafter for the Jerkin Head Roof:
= (Jerkin Head half span + eaves width + brick and cavity) x True length
per m run
= (1.350 + 0.450 + 0.150) x 1.164
= 1.950 x 1.164
= 2.270 m, Order 2.4 m
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