Chapter 3: Construct Advanced Roofs

CHAPTER
construct
LEVELLING AND
advancedOUT
roofs
SETTING
31
s
Dr Glenn P. Costin
Sa
m
pl
e
pa
ge
This chapter focuses on the construction of five common roof forms that fall within
the very broad advanced roofing category:
• Gambrel (Dutch gable)
• Jerkin head
• Skewed gable
• Oblique hip
• Unequal pitch.
For each of these, the mathematical, geometric and construction techniques
are described in detail, which will give you the ability to explore other advanced
roof types, such as octagonal ends, tapering spans and the comparatively simple
Mansard.
This chapter expands on—and at times challenges—the basic principles developed
in Chapter 5 of Site Establishment, Formwork and Framing (Laws 2009), which covered
the setting out and construction of basic roofs, such as the gable, broken hip, valley
and Scotch valley. We will start with a brief revision of these roofing basics.
BASIC
PRINCIPLES OF
ROOFING
The purpose of this revision is twofold.
First, it is to re-acquaint you with the
basic principles of roofing, including
the underpinning mathematics and
geometry; and second, it is to highlight
key assumptions within these basic
principles that are challenged in
Cheetham 03.indd 73
advanced roofing, particularly by
roofs with unequal pitches or tapering
spans. We suggest that you revisit
Chapter 5 of Site Establishment,
Formwork and Framing (Laws 2009)
and the PowerPoint slide show, ‘The
Seven Pillars of Roof’ (Costin 2009),
available with that text.
Having developed an understanding
of basic roofing, it is reasonable for
you to have formed the belief that the
following points hold true for all roofs:
13/11/12 1:16:32 PM
ADVANCED BUILDING AND JOINERY SKILLS
The mathematics: the ‘Seven Pillars’
revisited
0.466 m
1.000 m
CR rise/m run = tan 
Rise
tan 25° = 1.000
0.466 = rise
2. Common rafter length per metre run (CR factor)
3
1.10
0.466 m
CR/m run2 = 12 + 0.4662
CR/m run2 = 1 + 0.217
CR/m run = √1.217
CR/m run = 1.103 m
3. Common rafter set-out length (CR set-out length)
pl
m
Sa
Cheetham 03.indd 74
m
1.000 m
e
From the two texts referred to earlier (Laws 2009;
Costin 2009), the following is the basis of mathematics
underpinning roofing. While there are some notable
changes with regards to hip and valley length
calculations, these basic principles still hold, as does
the geometry that they are derived from.
This sample calculation for a basic hipped
roof is based upon a building with the following
characteristics:
Pitch = 25°
Span = 8010 mm
Eave width = 600 mm
Rafter sectional size = 125 3 45 mm
Hip sectional size = 175 3 35 mm
It is important that you develop a clear
understanding of this system of calculations and,
most importantly, the application of the common
rafter length per metre run (CR factor) and, similarly,
the hip length per metre run of common rafter (hip
factor). It is by using these factors that lengths of some
of the seemingly more difficult components are most
easily found.
1. Common rafter rise per metre run (CR rise/m
run)
s
1. Roofs are made up of right angled triangles
and are set out to centre lines (including hip
and valley rafters, centring rafters and
ridges).
2. Ridges run parallel and level to wall plates.
3. Regardless of the roof shape, rafters run at 90° to
the wall plates.
4. Hips and valleys bisect internal and external
corners regardless of the angles of those corners.
With the exception of point 3, all of the above will
be challenged at some point in the following pages.
And point 3, while not directly challenged here, can
not be viewed as a given in all roofing, for there are
occasions in some more complex architecture where
it may be rational to forgo this ‘rule’ also (though
generally in small areas where only light loads
apply).
pa
ge
74 4.419 m
3
1.10
m
0.466 m
1.000 m
4.005 m
(Half span)
CR set-out length = CR factor 3 half span
CR set-out length = 1.103 3 4.005
CR set-out length = 4.419 m
4. Common rafter order length (CR order length)
5.204 m
3m
1.10
0.466 m
1.000 m
4.605 m
(Half span  eave width)
R order length = [CR factor 3 (half span + eave
C
width)] + rafter depth
13/11/12 1:16:34 PM
75
CHAPTER 3 construct advanced roofs
CR order length = [ 1.103 3 (4.005 + 0.600)]
+ 0.125
= [1.103 3 4.605] + 0.125
= 5.079 + 0.125
= 5.204
CR order length = > 5.4 m
Remember to add the depth of the rafter
material (in this case 125 mm) to allow for the
bevel cut (see the ‘Seven Pillars’ PowerPoint for
clarification if you are unclear on this issue).
5. Hip length per metre run of common rafter (hip
factor)
7. Hip order length
7m
6.85
Allowance needed
for bevel cut
9
1.48
m
5m
4.60
With allowance
for bevel cut,
order 7.2 m hip
0.6
5m
4.0 0
Hip order length = [ hip factor 3 (half span + eave
Hip
s
width)] + hip depth
pa
ge
0.466 m
hip order length = [1.489 3 (4.005 + 0.600)]
1.414 m
1.000 m
00 m
1.000 m
9m
1.48
e
0.466 m
pl
1.414 m
Hip factor = √hip run + rise
hip factor2 = 1.4142 + 0.4662
hip factor2 = 2 + 0.217
hip factor = √2.217
hip factor = 1.489 m
6. Hip set-out length
2
Sa
m
2
+ 0.175
= [1.489 3 4.605] + 0.175
= 6.857 + 0.175
= 7.032
hip order length = > 7.2 m
Basic roofing geometry
From Chapter 5 of Constructing a Pitched Roof (Laws
2009), you will be aware that for basic roofing eight
bevels are required. The development shown in
Figure 3.1 has been taken directly from that text.
These eight bevels remain all that are required for
our first two roofs (gambrel and jerkin head), and
the basic principles hold for all the others. You need
to focus particular attention on the development of
5.963 m
the hip edge bevel. This is based upon the level line
Hip factor
1.489 m
1.414 m
(LL) principle (see Figures 3.2 and 3.3). This principle
Rise
works on the basis that any line running at 90° to the
1.000 m
4.005 m
component being considered is level. And as long as
the top edge of that component is square to a line
Hip set-out length = hip factor 3 half span
hip length = 1.489 3 4.005
hip length = 5.963 m
Cheetham 03.indd 75
running plumb or vertically through it (generally the
case with square or rectangular rafters, etc.), then this
line is also running along that surface.
13/11/12 1:16:37 PM
76 ADVANCED BUILDING AND JOINERY SKILLS
LBCR
Rise
Elevation view
Start near here or
on centre
PB hip
FB purlin
EB hip
se
Ri
LB hip
Start here
These two lines
and extend
same length
up
EP purlin
Plan view
pl
m
True bevel
Bevel is clearly not 45° as
LL1 and the true length
of X are not the same.
Sa
Figure 3.2
The level line
principle ‘fold out’
e
Eight roof bevels:
1 Plumb bevel common rafter  PBCR
2 Level bevel common rafter  LBCR
3 Plumb bevel hip  PB hip
4 Level bevel hip  LB hip
5 Edge bevel hip  EB hip
6 Edge bevel creeper  EB creeper
7 Face bevel purlin  FBP
8 Edge bevel purlin  EBP
The skill in developing roof bevels is always in
determining where to find the relevant right angled
triangle, i.e. where to place the level line, and where to
find the true length of the other side of that triangle.
After that it is ‘just’ (teachers love that word!) a matter
of putting the two lengths together as a right angled
triangle to form the ‘true’ bevel.
In Figures 3.2 and 3.3 the component we are dealing
with is a hip. Given that a level line in the plan view is
a true length, it is the side running up the hip that is
raking and so is actually longer than it appears. The
true length of this side is found in the side elevation
of the hip.
Using a layout similar to that modelled in
Figure 3.1, we can see one way of graphically putting
this into practice (see Figure 3.2).
True
length
of X
Rise
LL1
Ridge centre line
EB creeper
Figure 3.1
One approach to
the development
of roof bevels
s
PBCR
pa
ge
90° angle
LL1
Crown end centre line
X
Bevel we are looking for.
In plan appears as a 45°
angle as both X and LL1
are the same length.
Hip centre line
Hip centre line
Cheetham 03.indd 76
13/11/12 1:16:39 PM
77
CHAPTER 3 construct advanced roofs
Figure 3.3
The level line
principle—
pictorial view
(top) and plan
view (bottom)
(reproduced in
colour in the
Appendix)
Centre lines
Ridge
Hip
s
Centring rafter
pa
ge
Hip
pl
Sa
m
These lines are level as long as they run
‘square’ to the component (at 90º). In
bringing the line ‘over’ the centre line of
the component beside it, the resultant
triangle provides the edge bevel sought.
It is not the ‘true’ angle, however, until
the true length of the other side is found.
e
Crown end rafter
Study this concept carefully until you can fully
visualise what is being done. This concept will
occur repeatedly in developing the bevels for each
of the roofs that follow. It is also important for
you to realise that there are many ways that the
development of the above bevel may be laid out.
Cheetham 03.indd 77
However, each is working on the same principle
of recognising when a line is being seen in its true
length, and knowing how or where to find the true
length of those lines that are raking towards or
away from us.
13/11/12 1:16:41 PM
78 ADVANCED BUILDING AND JOINERY SKILLS
GAMBREL (DUTCH
GABLE) ROOF
In a gambrel roof, the ridge is extended as shown
in Figures 3.4 and 3.5. This means extra rafters are
installed and the hips shortened. The result is a
vertical gable in the hip end, located wherever the
builder, client or architect desires: usually at the most
convenient common rafter position. A waling plate is
fixed to the last set of common rafters to pick up the
ends of the jack rafters that fill out the end of the roof
(see Figure 3.6). Then the hips and any creepers are cut
in to finish the framing.
The gambrel or Dutch gable roof is basically a hipped
roof with an extended ridge forming small gables
at each end, i.e. the hips start from the plates at the
corners but don’t reach the apex. Sometimes the gable
end is built as a vent or has vents let into it. This is a
style that continues to be popular in contemporary
home design.
Extended ridge
Waling piece
pa
ge
s
Figure 3.4
Gambrel roof
frame
e
Figure 3.5
Plan of typical
gambrel
roof framing
(reproduced in
colour in the
Appendix)
Jack rafters
Shortened hip
m
pl
Last common rafter
Sa
Extension of ridge
A normal
hip end
Centring rafters
Common rafters
Jack rafters
Shortened
hip
Last common rafter
Cheetham 03.indd 78
13/11/12 1:16:43 PM
79
CHAPTER 3 construct advanced roofs
on the end wall plate; then all other rafter positions
moving away from these centre line marks at the
normal rafter spacings.
At this point you should do the development
of all your basic bevels (the eight bevels shown in
Figure 3.1) and the calculation of the first five of the
‘Seven Pillars’, as shown in the following table.
Extension of ridge
First five roofing pillars
In this case
(see pages
74 & 75 for
calculations)
Rise/m run of CR
CR length/m run (CR factor)
Set-out length of CR
Order length of CR
Hip length/m run of CR (hip
factor)
0.466 m
1.103 m
2.482 m
3.144 m
1.489 m
Setting out and constructing the
gambrel roof
pl
e
For simplicity, the roof being considered will have
similar characteristics to that used in the revision
exercises, but a narrower span i.e.:
Pitch = 25°
Span = 4500 mm
Eave width = 600 mm
Rafter sectional size = 125 3 45 mm
Hip sectional size = 175 3 35 mm
Begin by marking out the top plates as you would a
normal hipped roof: i.e. the centre line of the centring
rafters set back from the end of the build by the half
span; centre line of the crown end at the half span
From this information, you should create a pattern
rafter as per normal (see Laws 2009, pp. 141–145), but
exclude the creeper rafter lengths for now.
Setting out the ridge
Ridge set-out is done by marking it off the wall plate,
as you would in normal roofing practice, the only
difference being the allowance for the extension. This
is simply a matter of determining which rafter is going
to be the last common one.
Sa
m
1
2
3
4
5
pa
ge
Figure 3.6
Typical Dutch
gable showing the
ridge extension
s
Normal hip travel
Figure 3.7
Gambrel roof with
extended ridge
Ridge is extended past
original centring rafters
as required
Set out all other rafter positions
as normal: i.e. working away from
centring and crown end towards corners
Normal centring
rafter location
Centre line of crown
end as per normal
Centre line of
centring rafter
Half span
Half span
Cheetham 03.indd 79
13/11/12 1:16:46 PM
80 ADVANCED BUILDING AND JOINERY SKILLS
Figure 3.8
Ridge set-out
CL of centring rafter
Half span
Top plate
Ridge
Cut-off point for extended
ridge. Mark but do not cut
until roof is up.
s
Extension of ridge: in this case
3  450 mm rafter spacings, or 1350 mm
mm
pa
ge
967
Figure 3.10
Jack rafter set-out
m
pl
e
Figure 3.9
Main roof section
erected and
braced
Sa
In this case, the plan calls for an extension of
three rafters spaced at 450 mm centre to centre (see
Figure 3.8). If in your plan no dimension is given, you
will have to scale the length and then check with the
client or architect.
Standing the main roof section
Having marked out the ridge and
pattern rafter, the first part of the
roof may be cut out and assembled.
Be sure to install a temporary brace
at this point in case of unexpected
high wind gusts (see Figure 3.9).
The jack rafters and location of waling piece
To find the location of the waling piece, we will
need to set out and cut one of the jack rafters (see
Figure 3.10). This will then be used to mark where the
top edge of the jacks will finish on the common rafters
(see Figure 3.11).
The jack rafter is simply a shortened common
rafter (hence the name ‘jack’). To calculate the length of
Plumb line marked
from jack rafter
plumb cut
Top edge of
waling piece
Figure 3.11
Locating the
waling piece
Cheetham 03.indd 80
Jack rafter cut to length and
positioned for marking
13/11/12 1:16:49 PM
81
CHAPTER 3 construct advanced roofs
Line of plumb cut
Figure 3.12
Locating waling
piece detail
Position of top
edge of waling piece
Jack rafter held in
position for marking
Waling piece
s
In this case:
cut-off length = 877 3 1.103
cut-off length = 967 mm
Having cut at least one jack rafter as a pattern,
place the rafter as shown above and mark down the
plumb cut (see Figure 3.12). Do this on both sides of
the roof. Where this plumb line meets the bottom
edge of the common rafters is where you will position
the top edge of the waling piece.
967
mm
Sa
m
pl
e
pa
ge
any component that has the same pitch as a common
rafter, you need two easily found pieces of information:
the run, or plan length, of the component; and the CR
length/m run, or rafter factor.
The run of the jack is found by:
Half span – (ridge extension + half the thickness of
a common rafter*)
In this case:
jack run = 2250 – (1350 + 23*)
jack run = 877 mm
The cut-off length of the jack rafter is then
simply:
Cut-off length = component run 3 CR length/m
run
Option A: Notched rafters
967
967
Option B: Rafters shortened by thickness
of waler; waler positioned in line with
top of jack rafter
Line of plumb cut
Normal position
(Option A)
Cheetham 03.indd 81
Figure 3.13
Alternative waling
piece positions
mm
mm
Option C: Waler lowered to suit underside of
jack rafter; achieved by marking the thickness
of the waler back towards the wall plates from
the plumb line (see explanatory figure at right)
*Note: This reduction can be done later, as you
would for a crown end rafter. In taking this alternative
approach, you will be finding the ‘set-out’ length of the
jack rafter. You must then be sure to take off the half
thickness of CR (horizontally, and not down the length
of the rafter).
Thickness of
waling piece
Lower waler
position
Jack rafter held in
position for marking
13/11/12 1:16:51 PM
ADVANCED BUILDING AND JOINERY SKILLS
Hip rafters
Finishing the framing
Installation of the hips is much the same as normal,
though you will need to notch the top end of the hip
over the waling piece. This is easiest done by direct
measurement and use of the normal hip plumb and
level bevels. As with the jack rafters, be sure that
the top edge of the hip aligns correctly with the
edges of the jack and common rafters (see detail in
Figure 3.17).
Set-out, cutting and installation of the creepers
is now done, as for a standard hipped roof (see
Figure 3.18). As always, take care that all surfaces are
true (in wind and straight).
e
To calculate the length of hips we use, as with basic
hipped roofing, the run of the common rafter that is
required to obtain the same height. In basic hipped
roofing this is simply the half span. In this case, as is
clearly evident in Figure 3.16, the jack rafter obtains
the height we require. So it is the run of the jack rafter
that we require. We then simply multiply this length
by the hip length/m run of CR (the hip factor).
Hip set-out length = run of jack rafter 3 hip
length/m run of CR
In this case:
hip set-out length = 0.877 3 1.489
hip set-out length = 1.306 m
As we are using a jack run that already allows for
the half thickness of a common rafter, no further
reductions are required when setting out the hip. The
hip set-out is shown in Figure 3.16.
s
Alternatives for locating waling piece
Locating the waling piece as described above requires
that you notch the underside of each jack rafter as
shown on the previous page (Option A). Options B and
C offer alternatives to this approach (see Figure 3.13).
Figure 3.14 shows the waling piece installed.
The jack rafters can now be installed. Note that the
top outer edge of the two outer rafters should meet the
edge of the common rafters as shown in Figure 3.15.
pa
ge
82 Figure 3.15
Jack rafters
installed
Cheetham 03.indd 82
Sa
m
pl
Figure 3.14
Installing the
waling piece
Waling piece set level
and bolted or otherwise
fixed as per AS 1864
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83
CHAPTER 3 construct advanced roofs
Figure 3.16
Hip set-out
Hip edge bevel
(see pages 76–77)
X
1.306 m
Hip plumb bevel
(see page 76)
X is the plumb height
taken from above the
common rafter bird’s
mouth.
Figure 3.17
Hip position and
detail
Sa
m
pl
e
pa
ge
s
X
Figure 3.18
Completed
framing for
gambrel roof
Cheetham 03.indd 83
13/11/12 1:16:56 PM
84 ADVANCED BUILDING AND JOINERY SKILLS
Extended ridge
Soldier wall
s
Figure 3.20
Pictorial view of
jerkin head roof
framing
rafter components will be familiar to those with an
understanding of basic hipped roofing.
The skill that you need to develop in the case of
the jerkin head roof is how to determine the height
and width of the soldier wall. Figure 3.21 offers a
comparison between the framing of a ‘normal’ hip
end and the jerkin head. The length and position of
the soldier wall is shown as a pink line.
e
JERKIN HE AD ROOF
pa
ge
Figure 3.19
Ways of visualising
the jerkin head
roof (reproduced
in colour in the
Appendix)
Sa
m
pl
A jerkin head roof, like the gambrel, is an adaptation
of the hipped roof. This roof form is useful where
the attic or roof space of a house is required as the
living area, or where the architect desires to reduce the
imposing nature of a large gable. While an interesting
and pleasing roof from a construction point of view,
it is not much used in contemporary architecture.
Its inclusion in this chapter is based on its value in
developing skills useful for more advanced forms.
Like the gambrel, in constructing a jerkin head roof
the ridge is extended past the half span point where
the centring rafters would normally be positioned.
Unlike the gambrel, however, the hips and centring
rafters move with the end of the ridge. The result may
be described as a cut-off hip end (see Figure 3.19, blue
lines), or a truncated gable end wall. The hips therefore
do not intersect the corner of the building: instead, a
soldier, or shortened, wall is constructed to support
the crown end, creepers, and hips (see Figure 3.20).
As can be seen from the diagram in Figure 3.20,
the set-out and development of all the various
Cheetham 03.indd 84
Calculating the height and width of
the soldier wall
This is a very simple calculation once you understand
the basic geometry from which it derives. For this
discussion we will use the same characteristics as in
the gambrel roof example:
Pitch = 25°
Span = 4500 mm
Eave width = 600 mm
Rafter sectional size = 125 3 45 mm
Hip sectional size = 175 3 35 mm
And once more the ridge will be extended by 3 3
450 mm rafter spacings, or 1350 mm.
Now look closely at Figure 3.22 on the next page.
From Figure 3.22 you can see that the new hip and
the normal hip run parallel to each other. This means
that the red line (ridge extension) and the green line
(the horizontal run of the jerkin head rafter) are
equal, i.e. the run, or plan length, of the jerkin head
13/11/12 1:16:58 PM
85
CHAPTER 3 construct advanced roofs
Extension of ridge
Top plates
Common rafters
Creepers
A normal
hip end
Soldier wall
Normal centring
rafter position
Extension of ridge
pl
m
Run of jerkin
head rafter
Normal hip position
Normal centring
rafter position
rafter will always be equal to the amount by which
you extend the ridge.
In this case:
Run of jerkin head rafter = 1350 mm
As Figure 3.23 shows clearly, the width of the soldier
wall may now be determined by the following:
Cheetham 03.indd 85
Normal hip position
New centring
rafter position
Figure 3.21
Plan of framing
for a jerkin head
roof compared
to a ‘normal’ hip
end (reproduced
in colour in the
Appendix)
New hip position
Equal
Sa
Normal centring
rafter position
Half span
e
Equal
pa
ge
s
New hip position
Figure 3.22
Determining
the length of
the soldier wall
(reproduced in
colour in the
Appendix)
S oldier wall width = span – (2 3 ridge extension)
Or in this case:
soldier wall width = 4500 – (2 3 1350)
= 4500 – 2700
= 1800 mm
13/11/12 1:17:01 PM
86 ADVANCED BUILDING AND JOINERY SKILLS
Figure 3.23
Width of soldier
wall (reproduced
in colour in the
Appendix)
1350 mm
Ridge
extension
Figure 3.24
Rise/m run
principle
1350 mm
1350 mm
The height of the soldier wall (see Figure 3.25) is
therefore:
Height of the soldier wall = rise/m run of CR
3 ridge extension
In this case:
height of the soldier wall = 0.466 3 1.350
= 0.629 mm
 2.0  0.466
 0.932 m
1.0 m
2.0 m
Framing out the roof
From this point on, the setting out and cutting of the
crown end, hips and creepers is very much the same
as for a normal hipped roof: working, as always, with
plan lengths, and multiplying by the appropriate factor
(hip or rafter). Figure 3.26 shows how to determine
these lengths.
Crown end plan length = half span – ridge
extension
In this case:
crown end plan length = 2.250 – 1.350
= 0.900 m
Sa
m
pl
e
As with the gambrel, at this point you should do
the development of all your basic bevels (the eight
bevels shown in Figure 3.1) and the calculation of the
first five of the ‘Seven Pillars’. Because we are using the
same roof characteristics, these will be the same as
those for the gambrel roof (see pages 74 and 75).
The height of the soldier wall is now determined
using the first of the ‘Seven Pillars’, i.e. the rise/m run
of CR. Figure 3.24 shows how this works.
That is, for every 1.0 m of run, a rafter at 25° will
rise 466 mm. For 2.0 m, the rise will be twice as high
(2 3 0.466). For 1.5 m, the rise will be 1.5 times as high
(1.5 3 0.466), and so on.
pa
ge
s
0.466 m
Width of
soldier wall
25°
629 mm
466 mm
Figure 3.25
Height of soldier
wall
1000 mm
1350 mm
Cheetham 03.indd 86
13/11/12 1:17:03 PM
87
CHAPTER 3 construct advanced roofs
Ridge
extension
Half
span
Figure 3.26
Finding the crown
end plan length
(reproduced in
colour in the
Appendix)
Crown end run,
or plan length
Half rafter
thickness
993
s
Remember, this is your set-out length. As with a
normal hipped roof, you must make a horizontal
reduction on your pattern of half the thickness of the
common rafter material (see Figure 3.27). Positioning
of the crown end rafter is shown in Figure 3.28.
pa
ge
crown end set-out length = p
lan length 3 CR
length/m run (rafter
factor)
crown end set-out length = 0.900 3 1.103
= 0.993 m
993
Se
t-o
ut
len
gth
Sa
m
pl
e
mm
mm
Figure 3.27
Applying the
crown end
reduction
Note: Be sure to apply reductions horizontally or
square to plumb (i.e. at 90º to the plumb line),
not ‘down’ the rafter.
Centre lines
Ridge
Centring
rafters
Figure 3.28
Positioning the
crown end rafter
Crown end
rafter
Remember, in this case all
the roof components align to
a centre line (above).
Make sure the top edges of
the crown end rafter align
with the edges of the
centring rafters, as with a
normal crown end cluster
(right).
Cheetham 03.indd 87
13/11/12 1:17:06 PM
ADVANCED BUILDING AND JOINERY SKILLS
Half mitre thickness
of common rafter
Cutting and installing creepers
Traditionally creepers are set out using a rather
complicated reduction method to locate the long point
of the first (longest) creeper. Only after this may the
standard creeper reduction be used. The author has
developed a method which makes finding the lengths
of creepers for all roofs far simpler, and for roofs
such as this, particularly so. Once more, it is about
determining the plan length, or run, of a component,
and then multiplying this distance by the appropriate
factor (in this case, the rafter factor).
Figure 3.32 shows the measurements required for
this approach. Normally these would be taken directly
off the wall plates; however, it is possible to calculate
m
pl
e
Figure 3.29
Hip reductions
the same approach as would be used with a hip rafter
for a standard hipped roof. Positioning of the hips is
shown in Figure 3.31.
s
The hips
As with a standard hipped roof, the hip factor is
multiplied by the run of the appropriate rafter to gain
the hip set-out length, or our sixth pillar. In a normal
hipped roof, we would multiply by the half span,
i.e. the run of the common rafter. In this case, the
appropriate run, or plan length, is that of the crown
end rafter found previously:
Hip set-out length = crown end plan length 3 hip
factor
Hip set-out length = 0.900 3 1.489
Set-out is as per a normal hipped roof and, unlike
the gambrel, you must make your standard reduction
at the top of the hip of half the mitre thickness
of the common rafter. See Figure 3.29 for further
explanation.
The application of reductions and the full hip setout are shown in Figure 3.30. As stated earlier, this is
pa
ge
88 Sa
Draw a line at 45º on the edge of a piece of rafter
material as shown above. Measure and divide in
two.
Note: As with the jack rafter reduction, this must
be taken off square to plumb, i.e. at 90º to the
plumb bevel (see Figure 3.27).
Figure 3.30
Setting out the hip
Half mitre thickness of CR
taken off square or at 90°
to the plumb cut as shown
Hip edge bevel
(see pages 76–77)
1.34
0
m
Hip plumb bevel
(see page 76)
X
Cheetham 03.indd 88
X
X is the plumb height
taken from above the
common rafter bird’s
mouth.
13/11/12 1:17:08 PM
89
CHAPTER 3 construct advanced roofs
Figure 3.31
Positioning the hips
pa
ge
s
Note: As always, be sure
edges of hips align with
edges of rafters.
pl
X
m
Note: Assumes that rafter
spacings on jerkin head
end are the same as for
the rest of the roof.
e
Width of hip material
marked across corners
Figure 3.32
Determining
the plan length,
or run, of the
long side of the
first creeper
(reproduced in
colour in the
Appendix)
Plan length of long point of first
(longest) jerkin head creeper
Y
Sa
Plan length of long point of
first creeper (longest creeper)
them. Calculating these lengths is not difficult and is
important to know when dealing with larger roofs.
The formulas for calculating these lengths are given
after those for finding the set-out length for the first
creeper. All other creepers may be set out from these
long points (see Figure 3.33) using the standard creeper
shortening:
Creeper shortening = r after spacing 3 rafter
factor
In this case:
creeper shortening = 0.450 3 1.103
creeper shortening = 0.496 m
Note: This distance is applied ‘down’ the rafter
(see Figure 3.33).
Set-out length of first creeper = Y 3 rafter factor
Cheetham 03.indd 89
Set-out length of first jerkin head creeper = X 3
rafter factor
These distances are usually direct measured;
however, they may be found mathematically by:
Distance Y = (half span – half mitre thickness of
hip) – (rafter spacing – half rafter thickness)
Distance 3 = (half soldier wall width – half
mitre thickness of hip) – (rafter spacing – half rafter
thickness)
It is not necessary to carry out these calculations
here, as you are adequately equipped from the prev
ious workings to undertake this yourself. However,
it may be helpful to know the shorthand method of
finding the half mitre thickness of a component when
not direct measuring it:
13/11/12 1:17:10 PM
90 ADVANCED BUILDING AND JOINERY SKILLS
Figure 3.33
Setting out the
first (or longest)
creeper and
applying the
creeper shortening
Creeper
edge bevel
Creeper
shortening
Mea
sure
men
t is
to lo
ng p
oint
of b
eve
l
pl
m
Sa
SKE WED GABLE
Buildings are not always built as square or rectangular
structures. Sometimes the wall at the end of a
building will run at an angle other than 90°, making
what is known as a ‘splayed’ or ‘oblique’ end. This
has implications for roof design, requiring more
Cheetham 03.indd 90
Figure 3.35
Skewed gable
e
Figure 3.34
Completed roof
framing for the
jerkin head
pa
ge
Half mitre thickness = component thickness 4÷
1.414
This completes the main framing of the jerkin head
roof (see Figure 3.34). All other components, such as
purlins, strutting, barge trimmers and the like should
be familiar to you.
s
CR plumb bevel
(see page 76)
thought in the cutting of either gable or hip rafters
and creepers.
The skewed gable (see Figure 3.35) and the oblique
hip (see page 97) fall under the category of ‘splayed
ended roofs’. In this form, the skewed gable effectively
has creeper rafters to consider. For the moment, only
roofs with equal pitches will be dealt with, leaving
the issues arising from unequal pitches to the final
section.
As the diagram in Figure 3.36 demonstrates, the
skewed gable roof is aptly named as the gable end
is out of square to the building proper, that is to say,
it is skewed. The amount of skew is not relevant to
the setting out or construction as the mathematics
and geometry remain the same. Likewise, the main
roof may be constructed of unequal pitches without
altering the geometry that will be shown here.
The main part of this project is simply a gable roof,
which has been described previously in Laws (2009).
13/11/12 1:17:13 PM
1 12 ADVANCED BUILDING AND JOINERY SKILLS
SUMMARY
s
As said at the outset of this chapter, advanced roofing is a very large and broad
subject. This chapter, in exploring only five of the many forms possible—gambrel,
jerkin head, skewed gable, oblique hip and uneven pitch—has barely touched
the surface. It is hoped, however, that in choosing these particular forms, and
in examining them as completely as space would allow, you have been equipped
sufficiently for further exploration of your own. As with all skills, time and practice
are essential to their development. The tasks that follow are therefore designed
to assist the development of the skills explored in each of the various sections.
For those for whom roofing, and its associated geometry and mathematics, is a
challenge, take heart: mathematics is but one way of looking at the world; as you
have been shown here, there are others, and they are just as trustworthy.
Text and PowerPoint
presentation
pa
ge
References and Further
Reading
pl
e
Laws, A. (2009), Site Establishment,
Formwork and Framing, Pearson Australia,
Frenchs Forest, NSW.
Sa
m
Costin, G.P. (2009), Chapter 5,
‘Construct a pitched roof: the Seven
Pillars of roofing’. Support material to
Laws (2009), Pearson Australia, Frenchs
Forest, NSW.
Australian Standards
AS 1684.2: 2010 Residential Timberframed Construction—Non-cyclonic Areas
AS 1684.3: 2010 Residential Timberframed Construction—Cyclonic Areas
Cheetham 03.indd 112
Web-based resources
There are myriad sites that offer,
sometimes for a fee, solutions to the
geometric and mathematical challenges
presented by the more complex roof
types. Most, however, do not spell out
the ‘why’ behind the ‘how to’, offering
the ‘answer’ only. Because of this, no
sites are specifically mentioned here.
However, the internet is very useful
and you are encouraged to explore
it frequently for new materials, fresh
or alternative solutions and, most
importantly, alternative perspectives
on the ‘why’. Indeed, often the key to
understanding is often found simply by
viewing the issue from another angle
or by the solution being presented in a
different form. The internet can often
provide these alternatives (though be
aware that what is ‘out there’ is not
always correct).
13/11/12 1:18:04 PM
113
CHAPTER 3 construct advanced roofs
Worksheet 1
Student name:_________________________________________________________________
To be completed by teachers:
Enrolment year: ______________________________________________________________
Student competent
Class code:______________________________________________________________________
Student not yet competent
Competency name/Number:___________________________________________
Task
Roof type:
Jerkin head
1800 mm
s
Characteristics:
pa
ge
Pitch: 40°
Span: 5000 mm
Ridge extension: 1800 mm
Rafter spacing: 600 mm
e
Rafters: 125 3 35 mm
pl
Hips: 150 3 35 mm
Find:
m
Eave: 450 mm
Height and width of the soldier wall
2.
Set-out length of first creeper
Sa
1.
Cheetham 03.indd 113
▲
continued 13/11/12 1:18:05 PM

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