Gable Roof
BCGCA3007B
Gable Roof
• Flush with no eaves
• Flush with raked eaves
• Boxed
Flush Gable (No Eaves)
•Rafter finishes in line
with end wall
•Barge fixed to outside
wall
•No Overhang
Studwork to take sheeting or
support brickwork
Not required to support roof
Flush Gable with Raked Eaves
Ridge Extends to
Form Eave
Verge Trimmers to
Support Gable Rafter
Trimmers to Support
Verge Trims and
Roof Battens
Gable Stud supports
1. Second Rafter
2. Brick Work or
Wall Lining
Second Rafter to
Support Verge Trims
Top Plate may be
extended to support
Rafter
Roof Extends to Form eave
Boxed Gable
Verge Rafter
Gable Studs & Sheeting
Purlin & Lintel
Extended to Support
Verge Rafter
Minimum Back span
2 x Overhang
Design 3 x Overhang
Base of Gable Lined & Level
with U/S of Eaves
Verges
Is the Junction of the Roof and the Barge/Verge Board
Verge Detail for Tiled Roof
Verge Detail for Tiled Roof
Principles of Roofing
1. Ridges are Level
2. Rafters run at 90
3. Hips & Valleys bisect all internal & external Corners
4. Roof members are set out along centerlines
Roofing Members
Ridge
Common Rafter
Collar Tie
Top Plate
Hanging Beam
Purlin
Strut
Ceiling Joist
Top Plate
Strutting
Beam
Strut
Roof Members
Common Rafter –
• Main Sloping Roof Member
• Spacing 450mm or 600mm for Tiled Roofs
• 900mm for tiled roofs
• Supports Roof Battens which in turn support
roof coverings
• Must be in single lengths or joined over
supports
• Fixings 2 x 75mm Skew & 2 x 75mm into
Ceiling Joist (if joist is > 38, 90mm Nails)
Roofing Members
Ridge
Common Rafter
Collar Tie
Top Plate
Hanging Beam
Purlin
Strut
Ceiling Joist
Top Plate
Strutting
Beam
Strut
Ridge
•
•
•
•
•
Highest Part of the Roof
Will run full length of Gable Roof
Runs Parallel to TOP PLATE
Fixes Rafters at Top of Roof
Rafters are nailed either side and not offset by
more than 1 thickness
• In Uncoupled Roof, they act as beams
• Nailed to Rafters with 2 x 75mm Nails
Joining Ridge Boards
Roofing Members
Ridge
Common Rafter
Collar Tie
Top Plate
Hanging Beam
Purlin
Strut
Ceiling Joist
Top Plate
Strutting
Beam
Strut
Purlin
•
•
•
•
•
Also known as Underpurlin
Fixed to Underside of RAFTER
Runs parallel to ridge and wall plates
Reduce span of RAFTER
Will run full length of Gable Roof
Roofing Members
Ridge
Common Rafter
Collar Tie
Top Plate
Hanging Beam
Purlin
Strut
Ceiling Joist
Top Plate
Strutting
Beam
Strut
Struts
• Transfer Loads to Load Bearing Walls
Roofing Members
Ridge
Common Rafter
Collar Tie
Top Plate
Hanging Beam
Purlin
Strut
Ceiling Joist
Top Plate
Strutting
Beam
Strut
Collar Ties
Common Rafter Principles
Birdsmouth Max 1/3
Depth of Rafter
Common Rafter Principles
Span – From Birdsmouth to Birdsmouth
Half Span – From Birdsmouth to Centre
Common Rafter Principles
True Rise –From
Horizontal line that passes thru the Intersection of Birdsmouth Plumb Line &Top of Rafter
To : vertically plumb up to the Apex
NOTE – True Rise is not measured from TOP PLATE
A Closer Look
Top Of Rafter
Birds Mouth
Top Plate
Ceiling Joist
NOTE – True Rise is not measured from TOP PLATE
Common Rafter Principles
If Rise is taken from Top Plate to Apex pitch will be incorrect
Common Rafter Principles
Pitching Line
x-y line
Common Rafter Principles
Measured to
edge of Ridge
Measured to
centre of Ridge
Common Rafter Principles
True Length
Centre Line
Length
True Length is usually what we need
Determine Rafter Length
Mathematically
Determine Span
Determine Half Span
Determine Rafter Run
Half Ridge Thickness
= 2700mm
= 2700/2
= 1350mm
= 1350= 19/2 = 9.5
Rafter Run
= 1341.5
True Length Rafter = 1341.5 / Cos 25
= 1480
Rafter 90 x 45
Ridge 125 x 19
Overhang
Plan always from external wall
Determine Rafter Length
Mathematically
In this case Timber
Famed Wall
O/H = 450 / cos 25
= 497mm
•Roofing calculations are always measured from
Birdsmouth or pitching point
Rafter 90 x 45
Ridge 125 x 19
Determine Rafter Length
Mathematically
In this case Brick Veneer Wall
O/H = 450 + 40 + 110
cos 25
Total Rafter Length
Timber Frame = 1480 + 497 = 1977mm
Brick Veneer = 1480 + 662 = 2142mm
= 662mm
Rafter 90 x 45
Ridge 125 x 19
Determine Rafter Length
Graphically
1.
2.
3.
4.
Draw Roof Full-size
Measure members directly
Avoid using scaled drawing
Scale use only for angles
Rafter 90 x 45
Ridge 125 x 19
Determine Rafter Length
Using Roofing Square
Use Calculator
1. Press Tan 25 – what does this give you
2. Therefore for every 1 metre run there is
0.466m rise
3. Using the principle of similar triangles
we half the size of the triangle
Use Calculator
1. Press Tan 25 – what does this give you
2. Therefore for every 1 metre run there is
0.466m rise
3. Using the principle of similar triangles
we half the size of the triangle
Use Calculator
1. Press Tan 25 – what does this give you
2. Therefore for every 1 metre run there is
0.466m rise
3. Using the principle of similar triangles
we half the size of the triangle
Using Similar Triangles
1341/500
= 2.682 or
= 2 r 341
Using Similar Triangles
1341/500
= 2.682 or
= 2 r 341
Step out 2 full
triangles
Select Start
Point
Allowing for
O/H
Intersection of
Top of Rafter &
Edge of Square
Using Similar Triangles
1341/500
= 2.682 or
= 2 r 341
Step out 2 full
triangles
Select Start
Point
Allowing for
O/H
Step out 341
& use square
to extend
Determine Roofing Angles
used in Gable Roofs
Plumb Cut
Foot Cut
Determining Angles with
Roofing Square
When we set out rafter
previously we determined
plumb cut
Determining Angles with
Roofing Square
Plumb Cut
Foot Cut
Determining Angles
Mathematically
1. Extend Line 90⁰ From Rafter
Determining Angles
Mathematically
1. Extend Line 90⁰ From Rafter
2. Extend Plumb Line
Determining Angles
Mathematically
1. Extend Line 90⁰ From Rafter
2. Extend Plumb Line
3. Angle Formed is same as
roof pitch
Determining Angles
Mathematically
1. Extend Line 90⁰ From Rafter
2. Extend Plumb Line
3. Angle Formed is same as
roof pitch
4. Offset = Tan (pitch) x width
Determining Angles
Mathematically
1. Extend Line 90⁰ From Rafter
2. Extend Plumb Line
3. Angle Formed is same as
roof pitch
4. Offset = Tan (25⁰) x 90
= 42
Determining Angles Mathematically
Next we can Determine our Birdsmouth
1. Width across plumb cut = 90/ cos 25
2.
= 99
3. Therefore max Birdsmouth = 33
4. Distance from top plate to top of
Rafter = 66
Complete Q5 in Workbook
Determine Roof Angles
Graphically
Plan – View
We can only see rafter run
Determine Roof Angles
Graphically
Extend Top Plate &
Ridge
Determine Roof Angles
Graphically
Extend Top Plate &
Ridge
Mark Rise
Determine Roof Angles
Graphically
Plumb Cut
Extend Top Plate &
Ridge
Mark Rise
Foot Cut
Draw Hypotenuse
Steps In Construction
Gable Roof
1.
2.
3.
4.
5.
6.
Physically Confirm Span & Plates are Parallel
Calculate True Rafter, Rise & Plumbcuts
Mark out ceiling joists, rafters & ridge
Install Ceiling Joists
Cut Pattern Rafter & test to confirm
Cut required rafters & install
Estimating Gable Roof - Rafter
From Previous Ceiling Estimate
Pitch = 25⁰
Determine No of Ceiling Joists 12 250/ 600 = 20.4167
= 21 + 1
= 22 @6900
Therefore 22 set of Rafters
= 44 + 4 Verge
= 48
Determine Rafter Length
Span
= 6900
Determine Rafter Length
Span
= 6900
Half Span = 3450
Determine Rafter Length
Span
= 6900
Half Span = 3450
Run
= 3450 – 9.5 (Half Ridge)
= 3340.5
Determine Rafter Length
Span
= 6900
Half Span = 3450
Run
= 3450 – 9.5
= 3440.5
True Length = 3440.5 / cos 24
=3766
Determine Rafter Length
Span
= 6900
Half Span = 3450
Run
= 3450 – 9.5
= 3440.5
True Length = 3440.5 / cos 24
=3766
Overhang = 450/cos 24
= 493
Determine Rafter Length
Span
= 6900
Half Span = 3450
Run
= 3450 – 9.5
= 3440.5
True Length = 3440.5 / cos 24
=3766
Overhang = 450/cos 24
= 493
Total Rafter = 4259
Estimating Sheet
Order
Item
Rafters
Section Size
90 x 45
Material
Radiata
No.
48
Length
4.259
Linear
4.500
Total Linear
216.000
Rate / lm
$
2.30
$
Cost
496.80
Estimating Gable Roof - Purlin
Pitch = 24⁰
Overhang 450
Rafter 90 x 45 F7
600 c to c
Tiled Roof
Ridge 140 x 19
Purlin 90 x 70 F7
Purlin
Run full length of Roof
Do we need purlins
Span Required = 3766
In First Instance we
check single span
Max Span = 1900
Max Span 2600
3766/2600 = 1.4
Therefore 1 row required each side
One Row of Purlin Each Side
Estimating Gable Roof -Purlin
Pitch = 24⁰
Overhang 450
Rafter 90 x 45 F7
600 c to c
Tiled Roof with
Flush Gable &
Raking Eaves
Ridge 140 x 19
Purlin 90 x 70 F7
Purlins Run Full Length of Gable Roof
2 @ 12.500 + 2 x 150 (Joins)
= 12.800
Estimating Sheet
Order
Item
Rafters
Purlins
Section Size
90 x 45
90 x 70
Material
Radiata
Radiata
No.
48
2
Length
4.259
12.800
Linear
4.500
12.900
Total Linear
216.000
25.800
Rate / lm
$
2.30
$
3.80
$
$
Cost
496.80
98.04
Estimating Gable Roof - Ridge
Pitch = 24⁰
Overhang 450
Rafter 90 x 45 F7
600 c to c
Tiled Roof with
Flush Gable &
Raking Eaves
Ridge 140 x 19
Purlin 90 x 70 F7
Ridge Runs Full Length + Overhang
12.500 +2 x 450 (O/H)
13400 + 2 x 300 (allow for joins)
=14000mm
Note –
For Flush eaves there is no O/H ridge = 12500 + joins
Boxed eaves will require O/H ridge = 13400 + Joins
Estimating Sheet
Order
Item
Rafters
Purlins
Ridge
Section Size
90 x 45
90 x 70
140 x 19
Material
Radiata
Radiata
Radiata
No.
48
2
1
Length
4.259
12.800
14.000
Linear
4.500
12.900
14.100
Total Linear
216.000
25.800
14.100
Rate / lm
$
2.30
$
3.80
$
3.50
$
$
$
Cost
496.80
98.04
49.35
Estimating Gable Roof - Struts
Pitch = 24⁰
Overhang 450
Rafter 90 x 45 F7
600 c to c
Tiled Roof with
Flush Gable &
Raking Eaves
Ridge 140 x 19
Purlin 90 x 70 F7
Span 2700
Struts are Difficult to Estimate
Develop a Best Guest Method
Determine No Required
12500/2700 = 4.6 = 5 = 6 Each Side
Estimating Gable Roof - Struts
Pitch = 24⁰
Overhang 450
Rafter 90 x 45 F7
600 c to c
Tiled Roof with
Flush Gable &
Raking Eaves
Ridge 140 x 19
Purlin 90 x 70 F7
Span 2700
Struts are Difficult to Estimate
Develop a Best Guest Method
Determine No Required
12500/2700 = 4.6 = 5 = 6 Each Side
Rise (at Strut Pos) x √2 x √2
Tan 24⁰ x 1721 x 2
= 1532
Strut Position
Estimating Sheet
Order
Item
Rafters
Purlins
Ridge
Struts
Section Size
90 x 45
90 x 70
140 x 19
90 x 70
Material
Radiata
Radiata
Radiata
Radiata
No.
44
2
1
12
Length
4.259
12.800
14.000
1.532
Linear
4.500
12.900
14.100
1.800
Total Linear
198.000
25.800
14.100
21.600
$
$
$
$
Rate / lm
2.30
3.80
3.50
3.80
$
$
$
$
Cost
455.40
98.04
49.35
82.08
Estimating Gable Roof – Collar Ties
Pitch = 24⁰
Overhang 450
Rafter 90 x 45 F7
600 c to c
Tiled Roof with
Flush Gable &
Raking Eaves
Ridge 140 x 19
Purlin 90 x 70 F7
Span 2700
Collar Ties 90 x 35
Collar Ties on Every 2nd Rafter
On top of Purlins
22/2 = 11 @ HALF SPAN
11 @ 3450
Estimating Sheet
Order
Item
Section Size
Rafters
90 x 45
Purlins
90 x 70
Ridge
140 x 19
Struts
90 x 70
Collar Ties
90 x 35
Material
Radiata
Radiata
Radiata
Radiata
Radiata
No.
44
2
1
6
11
Length
4.259
12.800
14.000
3.064
3.450
Linear
4.500
12.900
14.100
3.300
3.600
Total Linear
198.000
25.800
14.100
19.800
39.600
Rate / lm
$
2.30
$
3.80
$
3.50
$
3.80
$
2.00
$
$
$
$
$
Cost
455.40
98.04
49.35
75.24
79.20
Estimating Gable Roof – Verge Trims
Pitch = 24⁰
Overhang 450
Rafter 90 x 45 F7
600 c to c
Tiled Roof with
Flush Gable &
Raking Eaves
Ridge 140 x 19
Purlin 90 x 70 F7
Span 2700
Collar Ties 90 x 35
True Length of Rafter
Rafter True length / 600
= 6.28 = 7 (Ridge Closes)
28 @ 1200 = 33.6
Say 33.9
Estimating Sheet
Order
Item
Section Size
Rafters
90 x 45
Purlins
90 x 70
Ridge
140 x 19
Struts
90 x 70
Collar Ties
90 x 35
Verge Trims
90 x 45
Material
Radiata
Radiata
Radiata
Radiata
Radiata
Radiata
No.
44
2
1
6
11
1
Length
4.259
12.800
14.000
3.064
3.450
33.900
Linear
4.500
12.900
14.100
3.300
3.600
33.900
Total Linear
198.000
25.800
14.100
19.800
39.600
33.900
Total
$
$
$
$
$
$
Rate / lm
2.30
3.80
3.50
3.80
2.00
2.30
$
$
$
$
$
$
Cost
455.40
98.04
49.35
75.24
79.20
77.97
$
835.20