BCGA3007B Construct Pitched Roofs (Simple). Types of Roofs we will Cover Coupled Roofs Gable Roofs.
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BCGA3007B
Construct Pitched Roofs(Simple)
Types of Roofs we will Cover
Coupled Roofs• Gable Roofs
Types of Roofs we will Cover
Non Coupled Roofs• Skillion Roofs• Lean to• Graded Battens
What is a Roof
Roof Structure Definitions
Roof Structure Definitions
Flat Roofing
• Pitch less than 1:11.5 or 5⁰• Ceilings usually directly fixed to underside.
– Member Sizes Adjusted Accordingly
• Usually Single Span, therefore large members• Due to low pitch roof coverings types are
limited
Skillion Roofs
• Freestanding• Wall at one end is
higher• Walls parallel to rafters
are tapered
Selecting Members
What determines rafter spacing ?1.Ceiling Linings – if direct fixing2.Roof Batten Span3.Required Rafter span4.Roof Material Mass5.Ceiling Mass – if direct fixing
Selecting Members
For types of roof we will cover today
Always Check Notes first to ensure it is suitable
Roof Battens Have Span Limits – it is not uncommon where ceilings are unlined to space rafters greater than 650 c to c. Note 70 x 35 will be unsuitable
Select Rafter SizePitch 3°Rafters 600 c to c
Step 1 – Determine Rafter Span
Span is defined in AS 1684.2cl 2.7.5.1 & 2.7.5.3 as distance between points of full support
Note Horizontal Distance is called run
Span is also used to determine rafters length mathematically.
In this case it is measured differently and will be explained later
Rafter SpacingRead Notes
Roof Load
190 x 45 with maximum overhang of 1550
Methods to Calculate Rafter Length
Rise 1 in 20Rafters 600 c to c
To Calculate Rafter Length we need to Determine the Run of the Rafter.
Generally in this type of roof it will be from back of birdsmouth to back of birdsmouth
Forming the above triangle
Methods to Calculate RafterRise 1 in 20Rafters 600 c to c
Rise = Every 20 meters, Rise of 1 metersRise per m = 1/20
= 0.05 x 4.6Rise = 0.230
Use Pythagoras to Calculate Rafter Length Rafter = √ (4.62 + 0.2302)
= 4.606
To Determine Pitch
Pitch =230/ 4600= 0.05= Inv Tan 0.05= 2.862°
Methods to Calculate RafterRise 1 in 20Rafters 600 c to c
If we are given the pitch = = 2.862°
Rafter = Run / Cos Pitch
Rafter = 4600/Cos = 2.862°
= 4600/0.9988
=4606
Complete FollowingRun Pitch Rise Rafter4.635 0.5⁰3.652 1.2⁰2.145 2.3⁰3.254 5⁰5.236 4.2⁰
Run Rise Rise Rafter Pitch4.635 1 in 203.652 1 in 252.145 1 in 403.254 1 in 305.236 1 in 50
AnswersRun Pitch Rise Rafter4.635 0.50 0.040 4.6353.652 1.20 0.076 3.6532.145 2.30 0.086 2.1473.254 5.00 0.285 3.2665.236 4.20 0.385 5.250
Run Rise Rise Rafter Pitch4.635 1 in 20 0.232 4.641 2.863.652 1 in 25 0.146 3.655 2.292.145 1 in 40 0.054 2.146 1.433.254 1 in 30 0.108 3.256 1.915.236 1 in 50 0.105 5.237 1.15
Alternative Set Out Length = 4606mm
Step 1 – Set out Rafter Length along one edge
Remember to allow for overhang
Step 2 – Extend Plumb Lines to other edge
Similar Triangles
If all angles are the same the triangles are similarAll sides are increased by same magnitude
Similar Triangles
Basic Steel Square Demo
Alternate Method – Using Similar TrianglesRoof Pitch = 3°Tan 3⁰ x 1m =0.052m (Answer is always value for 1)Remember angles stay the same in similar triangles
Rise for triangle half size == tan 3⁰ x 0.500m = 0.026m or 26mm
Plumb Cut
Roof Run = 4600Roof Rise = 210Ratio =210/4200
= 0.05
Square Run = 500mmRise = 500 x 0.05 = 25
1
3⁰
Graded Batten Roof
• Where direct fix ceiling is required to be level on flat roof
• Fall is achieved with Battens
Constructing a Skillion Roof
Place End Rafters, allowing for soldier trims
Place Rafters at maximum 600 centres (580) Provide Solid Blocking at 1800 centers max
Install Soldier Trims
Cut Overhangs & Install Verge Trims
Install Wall Blocking
Install Fascia
KliplockMinimum Pitch 1⁰
TrimdeckMinimum Pitch 2⁰
Custom Orb Generally Known as Corrugated Iron
Minimum Pitch 5⁰Not Suitable For Flat Roofs
Maximum Recommended Support Spacings
Type of Span Thickness (mm) BMT
0.42 0.48
RoofsSingle SpanEnd SpanInternal SpanUnstiffened Eaves OverhangStiffened Eaves OverhangWallsSingle Span End Span Internal SpanOverhang
700900 1200200300
18002500 2700 200
80013001700250350
180027002700250
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