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