1. Introduction to Roofing Concepts and Roof Framing
What’s in this presentation:
Basic roof shapes
Reading roof shapes from lines on a drawing
Explaining roof lines
Focus on gable, hip, dutch gable and valley roofs
Generic approaches to roof framing
Differences between pitched and trusses roofs
Roof load width
Loads on roof framing
Transferring loads
Typical bracing requirements
Links to presentations on pitched and trussed roofs

2. Basic Roof Shapes
The footprint of a building generally consists of a rectangular block or multiple blocks joined together
Roof shapes are made to cover the footprint while also providing sloping planes able to shed water
Skillion
Common roof shapes used to cover the required area are shown
Gable (Cathedral or flat ceiling)
Hip
Dutch Hip (or Dutch Gable)
Hip and valley

3. Reading Roof Shapes From Lines on a Drawing
In technical drawings, roof planes are defined using lines describing the boundaries of roof planes or lines between them, including:
Ridge Lines
Gable Lines
Eaves lines
Hip Lines
Valley Lines
Being able to read these lines is important because they show:
Where roof shapes are positioned in the overall roof plan
The span and length of each individual roof shape
How each individual roof shape links in with others
This information is important in roof framing setout.

4. Example of Roof lines
Each of the roof lines below, are explained in more detail on the following slides
Gable Line
Gable line
Ridge Line
Walls
Valley line
Hip line
Eaves Line

5. Eaves lines define how much roof planes overhang support walls
Gable line
Ridge Line
Gable Line
Ridge Line
Valley line
Hip line

6. Eaves Line
Walls
Ridge Lines define where two opposing roof planes meet at the highest point
Gable line
Ridge Line
Gable Line
Ridge Line
Valley line
Hip line

7. Eaves Line
Walls
Gable Lines occur where the ends of roof planes run at 900 to the ridge line. They may be flush with end walls or form an overhang
Gable line
Ridge Line
Gable Line
Ridge Line
Valley line
Hip line

8. Hip lines typically connect to the outer end of a ridge line.
Eaves Line
Walls
Gable line
Ridge Line
Gable Line
Ridge Line
Valley line
Hip line
Hip Lines are created by the meeting of two roof planes forming an external corner. The planes are usually at 900 to each other and where they intersect, a bisecting 450 hip line is formed.
Hip lines typically connect to the outer end of a ridge line.

9. Eaves Line
Walls
Gable line
Ridge Line
Gable Line
Ridge Line
Valley line
Hip line
Valley Lines often run parallel to hip lines but always occur at internal corners not external corners. In addition, roof planes fall into valleys rather than falling away from hips. Valleys connect to the ridge line but always at the inner end.

10. Gable Roof Shapes
The Gable is one of the simplest and most common types of roof. Its supported by the side walls of a rectangular wall layout e.g. like two playing cards leaning against each other.
There are different types of gable ends:
Flush gables are signified by the gable line being flush with the end wall
Open gables are signified by the gable line overhanging the end wall and following the slope of the roof
Boxed gables overhang the end wall but the outer face of the gable is enclosed by cladding (as shown below).
Gable

11. Open or boxed Gable Line
Side Elevation
End Elevation
Walls
Ridge Line
Open or boxed Gable Line
Flush Gable Line
Eave Line
Plan View
Boxed Gable Line
Open Gable line

12. Hip Roof Shapes
The Hip Roof contains roof planes sloping down to side and end walls
The perimeter eave continues at the same level on all sides
If the pitches of all roof planes are the same and the support walls are in a rectangular shape, the hip lines are at 45 degrees to the side walls.
Hip

13. Side Elevation End Elevation Eave Line Walls Ridge Line Hip Line
Plan View

14. Dutch Gable Roof Shapes
The Dutch Gable is a combination of the gable and hip roof shapes. Imagine it as a hip roof but with shortened hip lines, an extended ridge line, and a a gable line linking the ridge and hips together.
The proportional appearance of the hip compared to the gable can be changed to suit architectural requirements.
Dutch Gable

15. Eave Line Walls Gable Line Gable Line Ridge Line Hip Line Hip Line
Side Elevation
End Elevation
Eave Line
Walls
Gable Line
Gable Line
Ridge Line
Hip Line
Hip Line
Plan View

16. Valley Shapes
A Valley occurs where two roofs perpendicular to each other join together.
More specifically, the ridge from the smaller roof extends inwards until it butts into the larger roof. Valleys form on the side(s) of the ridge where running down to internal corners in the building layout.

17. End Elevation
Ridge Line
Valley line
Walls
Side Elevation
Plan View

18. Generic Approaches to Roof Framing
The previous roof shapes can be framed using two approaches
Pitched roofs (i.e. raftered roofs cut and erected on site)
Trussed roofs (engineered frames made in a factory, erected on-site)
Click above to see a video

19. Differences Between Pitched and Trussed Roofs
Pitched roofs have evolved from traditional origins - rafters are pitched onsite like raking beams supported by external walls; inner roof framing members provide additional support and are supported by internal walls
Trussed roofs utilise contemporary engineering principles - each piece of timber in the truss is designed to be axially loaded (stretching or squashing it along it’s axis) instead of bending like a beam. Long spans are possible and internal walls aren’t required for support
Pitched (raftered roof)
Trussed roof

20. Pros and Cons of Pitched and Trussed Roofs
Site focused process (bad weather can restrict progress)
Trade skills and site crafting are important to calculate and cut the required roof geometry
The site based process has greater ability to deal with unexpected design problems and variations
More labour intensive than trussed roofs
Trussed
Factory environment involves a more automated and repetitious process - therefore potential for better quality control
Less site work means less affected by bad weather
Makes maximum structural use of the timber
Capable of long spans
Internal walls are usually non-loadbearing therefore lighter weight internal walls are possible

21. Roof Load Width
Irrespective or the method of framing, trusses or rafters are set up at regular intervals to form the three dimensional shape of the roof
Each supports loads from a certain contributing area of the roof and this influences the size of the members used
The contributing area is usually a strip whose width is defined by the mid-lines between adjacent rafters of trusses (as shown)
Trusses and rafters are often spaced 600mm or more according to local practise and the type of roofing materials (e.g. sheet metal roof)

22. Specific Loads on Roofs
Loads falling within the roof load width include:
Gravity Dead Loads including roof and ceiling materials
Gravity Live Loads including people working on the roof and stuff stacked on it
Wind loads including downward pressure or suction that lifts upwards – these are only felt some of the time but downward pressure adds to the above gravity loads, while uplift works in the opposite directions

23. Gravity Dead Load
The weight of the roofing material can be expressed as weight (kg) per unit area of roof (square metres), ie. (kg/m2)
The weight of a tiled roof with battens, a plasterboard ceiling and insulation is approximately 75 kg/m2
The weight of a sheet metal roof with softwood ceiling and insulation is approximately 20 kg/m2
DEAD LOAD (structure)

24. Gravity Live Loads
Live loads result from the occasional presence of people and materials on the roof
We must allow for the weight of a large person standing anywhere on the roof.
Live loads
(people,)
Construction loads
(people, materials)

25. Wind loads
Wind loads push against the roof but can also cause uplift and suction
The amount of wind load which acts on the roof depends on several things - the most important being the speed of the wind
Suction
Internal
Wind
Suction

26. As the wind speed increases so does wind load – this load is spread over the area of the building exposed to the wind

27. When the wind passes over a roof it can cause a suction
When the wind passes over a roof it can cause a suction. When it gains access to the interior it can cause an uplift
The roof must be strong enough to resist the load developed by suctions and uplift. The frame must be attached adequately to the rest of the structure so the whole roof is not sucked off.
Suction
Internal
pressure
Suction (uplift)
Wind

28. Combinations of loads
More than one type of load can be acting on the roof at the same time
This may be a combination of gravity dead loads plus gravity live load, plus wind loads – all acting downwards.
In other instances wind may be acting upwards (where suction and uplift occur), therefore acting in the opposite direction to gravity dead and live loads.
In high wind areas, wind uplift can easily exceed downward gravity loads. For resisting uplift, the heavy dead load from a tiled roof is useful.

29. Transferring Loads to Pitched and Trussed Roofs
2. Battens - take roofing loads and transfers them to the rafters/trusses
3. Rafters/Trusses – take batten loads and transfers them to the support structure below e.g. walls
Support wall
1. Roofing materials - take live/dead/wind loads and transfers them to the battens

30. Typical Bracing for Pitched Roofs
Bracing is essential for providing stability to the roof frame under all loading conditions. Bracing for a gable roofing is shown above. Though not shown, hip ends provide a self bracing effect.

31. Typical Bracing for Trussed Roofs
Bracing in trussed roofs make significant use of “steel brace” applied in “X” and “V” patterns across the roof planes. Trussed roofs must especially prevent buckling of members and must address wind uplift

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