introduction of roof framing system and concept

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
Hip
Dutch Hip
(or Dutch Gable)
Skillion
Hip and valley
Common roof shapes
used to cover the
required area are
shown
Roof shapes are made to cover the
footprint while also providing sloping
planes able to shed water
Gable
(Cathedral or flat ceiling)

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

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

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

7. 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
Gable
line
Ridge Line
Walls
Ridge
Line
Eaves Line

8. 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.
Gable
Line
Gable
line
Ridge Line
Walls
Ridge
Line
Eaves Line

9. 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.
Gable
Line
Gable
line
Ridge Line
Walls
Eaves Line
Ridge
Line

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

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
Ridge
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
Pitched
 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. 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)
Roof Load Width

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 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
1. Roofing materials - take
live/dead/wind loads and
transfers them to the
battens
Support wall

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

32. Go to back to the next presentation on “Pitched roofs”
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