Roofing concepts

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Introduction to Roofing Concepts and Roof Framing

• Whats 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

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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
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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.

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Example of Roof lines
Each of the roof lines below, are explained in
more detail on the following slides
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Eaves Line
Eaves lines define how much roof planes overhang
support walls
Walls
Ridge Line
Gable line
Gable Line
Valley line
Ridge Line
Hip line
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Eaves Line
Walls
Ridge Lines define where two opposing roof
planes meet at the highest point
Ridge Line
Gable line
Gable Line
Valley line
Ridge Line
Hip line
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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
Ridge Line
Gable line
Gable Line
Valley line
Ridge Line
Hip line
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Eaves Line
Walls
Ridge Line
Gable line
Gable Line
Valley line
Ridge 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.
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Eaves Line
Walls
Ridge Line
Gable line
Gable Line
Valley line
Ridge 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.
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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
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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
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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
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Side Elevation
End Elevation
Eave Line
Walls
Gable Line
Gable Line
Ridge Line
Hip Line
Hip Line
Plan View
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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.

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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
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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 its axis) instead of bending
  like a beam. Long spans are possible and
  internal walls arent required for support

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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

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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)

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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

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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

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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.

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

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• 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.

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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.

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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
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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.

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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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