Introduction to Pitched Roof Framing

1
Introduction to Pitched Roof Framing

• Whats in this presentation
• Basic concepts
• Pitched roofs can be coupled or non-coupled
  systems
• Treatment of skillion roofs
• Treatment of cathedral roofs
• Treatment of gable roofs
• Treatment of hip and valley roofs
• Treatment of wide hip and gable roofs

2
Basic Concepts

• Pitched roofs rely on rafters to provide the main
  skeleton of the frame
• Distribution of roof loads is determined by
  rafter spans, overhangs and rafter spacings
• Ridge, intermediate and wall supports share the
  load
• Intermediate supports break rafters into two
  spans, thus making them continuous rather than
  single span elements

3
Pitched Roofs can be Coupled or Non-coupled
Systems

• Coupled roofs use a horizontal member to couple
  the base of the two opposing rafters together
  thus resisting loads running down the rafters
• Non-coupled roofs dont have a horizontal member
  thus loads must be resolved differently e.g.
  via vertical supports from walls and ridge beams

4
Treatment of Skillion Roofs

• Skillion roofs are classified as non-coupled
  roofs
• Skillions have evolved from the most basic of
  shelter structures
• Building width is limited by the available size
  of rafter timbers
• Rafters slope is one directional
• Rafters take both roof and ceiling loads
• No horizontal framework coupling opposing sides
  of the roof together

5
Treatment of Cathedral Roofs

• Cathedral roofs have some similarities with
  Skillions
• Rafters take both roof and ceiling loads
• They are non-coupled roofs i.e. no horizontal
  framework coupling opposing sides of the roof
  together

• Cathedral roofs also have
• Two directional opposing roof planes (these may
  have different pitches and rafter lengths)
• Greater emphasis on beams to increase rafter span
• ridge beams support the tops of rafters
• intermediate beams break rafter lengths into two
  spans
• beams may be supported by posts instead of walls

6
Treatment of Gable Roofs

• Gable roofs typically utilise the coupled roof
  concept i.e
• Two equal and opposing rafters push against each
  other (via the ridge board which locates and
  stabilises the rafter ends)
• Some loads run down the rafters causing the
  support walls to splay outwards (like a
  collapsing roof made of cards) but ceiling joists
  and collar ties prevent this action by coupling
  the two sides together
• Rafters support the roofing but ceiling joists
  support the ceiling

7
Treatment of Hip and Valley Roofs

• Hips are typicallyadded to the ends of gable
  roof sections
• Broken hips and valleys occur where the main roof
  branches off into smaller roofs

• Different rafter types are used in hips and
  valleys compared to gable roof sections
• Hip and Valley rafters define the main
  changes in roof lines
• Creeper rafters - which graduate in length -
  fill between Hip and Valley rafters

8
Intermediate Support for Wider Buildings

• Roofs for wider buildings (including gable and
  hip roofs) can be achieved by adding
  underpurlins a type of beam offering
  intermediate support. This is often more
  efficient than using large rafters to span large
  distances
• Underpurlins break rafter lengths into two spans
• Struts (a type of raking column) support
  underpurlins along the length
• Struts are supported directly by internal walls
  or strutting beams supported by internal walls
  (ceiling joists are not involved)
• Other forms of support for struts are also
  possible

9
Treatment of Wider Hip and Gable Roofs

• Wider hip and gable roofs can be achieved by
  adding underpurlins a type of beam offering
  intermediate support.
• Underpurlins break rafter lengths into two spans
• Underpurlins are often efficient than large
  rafters required to make single spans
• Struts (a type of raking column) support
  underpurlins
• Struts are supported by internal walls or by
  strutting beams which are in turn supported by
  internal walls
• Ceiling joists are not structurally involved in
  supporting struts

10
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