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Introduction to Pitched Roof Framing

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Introduction to Pitched Roof Framing What s in this presentation: Basic concepts Pitched roofs can be coupled or non-coupled systems Treatment of skillion roofs – PowerPoint PPT presentation

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Title: 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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    roof framing)
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