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 Treatment of cathedral roofs Treatment of gable roofs Treatment of hip and valley roofs Treatment of wide hip and gable roofs

Continuous span rafter 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 Single span rafter Rafter Spacings Ridge support span Intermediate support Continuous span rafter Wall support Overhang

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 don’t have a horizontal member – thus loads must be resolved differently e.g. via vertical supports from walls and ridge beams

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 Wall supports resist rafter loads

Treatment of Cathedral Roofs Ridge beam Intermediate beam Rafters Wall support Post support 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

Treatment of Gable Roofs Ridge board Collar tie Ceiling Joist Outward forces resisted by ceiling joists and collar ties Rafter 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

Treatment of Hip and Valley Roofs Creeper rafter Valley rafter Cripple creeper rafter Broken hip Hip rafter Valley creeper rafter Jack rafter (crown end) “Common” rafters (used in gabled areas) Hips are typically added 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

Intermediate Support for Wider Buildings Underpurlin Strut Load from the underpurlin is transferred down the strut and into the Internal wall Ceiling Joist Rafter Ridge board Collar tie 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

Treatment of Wider Hip and Gable Roofs Internal wall Underpurlin Strut Strutting beam Ceiling joist 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

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