1. 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

2. 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

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 don’t 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
Wall supports resist rafter loads

5. 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

6. 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

7. 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

8. 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

9. 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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