Introduction to Roofing Introduction to Roofing Concepts and Roof Concepts and Roof FramingFraming

What’s in this presentation:What’s in this presentation:

Basic roof shapesBasic roof shapes

Reading roof shapes from lines on a drawingReading roof shapes from lines on a drawing

Explaining roof linesExplaining roof lines

Focus on gable, hip, dutch gable and valley roofsFocus on gable, hip, dutch gable and valley roofs

Generic approaches to roof framing Generic approaches to roof framing

Differences between pitched and trusses roofsDifferences between pitched and trusses roofs

Roof load widthRoof load width

Loads on roof framingLoads on roof framing

Transferring loads Transferring loads

Typical bracing requirementsTypical bracing requirements

Links to presentations on pitched and trussed roofs Links to presentations on pitched and trussed roofs

Basic Roof ShapesBasic Roof ShapesThe footprint of a building generally consists of a The footprint of a building generally consists of a rectangular block or multiple blocks joined togetherrectangular block or multiple blocks joined together

Hip

Dutch Hip (or Dutch Gable)

Skillion

Hip and valley

Common roof shapes Common roof shapes used to cover the used to cover the required area are required area are shownshown

Roof shapes are made to cover the Roof shapes are made to cover the footprint while also providing sloping footprint while also providing sloping planes able to shed waterplanes able to shed water

Gable (Cathedral or flat ceiling)

Reading Roof Shapes From Lines Reading Roof Shapes From Lines on a Drawingon a Drawing

In technical drawings, roof planes are defined using lines describing In technical drawings, roof planes are defined using lines describing the boundaries of roof planes or lines between them, including:the boundaries of roof planes or lines between them, including:

Ridge LinesRidge Lines Gable LinesGable Lines Eaves linesEaves lines Hip LinesHip Lines Valley LinesValley Lines

Being able to read these lines is important because they show:Being able to read these lines is important because they show: Where roof shapes are positioned in the overall roof planWhere roof shapes are positioned in the overall roof plan The span and length of each individual roof shapeThe span and length of each individual roof shape How each individual roof shape links in with othersHow each individual roof shape links in with others

This information is important in roof framing setout.This information is important in roof framing setout.

Each of the roof lines below, are explained in more detail on the Each of the roof lines below, are explained in more detail on the following slidesfollowing slides

Example of Roof lines Example of Roof lines

Gab

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Gab

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Ridge Line

Walls

Valley line

Hip line

Ridge Line

Eaves Line

Eaves linesEaves lines define how much roof define how much roof planes overhang support wallsplanes overhang support walls

Gab

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Gab

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Ridge Line

Walls

Valley line

Hip line

Ridge Line

Eaves Line

Ridge LinesRidge Lines define where two define where two opposing roof planes meet at opposing roof planes meet at the highest pointthe highest point

Gab

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Gab

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Ridge Line

Walls

Valley line

Hip line

Ridge Line

Eaves Line

Gable LinesGable Lines occur where the occur where the ends of roof planes run at 90ends of roof planes run at 9000 to the ridge line. They may be to the ridge line. They may be flush with end walls or form an flush with end walls or form an overhangoverhang G

able

Lin

e

Gab

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Ridge Line

Walls

Valley line

Hip line

Ridge Line

Eaves Line

Hip LinesHip Lines are created by the meeting of are created by the meeting of two roof planes forming an external corner.two roof planes forming an external corner.The planes are usually at 90The planes are usually at 900 to each other to each other and where they intersect, a bisecting 45and where they intersect, a bisecting 450 hip hip line is formed.line is formed.

Hip lines typically connect to the outer end of a Hip lines typically connect to the outer end of a ridge line.ridge line.

Gab

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Gab

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Ridge Line

Walls

Valley line

Hip line

Ridge Line

Eaves Line

Valley LinesValley Lines often run parallel to hip lines but always occur at often run parallel to hip lines but always occur at internal corners not external corners. In addition, roof planes fall internal corners not external corners. In addition, roof planes fall into valleys rather than falling away from hips. Valleys connect into valleys rather than falling away from hips. Valleys connect to the ridge line but always at the inner end.to the ridge line but always at the inner end.

Gab

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Gab

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Ridge Line

Walls

Valley line

Hip line

Eaves Line

Ridge Line

Gable Roof Shapes Gable Roof Shapes The Gable is one of the simplest and most common types of roof. Its 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 supported by the side walls of a rectangular wall layout e.g. like two playing cards leaning against each other. playing cards leaning against each other.

There are different types of gable ends:There are different types of gable ends: Flush gables are Flush gables are signified by the gable line being flush with the end wall signified by the gable line being flush with the end wall OOpen gablespen gables are signified by the gable line overhanging the end wall and are signified by the gable line overhanging the end wall and

following the slope of the rooffollowing the slope of the roof Boxed gablesBoxed gables overhang the end wall but the outer face of the gable is overhang the end wall but the outer face of the gable is

enclosed by cladding (as shown below). enclosed by cladding (as shown below).

Gable

Side Elevation End Elevation

Walls

Ridge Line

Open or boxed Gable Line

Flush Gable Line

Eave Line

Plan View

Boxed Gable Line

Open Gable line

Hip Roof ShapesHip Roof Shapes

The Hip Roof contains roof planes sloping down to side and end The Hip Roof contains roof planes sloping down to side and end walls walls

The perimeter eave continues at the same level on all sidesThe perimeter eave continues at the same level on all sides

If the pitches of all roof planes are the same and the support walls 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 are in a rectangular shape, the hip lines are at 45 degrees to the side walls.side walls.

Hip

Side Elevation End Elevation

Walls

Ridge Line

Hip LineHip Line

Eave Line

Plan View

Dutch Gable Roof ShapesDutch Gable Roof Shapes

The Dutch Gable is a combination of the gable and hip 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 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. ridge line, and a a gable line linking the ridge and hips together.

The proportional appearance of the hip compared to the gable can The proportional appearance of the hip compared to the gable can be changed to suit architectural requirements. be changed to suit architectural requirements.

Dutch Gable

End ElevationSide Elevation

Walls

Ridge Line

Hip LineHip Line

Eave Line

Plan View

Gable Line Gable Line

Valley ShapesValley Shapes

A Valley occurs where two roofs perpendicular to each other join A Valley occurs where two roofs perpendicular to each other join together. together.

More specifically, the ridge from the smaller roof extends inwards 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 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. ridge where running down to internal corners in the building layout.

End Elevation

Ridge Line

Valley line

Ridge Line

Valle

y lin

e

Walls

Side Elevation

Plan View

Generic Approaches to Roof Generic Approaches to Roof FramingFraming

The previous roof shapes can be framed using two approachesThe previous roof shapes can be framed using two approaches Pitched roofs (i.e. raftered roofs cut and erected on site)Pitched roofs (i.e. raftered roofs cut and erected on site) Trussed roofs (engineered frames made in a factory, erected on-site) Trussed roofs (engineered frames made in a factory, erected on-site)

Click above to see a video

Differences Between Pitched Differences Between Pitched and Trussed Roofsand Trussed Roofs

Pitched roofs have evolved from traditional origins - rafters are pitched onsite Pitched roofs have evolved from traditional origins - rafters are pitched onsite like raking beams supported by external walls; inner roof framing members like raking beams supported by external walls; inner roof framing members provide additional support and are supported by internal wallsprovide additional support and are supported by internal walls

Trussed roofs utilise contemporary engineering principles - each piece of Trussed roofs utilise contemporary engineering principles - each piece of timber in the truss is designed to be axially loaded (stretching or squashing it timber in the truss is designed to be axially loaded (stretching or squashing it along it’s axis) instead of bending like a beam. Long spans are possible and along it’s axis) instead of bending like a beam. Long spans are possible and internal walls aren’t required for support internal walls aren’t required for support

Pitched (raftered roof)

Trussed roof

Pros and Cons of Pros and Cons of Pitched and Trussed RoofsPitched and Trussed Roofs

PitchedPitched Site focused process (bad Site focused process (bad

weather can restrict progress)weather can restrict progress) Trade skills and site crafting are Trade skills and site crafting are

important to calculate and cut the important to calculate and cut the required roof geometry required roof geometry

The site based process has The site based process has greater ability to deal with greater ability to deal with unexpected design problems and unexpected design problems and variationsvariations

More labour intensive than More labour intensive than trussed roofstrussed roofs

Trussed Trussed Factory environment involves a more Factory environment involves a more

automated and repetitious process - automated and repetitious process - therefore potential for better quality therefore potential for better quality controlcontrol

Less site work means less affected by Less site work means less affected by bad weather bad weather

Makes maximum structural use of the Makes maximum structural use of the timbertimber

Capable of long spansCapable of long spans Internal walls are usually non-Internal walls are usually non-

loadbearing therefore lighter weight loadbearing therefore lighter weight internal walls are possibleinternal walls are possible

Irrespective or the method of framing, tIrrespective or the method of framing, trusses russes or rafters or rafters are set up at regular intervals to form the are set up at regular intervals to form the three dimensional three dimensional shape of the roofshape of the roof

Each supports loads from a certain contributing area of the roofEach supports loads from a certain contributing area of the roof and this influences the size of the members used 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 The contributing area is usually a strip whose width is defined by the mid-lines between adjacent rafters of rafters of trussestrusses (as shown) (as shown)

TTrusses russes and rafters and rafters are often are often spaced spaced 600mm 600mm or more according to local practise and the type of roofing materials (e.g. sheet metal or more according to local practise and the type of roofing materials (e.g. sheet metal roof)roof)

Roof Load WidthRoof Load Width

Specific Loads on RoofsSpecific Loads on Roofs

LLoads oads falling within the roof load width include:falling within the roof load width include: Gravity Dead LoadsGravity Dead Loads including including roof and ceiling materialsroof and ceiling materials Gravity Live LoadsGravity Live Loads including including people working on the roof and stuff people working on the roof and stuff

stacked on itstacked on it Wind loadsWind loads including downward pressure or suction that lifts upwards – including downward pressure or suction that lifts upwards –

these are only felt some of the time but downward pressure adds to the these are only felt some of the time but downward pressure adds to the above gravity loads, while uplift works in the opposite directionsabove gravity loads, while uplift works in the opposite directions

Gravity Dead LoadGravity Dead Load

The weight of the roofing material The weight of the roofing material can be expressed as weight (kg) per can be expressed as weight (kg) per unit area of roof (square metres), ie. unit area of roof (square metres), ie. (kg/m2)(kg/m2)

The weight of a tiled roof The weight of a tiled roof with with battens, a plasterboard ceiling and battens, a plasterboard ceiling and insulation insulation is approximately is approximately 7575 kg/m2 kg/m2

The weight of a sheet metal roof The weight of a sheet metal roof with softwood ceiling and insulation with softwood ceiling and insulation iis approximately s approximately 2020 kg/m2 kg/m2

DEAD LOAD (structure)

Gravity Live LoadsGravity Live Loads

Live loads result from the Live loads result from the occasional presence of people occasional presence of people and materials on the roofand materials on the roof

We must allow for the weight of a We must allow for the weight of a large person standing anywhere large person standing anywhere on the roof.on the roof.

Live loads (people,)Construction loads

(people, materials)

Wind loadsWind loadsWind loads push against the roof but can also cause uplift and Wind loads push against the roof but can also cause uplift and suctionsuction

The amount of wind load which acts on the roof depends on several The amount of wind load which acts on the roof depends on several things - the most important being the speed of the windthings - the most important being the speed of the wind

Suction

Internal Wind

Suction

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

Wind load on a flat surface vs speed

0

5

10

15

20

25

30

35

40

0 20 40 60 80 100

Wind speed (kph)

Loa

d (k

g/m

)

When the wind passes over a roof it can cause a suction. When it When the wind passes over a roof it can cause a suction. When it gains access to the interior it can cause an upliftgains access to the interior it can cause an uplift

The The roof roof must be strong enough to resist the load developed by must be strong enough to resist the load developed by suctions and uplift. Thesuctions and uplift. The frame frame must be attached adequately to the must be attached adequately to the rest of the structure so the whole roof is not sucked off.rest of the structure so the whole roof is not sucked off.

Suction

Internal pressure

Suction (uplift)

Wind

Combinations of loadsCombinations of loads

MMore than one type of load can be acting on ore than one type of load can be acting on the roof the roof at the same at the same timetime

This may be a combination of gravity dead loads plus gravity live This may be a combination of gravity dead loads plus gravity live load, plus wind load, plus wind loads – all loads – all acting downwards. acting downwards.

In other instances wind may be acting upwards (where suction and In other instances wind may be acting upwards (where suction and uplift occur), therefore acting in the opposite direction to gravity uplift occur), therefore acting in the opposite direction to gravity dead and live loads. dead and live loads.

In high wind areas, wind uplift can easily exceed downward gravity In high wind areas, wind uplift can easily exceed downward gravity loads. For resisting uplift, the heavy dead load from a tiled roof is loads. For resisting uplift, the heavy dead load from a tiled roof is useful.useful.

Transferring Loads to Pitched and Transferring Loads to Pitched and Trussed Roofs Trussed Roofs

2. Battens - take roofing loads and transfers them to the rafters/trusses3. Rafters/Trusses – take batten loads

and transfers them to the support structure below e.g. walls

1. Roofing materials - take live/dead/wind loads and transfers them to the battens

Support wall

Typical Bracing for Pitched RoofsTypical Bracing for Pitched Roofs

Bracing is essential for providing stability to the roof frame under all Bracing is essential for providing stability to the roof frame under all loading conditions. Bracing for a gable roofing is shown above. loading conditions. Bracing for a gable roofing is shown above.

Though not shown, hip ends provide a self bracing effect.Though not shown, hip ends provide a self bracing effect.

Typical Bracing for Trussed RoofsTypical Bracing for Trussed Roofs

Bracing in trussed roofs make significant use of “steel Bracing in trussed roofs make significant use of “steel brace” applied in “X” and “V” patterns across the roof brace” applied in “X” and “V” patterns across the roof

planes. Trussed roofs must especially prevent buckling of planes. Trussed roofs must especially prevent buckling of members and must address wind upliftmembers and must address wind uplift

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