Introduction to Roofing Concepts and Roof Framing What’s in this presentation: Basic roof shapes Reading roof shapes from lines on a drawing Explaining roof lines Focus on gable, hip, dutch gable and valley roofs Generic approaches to roof framing Differences between pitched and trusses roofs Roof load width Loads on roof framing Transferring loads Typical bracing requirements Links to presentations on pitched and trussed roofs

Basic Roof Shapes The footprint of a building generally consists of a rectangular block or multiple blocks joined together Roof shapes are made to cover the footprint while also providing sloping planes able to shed water Skillion Common roof shapes used to cover the required area are shown Gable (Cathedral or flat ceiling) Hip Dutch Hip (or Dutch Gable) Hip and valley

Reading Roof Shapes From Lines on a Drawing In technical drawings, roof planes are defined using lines describing the boundaries of roof planes or lines between them, including: Ridge Lines Gable Lines Eaves lines Hip Lines Valley Lines Being able to read these lines is important because they show: Where roof shapes are positioned in the overall roof plan The span and length of each individual roof shape How each individual roof shape links in with others This information is important in roof framing setout.

Example of Roof lines Each of the roof lines below, are explained in more detail on the following slides Gable Line Gable line Ridge Line Walls Valley line Hip line Eaves Line

Eaves lines define how much roof planes overhang support walls Gable line Ridge Line Gable Line Ridge Line Valley line Hip line

Eaves Line Walls Ridge Lines define where two opposing roof planes meet at the highest point Gable line Ridge Line Gable Line Ridge Line Valley line Hip line

Eaves Line Walls Gable Lines occur where the ends of roof planes run at 900 to the ridge line. They may be flush with end walls or form an overhang Gable line Ridge Line Gable Line Ridge Line Valley line Hip line

Hip lines typically connect to the outer end of a ridge line. Eaves Line Walls Gable line Ridge Line Gable Line Ridge Line Valley line Hip line Hip Lines are created by the meeting of two roof planes forming an external corner. The planes are usually at 900 to each other and where they intersect, a bisecting 450 hip line is formed. Hip lines typically connect to the outer end of a ridge line.

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

Gable Roof Shapes 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 playing cards leaning against each other. There are different types of gable ends: Flush gables are signified by the gable line being flush with the end wall Open gables are signified by the gable line overhanging the end wall and following the slope of the roof Boxed gables overhang the end wall but the outer face of the gable is enclosed by cladding (as shown below). Gable

Open or boxed Gable Line 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 Shapes The Hip Roof contains roof planes sloping down to side and end walls The perimeter eave continues at the same level on all sides 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 side walls. Hip

Side Elevation End Elevation Eave Line Walls Ridge Line Hip Line Plan View

Dutch Gable 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 ridge line, and a a gable line linking the ridge and hips together. The proportional appearance of the hip compared to the gable can be changed to suit architectural requirements. Dutch Gable

Eave Line Walls Gable Line Gable Line Ridge Line Hip Line Hip Line Side Elevation End Elevation Eave Line Walls Gable Line Gable Line Ridge Line Hip Line Hip Line Plan View

Valley Shapes A Valley occurs where two roofs perpendicular to each other join together. 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 ridge where running down to internal corners in the building layout.

End Elevation Ridge Line Valley line Walls Side Elevation Plan View

Generic Approaches to Roof Framing The previous roof shapes can be framed using two approaches Pitched roofs (i.e. raftered roofs cut and erected on site) Trussed roofs (engineered frames made in a factory, erected on-site) Click above to see a video

Differences Between Pitched and Trussed Roofs Pitched roofs have evolved from traditional origins - rafters are pitched onsite like raking beams supported by external walls; inner roof framing members provide additional support and are supported by internal walls Trussed roofs utilise contemporary engineering principles - each piece of 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 internal walls aren’t required for support Pitched (raftered roof) Trussed roof

Pros and Cons of Pitched and Trussed Roofs Site focused process (bad weather can restrict progress) Trade skills and site crafting are important to calculate and cut the required roof geometry The site based process has greater ability to deal with unexpected design problems and variations More labour intensive than trussed roofs Trussed Factory environment involves a more automated and repetitious process - therefore potential for better quality control Less site work means less affected by bad weather Makes maximum structural use of the timber Capable of long spans Internal walls are usually non-loadbearing therefore lighter weight internal walls are possible

Roof Load Width Irrespective or the method of framing, trusses or rafters are set up at regular intervals to form the three dimensional shape of the roof Each supports loads from a certain contributing area of the roof 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 rafters of trusses (as shown) Trusses and rafters are often spaced 600mm or more according to local practise and the type of roofing materials (e.g. sheet metal roof)

Specific Loads on Roofs Loads falling within the roof load width include: Gravity Dead Loads including roof and ceiling materials Gravity Live Loads including people working on the roof and stuff stacked on it Wind loads including downward pressure or suction that lifts upwards – these are only felt some of the time but downward pressure adds to the above gravity loads, while uplift works in the opposite directions

Gravity Dead Load The weight of the roofing material can be expressed as weight (kg) per unit area of roof (square metres), ie. (kg/m2) The weight of a tiled roof with battens, a plasterboard ceiling and insulation is approximately 75 kg/m2 The weight of a sheet metal roof with softwood ceiling and insulation is approximately 20 kg/m2 DEAD LOAD (structure)

Gravity Live Loads Live loads result from the occasional presence of people and materials on the roof We must allow for the weight of a large person standing anywhere on the roof. Live loads (people,) Construction loads (people, materials)

Wind loads Wind loads push against the roof but can also cause uplift and suction The amount of wind load which acts on the roof depends on several things - 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 over the area of the building exposed to the wind

When the wind passes over a roof it can cause a suction When the wind passes over a roof it can cause a suction. When it gains access to the interior it can cause an uplift The roof must be strong enough to resist the load developed by suctions and uplift. The frame must be attached adequately to the rest of the structure so the whole roof is not sucked off. Suction Internal pressure Suction (uplift) Wind

Combinations of loads More than one type of load can be acting on the roof at the same time This may be a combination of gravity dead loads plus gravity live load, plus wind loads – all acting downwards. In other instances wind may be acting upwards (where suction and uplift occur), therefore acting in the opposite direction to gravity dead and live loads. In high wind areas, wind uplift can easily exceed downward gravity loads. For resisting uplift, the heavy dead load from a tiled roof is useful.

Transferring Loads to Pitched and Trussed Roofs 2. Battens - take roofing loads and transfers them to the rafters/trusses 3. Rafters/Trusses – take batten loads and transfers them to the support structure below e.g. walls Support wall 1. Roofing materials - take live/dead/wind loads and transfers them to the battens

Typical Bracing for Pitched Roofs Bracing is essential for providing stability to the roof frame under all loading conditions. Bracing for a gable roofing is shown above. Though not shown, hip ends provide a self bracing effect.

Typical Bracing for Trussed Roofs Bracing in trussed roofs make significant use of “steel brace” applied in “X” and “V” patterns across the roof planes. Trussed roofs must especially prevent buckling of members and must address wind uplift

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