Framing Roof Sections, Building Ends and Junctions
What follows is a description of the framing of different roof sections, building ends and junctions, giving a general indication of the positions and profiles of trusses required to form these sections. The actual number of trusses and the truss types required in any particular roof section depends on the span, pitch and maximum truss spacing for the roof cover used on that roof.
The roof designer should begin by identifying the Load Bearing Walls, on which the roof trusses will be supported. Wall Plates are placed on top of load bearing walls to help spread the weight of the trusses evenly over the brickwork. Trusses are bearing on the wall plates and tied down to the superstructure with wire or hoop iron that is built into the brickwork.
Trusses are placed perpendicular to the load bearing walls, with their apexes forming the ridgeline of the roof section and their overhangs extending to the eaves lines outside of the load bearing walls.
The roof covering is connected to Purlins (for sheeting) or Battens (for tiles), which are small-section timber members securely fixed to the top of the top chords of the trusses with 2.4mm wire ties or system-produced wire connectors. Purlins or battens run horizontally along roof planes, parallel to the load bearing walls, and are normally spaced equally between the ridgeline and the ends of the eaves overhangs.
The maximum spacing of trusses is limited by the weight of the roof covering and the maximum distance that the purlins or battens can span. All truss centers – in building ends and junctions, as well as in the straight building sections between them, must be less than or equal to the maximum centers specified for the roof cover. At outside corners of buildings the spacing between the ends of the overhangs should also not exceed the maximum centers.
Roof Truss Sections
In addition to Plan and Elevation views, architects’ drawings may also include Sections.
A Section is a “slice” through a building, showing the internal features of the building. The position of this “slice” is indicated on the roof plan by dotted lines, as shown here. The arrows through the dotted lines indicate the direction in which the section is viewed.
Note: A Fink type truss is shown here. The truss type selected in a specific roof depends on the span, pitch and roof cover, as well as on an understanding of structural roof design.
Web configurations included on an architect’s section drawings may usually be ignored.
Section drawings are useful in illustrating the profiles of non-standard trusses, such as on cantilevered or stubbed sections of a building, as shown below.
Cantilevered and Stubbed areas of a building can be identified on a plan by the position of the overhang line – at a cantilever the overhang line is flush with that of the standard building section, while at a stub the overhang line is stepped in with the stub wall, as shown on the plan drawings above. Cantilevered and Stubbed areas of a building can be identified on elevation drawings by the height of the overhang line – the overhang line maintains the standard height along a cantilever wall, while it is higher along a stub wall, as shown on the West Elevations in the drawings above.
Roof Truss Elevations
Roof Truss Elevations drawings are side views of a building, named for the direction from which the building is viewed:
The South Elevation shows the south side of the building, viewed from the south looking north;
The East Elevation shows the east side of the building, viewed from the east looking west;
The North Elevation shows the north side of the building, viewed from the north looking south;
The West Elevation shows the west side of the building, viewed from the west looking east;
In the drawings of Roof A below, and Roof B on the following page, the elevations are shown rotated around the plan drawing, so that each elevation is positioned on that side of the plan which it depicts.
This is normally done on architectural drawings, but is done here so that the position of hip or valley apexes, wall lines or overhang lines in the elevations may easily be checked by simply laying a ruler or other straight edge across from the plans to the elevation drawings.
The four elevations of Roof A and Roof B are shown again on the following pages, this time with the elevations in the upper right position and, except for the south elevations, the plans have been rotated. In each drawing the elevation depicts the side of the plan which is shown directly above it.
Again, this allows the position of lines or points in the elevations to be checked by laying a straight edge across from the plans to the elevation drawings.
The horizontal roof lines in this elevation correspond with the horizontal ridgelines on the plan. The apex of the hip at the left of the elevation corresponds with the vertical ridgeline on the plan.
The sloping roof lines from the highest section of the roof elevation connect this higher roof in the centre to the lower roofs on either side, corresponding with the partial hip lines in the plan.
The South Elevation shows the south side of the building, viewed from the south looking north.
The horizontal roof lines in this elevation correspond with the horizontal ridgelines on the plan.The apex of the parapet wall on the left of the elevation corresponds with the vertical ridgeline on the plan.The height of the central section extends from the apex of the left section in the elevation, where the roof is obscured by the parapet wall, to the under-gable junction.
Note the change in height of the roof at this junction, with a continuous roof plane extending across the front of the two sections on the right of this elevation.
The East Elevation shows the east side of the building, viewed from the east looking west. The roof plans have been rotated through 90o (clockwise), so that the view illustrated by the elevation reflects the lower side of the plan drawing above it.
The hip end of the central section of the roof plan is visible in the elevation drawing behind the front hip end. The two vertical ridgelines on this plan drawing correspond with the apexes of these two hips in the elevation. The horizontal roof line at the left of the elevation drawing extends from the apex of the hip at the left to the apex of the valley at the side of the higher hip end, corresponding with the horizontal ridgeline at the top of the plan drawing.
The partial gable wall at the under- gable junction is visible behind the front gable wall in the elevation. The two vertical ridgelines on this plan drawing correspond with the apexes of these two gable walls in the elevation. The horizontal roof line in this elevation drawing extends from the parapet wall on the left to the apex of the higher roof section on the right,
corresponding with the horizontal ridgeline in the plan drawing.
The North Elevation shows the north side of the building, viewed from the north looking south.
The roof plans have been rotated through another 90o (a total of 180o
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The horizontal roof lines at the top of each roof section in this elevation correspond with the
horizontal ridgelines on the plan. The section at the top right of the roof plan is not visible in the elevation drawing, as it is obscured by the higher central section of the roof.
The horizontal roof lines in this elevation correspond with the horizontal ridgelines on the plan. The right hand horizontal ridgeline on the plan extends from the overhang line at the under-gable junction to the vertical ridgeline on the right, corresponding with the horizontal roof line of the higher roof in the elevation drawing. Note the change in height of the roof at the Under-gable junction, with a break in the roof planes across the two sections of the building shown in this view. At the right of the elevation the parapet wall is visible above the sloping roof.
The West Elevation shows the west side of the building, viewed from the west looking east.The roof plans have been rotated a total of 270o relative to the original orientation.
The lower vertical ridgeline on this plan corresponds with the hip apex of the left section of the roof in the elevation drawing. The horizontal ridgeline on the plan extends from the apex of the valley at the side of the hip end to the apex of the hip end at the right, corresponding with the horizontal roof line in the elevation drawing.
The lower vertical ridgeline on this plan drawing corresponds with the apex of the sloping end of the roof in the elevation drawing.The horizontal ridgeline on the planextends from the vertical ridgeline to the parapet wall on the right,Corresponding with the horizontal roof line in the elevation drawing.
Plan View Drawings
It is important to identify the Load Bearing Walls on which the roof trusses will be supported (in the following plan view drawings all load bearing walls are shown shaded).
The Roof Lines – ridgelines, hip lines, valley lines, gable and eaves overhang lines – at the ends of building sections and at junctions, provide information regarding the required shape of the roof.
In the two examples shown below the wall plans are identical (apart from the parapet wall in Roof B), yet the two roof shapes are quite different, as indicated by the roof lines on the plan drawings.
A ridgeline marks the apex of each of the three roof sections on this roof plan. Diagonal lines in the outside corners of the building indicate hip ends. The two partial hip lines in the central section connect the higher ridgeline to the two lower ridgelines on either side of it.The diagonal lines in the two inside corners of the building are valley lines formed where two sections intersect.
A ridgeline marks the apex of each of the three roof sections. Gable ends are identified by theridgeline extending over the end wall to the gable overhang line. At a parapet end (at lower left), the ridgeline and roof end at the inside edge of the parapet wall, which extends above the roof.
The parapet and gable walls are not load bearing. The vertical ridgeline meets the left horizontal ridgeline at a point, indicating that these two ridge-lines are at the same height.
This means that the narrower section must have a steeper pitch than the wider section in order to reach the same ridge height. The difference between the pitches of the two sections is also clear from the fact that the hip and valley lines across the corner are not at
45o to the walls and ridgeline.
The following Plan Views of Roof A and Roof B show the building End Types and Junction.
Types of these two different roof shapes.
The hatching on these drawings runs horizontally along each roof plane.
The bottom left section starts with a hip at the bottom and ends in a valley, which sits on top of the side plane of the top section, forming a Valley junction. The top left section has a hip at both ends, while the top right section has a valley at the left and a hip at the right end. At the junction of the two parallel sections the valley end of the narrower section sits on top of the hip end of the wider section.
Note: In both junctions the second valley line coincides with the lower portion of a hip line, but these lines are not shown because the roof planes on that side of the junctions are continuous across the junctions.
At the bottom end of the building the roof ends at the inside face of the parapet wall. At the right end of the building the roof extends over the gable wall. At the under-gable junction the narrower section of the roof is lower and extends to the partial gable wall, while the roof of the wider section is higher and extends to a gable overhang above the lower roof – on one side only. On the other side of this under-gable junction the roof plane is continuous.
Note: Since the pitch differs on either side of the corner, this corner junction will require a customized hip and valley set. Alternatively, a Hip-Valley junction may be used – see Hip-Valley.