1. Wood Structures Topic 6 Roof Framing Trusses
Roof Framing part 1
Roof Types and Terms
Trusses
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2. Roof Types Shed Gable Hip Intersecting Gambrel Mansard Butterfly
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4. Wood Structures Topic 6 Roof Framing Trusses
Introduction
The roof protects the house from weather.
It resists snow and wind loads, sheds rain, and helps control the movement of heat into and from the house.
Strength and rigidity are important characteristics.
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5. Introduction Two methods to frame roofs
Roof trusses. Prefabricated in the factory, may be erected on location to frame the roof assemblage.
Conventional framing. Individual members, such as rafters and joists, may be framed in their final location.
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6. Trusses
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7. Conventional method/rafters
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8. Introduction Most roofs are built with trusses.
They save time and use less material.
They enable a clear span between outside bearing walls without the need for intermediate support.
Conventional framing may be more convenient or practical in certain circumstances.
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9. Topics covered Roof assemblage: Framing requirements:
Shapes, trusses, conventional framing
Framing requirements:
Structure, spans, nailing
Truss installation
Sheathing
Notching and drilling
Ventilation
Attic access
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10. Shapes or Types
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11. Shapes Shed roof is the easiest to build.
Gable and hip roofs are most commonly used.
A combination of roof shapes can be used.
Intersecting roofs may have different slopes.
Roof shapes may be modified to include dormers.
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12. Trusses
A truss is a structural framework relying on a triangular arrangement of webs and chords to transfer loads.
Light frame trusses are made from dimension lumber of various sizes, with the webs and chords connected together by toothed connector plates stamped from galvanized sheet steel.
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13. Wood Structures Topic 6 Roof Framing Trusses
Roof Trusses
Used mostly in new construction
Almost any type of roof can be built with trusses
Trusses are engineered for each job
The truss fabricator will provide engineering certification; framing plan and design drawings for each truss; instructions on installation and bracing.
Parallel chord, open web trusses are used in floor and flat roof systems.
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14. Wood Structures Topic 6 Roof Framing Trusses
Roof Trusses
Advantages:
Faster to erect
Less skilled labour required
Closure against weather faster because often both trusses and sheathing can be done in a day
Trusses mean more flexible floor plans because they can span longer distances without interior bearing walls than conventional roofs
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15. Wood Structures Topic 6 Roof Framing Trusses
Roof Trusses
Advantages:
Trusses are efficient in their use of lumber, where a framed roof may require 40 x 185 mm (2” x 8”) 400 mm (16”) O.C. a trussed roof might be made of 40 x 90 mm (2”x4”) 600 mm (24”) O.C. and use 15% to 25% less wood
Though they look spindly, trusses are, when installed correctly, stronger than a traditional roof
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16. Wood Structures Topic 6 Roof Framing Trusses
Roof Trusses
Disadvantages:
Trusses must be handled with extreme care
They must be stored so as not to cause damage
Trusses are strong vertically but weak when on the horizontal
They can’t be banged around a job site like dimensional lumber can
Proper installation techniques and bracing are critical
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17. Wood Structures Topic 6 Roof Framing Trusses
Roof Trusses
Disadvantages
Trusses cannot be modified in the field without changing engineering requirements
None of the truss members can be removed or altered without an engineers approval
No notching or drilling of truss members without engineers approval
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18. Wood Structures Topic 6 Roof Framing Trusses
Roof Trusses
Disadvantages:
May be difficult to store
May be difficult to maneuver into place
May require a crane to lift into place
May be problematic in renovations when tying into an existing roof
May have lengthy wait while trusses are manufactured
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19. Wood Structures Topic 6 Roof Framing Trusses
Truss rafter designs
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20. Wood Structures Topic 6 Roof Framing Trusses
Cost
Trusses are usually more economical in new construction
Conventional roofs are more economical for renovations
Infill Construction usually requires looking at the circumstances and costs individually
Less skilled workers required with truss roofs
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21. Wood Structures Topic 6 Roof Framing Trusses
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22. Wood Structures Topic 6 Roof Framing Trusses
Trusses and Triangles
From the pyramids to the present we know that building with triangles is one of the strongest methods of construction
Conventional roofs use triangles, from the rafters to the ceiling joists and collar ties
Roof trusses use a series of triangles. Cut any one of them and the structure is weakened and compromised
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23. Wood Structures Topic 6 Roof Framing Trusses23
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24. Wood Structures Topic 6 Roof Framing Trusses
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26. Truss Design and Manufacture
Wood Structures Topic 6 Roof Framing Trusses
Truss Design and Manufacture
Trusses are usually designed using computer software that considers:
Building Code Requirements
Snow Loads
Roof and Ceiling loads
Any special conditions such as wind considerations, hurricane zone, seismic consideration
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27. Wood Structures Topic 6 Roof Framing Trusses
Truss Integrity
Integrity of the truss depends on the integrity of the metal plate connectors (gusset plates)
Metal plate connectors stamped from 16 to 20 gauge structural steel coated with zinc plates having many integral teeth
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28. Delivery and Installation
Wood Structures Topic 6 Roof Framing Trusses
Delivery and Installation
Trusses must be stored off the ground and preferably protected with a tarp
One storey homes can have trusses efficiently installed manually
Two storey homes can have trusses most efficiently installed using a crane or forklift
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31. Wood Structures Topic 6 Roof Framing Trusses
Installation
Installation begins with a gable truss (gable roofs) braced in place
Other trusses are than lifted into place and raised using a y shaped pole
A line used along the eaves will ensure a straight fascia board
Remember to place plenty of temporary bracing until sheathing is all in place
Sheathing provides rigidity for top chords
Bottom chords may require bracing as shown in engineers drawings
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36. Wood Structures Topic 6 Roof Framing Trusses
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37. Wood Structures Topic 6 Roof Framing Trusses
Damaged Trusses
When trusses are damaged you should consult the design engineer for a remedy
Why?
When a truss is damaged it does not work like a truss
Whoever does the repair takes responsibility for the truss
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