Building roofs may be the most complicated of all construction projects. There are only about a dozen different roof styles but there are many variations on how to build them. Gable roofs that slope to two sides and hip roofs that slope to four sides are the most common styles. Both of those styles and others can be built with joists and rafters or with trusses. Trusses are most common now. They are less cumbersome to install and offer great strength and stability. Many truss design options are available, depending on the style of roof and finished interior.
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Things you need
- Weight and wind load tables
- Tape measure
- Basic truss style reference
Measure the roof span, the width a truss will have to cover. Determine the style of roof, both external and internal. The external style is the basic design, such as gable or hip, and the pitch or slope of the roof. Internal style covers options like flat ceilings, vaulted or arched ceilings or some variation.
Calculate the loads a truss will have to support. That starts with the weight of the roofing, which will vary with the size of the roof and the type. Shingles will weigh far less than tile. Weight will help decide the vertical load a truss must support. Find the wind loads, the pressure a roof must endure from winds blowing against it, from tables available online and other resources. These vary by locale.
Choose a basic truss style. The two most common are fink and double w. A fink truss has a bottom girder, two slanted rafters and two sets of diagonal braces between girder and rafters; a double w uses the same technique but with four sets of internal braces. Fan and howie girder options use a combination of diagonal and upright vertical supports between girder and rafters. These all provide a flat ceiling.
Design trusses for internal space, such as an upstairs room or an attic, using open plan or storage options. These trusses have vertical side braces with another cross brace at the top of the space, a vertical from the cross brace to the roof peak and diagonal braces between the side braces and the rafters.
Provide more internal space or different ceilings with raised tie, scissors or raised tie scissor trusses. Raised ties move the bottom cross brace between the rafters higher, with a single diagonal brace connecting girder and rafters. It relies on outside walls to provide support on the edges. Scissor braces employ double diagonal girders from the rafters to a vertical support from the peak and diagonal braces in between. A raised tie scissor combines those styles, with a horizontal girder in the centre, angled girders on each side, with vertical braces at the peak and where girders meet.
Tips and warnings
- Designing roof trusses is a complex matter and may require the services of a structural engineer.
- The critical element in truss design is the connector, usually steel plates with many sharp points that penetrate the wood and hold truss pieces securely. These normally are installed in a factory. Plywood gussets, screwed to both sides of truss pieces, are used sometimes where trusses must be built at a job site.
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