Roof Truss Design

Whether you are a roofing contractor or a home owner looking into roofing for your home you may need to get roof trusses. Therefore having as much information on them as possible or a roof truss design on hand can help your decision making whether it be a more complex attic extension or just for a simple building like a garden wooden shed.

This article presumes your build was from a set of architect drawings and that you can refer to more detail if you need on those plans. For more complex builds such plans may also have engineer specifications for details such as loadings and wind bracing. While the various types of roof structures are not looked at here it is presumed that you have resolved such issues as whether you are using the loft space for living accommodation, whether you are having flat or vaulted ceilings, and whether you are having a gabled or hipped roof design. Below is a look at the roof truss and details about roof trusses and hopefully explains some of the terminology that is used related to them.

Roof truss design

About a Roof Truss

When you are talking about a simple roof truss it is a triangle with two top rafters or chords at the same pitch that meet at an apex and then connect with a ceiling tie or bottom chord at their base. It is sometimes referred to as a close couple system and is the basic form from which nearly all more complicated roof truss designs begin. Connecting the tie and rafters in the truss are several webs which also make triangular patterns. This is to help distribute the forces known as the triangulation of forces. Homes most often use the fink roof truss which has webs that form a W shape.

The Span of a Roof Truss

The span over the wall plates if what males the overall span of the truss and it is measured in millimeters. The span is defined as the distance between the two supporting wall plates’ outer edges. In most roofs the overall span equals the ceiling tie’s length.

Pitch or Height

The angle created by the rafters to the horizontal is measured in degrees and is the pitch of the roof truss. If you look at mono pitch trusses they have one rafter so just one pitch – much like a right angled triangle for example. The fink roof truss and other common trusses are dual pitch so that have two pitches, one on each side. In some cases a dual pitch but with angles that are different may be needed, perhaps because of a design detail or because you are trying to match a roof line that already exists. If you go to a truss designer and you cannot give them the roof pitch he or she can use the height of the truss which is the measurement from the apex (the top most point of the truss) to the end of the ceiling tie.

Roof Truss Spacing

If you have a timber frame building the popular roof truss spacing is 600 millimeters. This will save money as less roof trusses are needed. But they can need larger nail plates and timber sections. There is also the possibility of deflection in battens that hold heavier roof materials such as slate.

roof truss designs

Fascia, overhang and eaves

When the rafter or ceiling tie is extended past the wall plate or its support this creates an overhang. Where that overhang goes past the outside face of the build this is the eaves. Having those there gives rooms for the soffit to be placed which is board attached under the eaves to hid the timber. The fascia is a horizontal board that is put in along the length of a build and then fixed to the end of the overhang. Usually the fascia board is made from a derivative of plastic and is made to prevent sun deterioration.

Roof truss loading

In a roof truss design, the designer has to think of several loading factors other than just storage within the loft space. There is design software that factors in these loads during the design stage and when you tell them where the build is things like the snow load and wind load are accounted for. Other information you may need to give is the type of roofing material you intend to use.

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