Mansard trusses [Fig. (f)] are variants of fink trusses that, unlike fink and fan trusses, have shorter leading diagonals even in extremely long span trusses. The pitched roof trusses' economical span lengths, excluding mansard trusses, vary from 6 m to 12 m. Longer spans are achieved by using multiple boxes or by adding further diagonal members. Truss designs with longer main beams and/or taller side walls are often called "gable-end" trusses. These terms do not imply that only one end of the truss is gabled; rather, the entire structure is angled.
The most common type of roof truss is the flat truss, which has no rise or fall between each joint in the truss. This type of truss can be built out of wood or steel. Flat trusses are used for roofs that are covered with tiles or other materials that require a level surface for safety and ease of replacement if it wears out. Flat trusses are also useful for covering large open areas without blocking air flow through the building. Flat trusses usually have metal straps attached to the top chord members to hold the truss plates together while they're being joined into a single piece.
Trusses with falls or rises between each joint are called curved trusses. These trusses can be built out of wood or steel.
Pitched and flat roof trusses with average loads and spacing of 15 to 20 ft are rarely utilized for spans more than 80 ft. Economical spans are often restricted by the sizes and lengths of available solid sawed or glued-laminated timber, as well as the possible capacity of the web-member connections. Larger trusses can be built from multiple subtrusses connected with gussets. The overall strength of a truss depends on the quality of its components and their adherence to the design criteria for tension vs compression in bending and torsion.
The actual load that a truss can support depends on its design, but an average-sized truss used for framing buildings or bridges should be strong enough to carry its intended load. As with any mechanical structure, larger trusses will be stronger than smaller ones of the same type and construction quality. Also, higher-quality trusses will be stronger than lower-quality ones of the same size. Engineers design trusses with respect to their expected loading conditions and then specify them for use under actual working conditions. For example, trusses used on roofs are usually specified as able to carry a certain amount of weight before failure occurs; however, during periods of heavy snowfall or high wind speeds, they may need to be strengthened to handle greater loads.
The design of a truss depends on its application.
Standard trusses may span up to 11 meters in 35mm wood and 15 meters in 47mm wood. Spans greater than this can be created, but are often supplied as many trusses joined together. As a rule of thumb, the number of trusses required equals 10 times the distance that the roof will be above ground level.
The type of wood used to make trusses depends on how much weight it is going to have to bear. If you need a light frame, then use 30-year-old lumber; if you want it to last longer than that, go with 45- or even 60-year-old wood. The actual thickness doesn't matter so much as the species: avoid pine, which tends to be soft, instead use hardwood such as maple or oak.
The best way to figure out how many trusses you'll need is by taking measurements from the ground up to the highest point on the roof. If your roof is very steep, you might need more trusses to keep the load distributed evenly.
Once you know how many trusses you need, get the measurements for that length of stringers (the boards that connect one truss to the next) and divide it by the number of trusses. That's how many meters each stringer should be.
Although the usual spacing of roof trusses is similar to that of rafters, with 24-inch spacing being a frequent choice, the design of a truss may allow for the use of a cheaper grade of lumber than would be necessary for rafters with identical spacing and span. For example, some truss designs can be built with 16- or 18-gauge steel wire instead of the usual 2- or 3-gauge used for rafters. Other factors such as weight and loading considerations will determine the appropriate material selection.
Without support, a roof truss can span up to 80 feet. That distance, however, would be impossible and prohibitively expensive in any home. Trusses are intended to span expanses without the need for interior support, with spans of up to 40 feet being the most frequent in today's homes. Spans longer than that require additional trusses attached to the original structure.
The maximum allowable span for a roof truss depends on how the truss is constructed. For example, a through truss has no limit on its span because it uses vertical posts on each end to support the load. A half-through truss has no limit on its span if it is built properly. A through truss with one post at each end supporting a horizontal beam does not have a limit on its span. However, there must be enough space beneath the truss for a carpenter to work without hitting any obstacles such as pipes or cables.
A house built before 1978 was likely to have roof trusses that were tied together with wire instead of using nails. These trusses could not be taken down easily and might have spanned 60 feet or more. The old trusses were usually replaced when properties were resold or upgraded to larger houses with new trusses that allowed for wider spans.
The typical single-story house has a roof truss with two equal parts: a headboard and a footer.