History of wood roof trusses
The wood trusses that we so commonly see today are frames of triangulated lumber joined together with galvanized steel connector plates, commonly referred to as truss plates. The shape of roof trusses has not changed over the years but the make-up, spans possible and the safety has changed.
The truss shape has been in use since man has used pieces of logs and then, centuries later, sawn lumber. In modern times, the first light wood frame trusses were built on the construction site using nailed boards or plywood gusset plates at the joints. Use of these early timber trusses offered longer spans, more cheaply, that un-trussed lumber but took a long time to build on the construction site.
In 1952, in Pompano Beach Florida, after experimenting with plywood gusset plates and varying concoctions and combinations of glue, staples, nails and screws, the metal plate connected engineered wood truss was invented and patented. The inventor, A. Carroll Sanford, founder of Sanford Industries, marked the beginning of the truss industry, that is still changing home, apartment and commercial building construction all over the world.
Modern roof and floor trusses, clear-spanning between the perimeter walls, allows larger more open room designs, particularly in areas of great snow loads. This roof system proved to be faster and more cost effective than earlier practices, much less material and less labor cutting and fitting materials in the field.
Wood roof trusses and floor trusses today
Wood trusses are widely used in single and multi-family residential, institutional, agricultural and commercial construction. Their high strength-to-weight ratios permit long spans, offering greater flexibility in floor plan layouts. They can be designed in almost any shape or size, restricted only by manufacturing capabilities, shipping limitations and handling considerations.
The design and manufacture/fabrication of metal plate connected wood trusses is, in modern truss operations, accomplished by computer. In Design, the first decision to be made is the overall look of the roof or floor system to be produced followed by a decision as to the overall span. Then the computerization of the process takes over. Common standard engineering principles along with building code required roof, roofing material, ceiling, wind and snow loads, loading of the floor above, as well as any extraordinary loading or stress conditions are taken into consideration. These technical details and a few others, are then entered into the computer.
An engineering drawing is produced by the computer detailing the forces that develop in each chord and web under the given design loads. Engineering specifications also include the timber truss dimensions and pitch, lumber sizes, species, dimension/size and grade of wood for each chord and web. The gauge, size and orientation of each metal connector plate is precisely specified as well as the size, strength and location of permanent bracing.
The resultant engineering drawings are supplied by the truss frame designer/manufacturer to the architect, engineer and to the building contractor. The carpentry contractor responsible for setting the timber trusses in place, must also get a copy of the these drawings to make certain that the design conditions are met.
The strength and integrity of the roof truss depends on the integrity of its metal plate connectors. Stamped from light (16,18, 20) gauge structural steel coated with zinc, most wood truss connector plates have many integral teeth, 5/16 inch to 9/16 inch in length. Normally, there are eight teeth per square inch. These plates are sized according to the level of stress that they have to transfer between members of the timber truss.
Timber trusses are, in the USA, most often made with southern pine dimension lumber. Our preference is Southern Yellow Pine (SYP), however, from time to time, and in some areas, Douglas fir and the woods of the spruce-pine-fir (SPF) group are used.
Typically the truss frame manufacturing process starts by carefully cutting nominal 2" (actually 1-1/2") thick members so that they are exactly the specified length and have the exactly the correct angles at the ends. Many roof truss manufacturers use computer-driven saws that produce multiple cuts quickly and precisely. The species, the size and the grade of lumber for each piece on the component cutting list (cut sheet) is based on the magnitude of force that each must resist while under potential maximum design load. Highly stressed, top and bottom chords are usually made of lumber that has been stress-rated, either visually or by machine, with rules mandated by code and industry association, to ensure specified performance. The webs of the wood truss, because they are usually subjected to lower stresses, are more often made from a lesser grade of lumber, such as #2, #3 grades.
The storage and handling of the raw material component parts of a wood truss is an important factor in the overall quality of the finished product. The lumber, before and during the manufacturing process must be kept out of the rain and/or snow and, in many cases, the humidity in the air of the storage area must be controlled.
Improper handling during delivery and during installation are the most frequently cited reasons for failure of the wood floor or roof truss system.