Engineered trusses are the most common type of framing for roofs. They are made in factories where computers control angles precisely and machines fasten joints between boards. A builder tells a manufacturer the key dimensions and style of truss and a computer program calculates the angles, lengths and other elements.
The first basic figure for roof truss calculation is the span or width of the truss from wall to wall. The span will determine the basic type of truss. Trusses can be made to span structures from garden sheds to buildings at least 50 feet wide. The most common house trusses are calculated to span 33 to 36 feet. Span also is affected by how the truss will fit on the roof, whether the angled rafters or the horizontal bottom chord will be what's called the bearing point on the wall.
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Pitch or slope of the roof from peak to wall is the next key factor. This is determined by the style of roof, the amount of internal space needed, insulation requirements and type of ceiling desired inside the house. Pitches are figured in inches of rise per foot of run or the distance each rafter side must support. For example, the pitch would be 5/12 for a rise of 5 inches per foot. The run is half the width or span.
Three types of loads exist. Dead load is the weight of the roof, which includes the framing members and roof covering. These can vary from light aluminum to heavy clay tile. Live load is weight added by external elements, mainly snow and ice. Wind load is force exerted by wind striking a roof. Locales with heavy snow loads tend to steeper pitches, which shed snow and ice more effectively. Flat or low-slope roofs work in areas of strong winds but are more subject to snow loads.
Another factor affecting truss calculations is the type of interior. Open beam ceilings usually use a scissor truss style, which creates an open vaulted ceiling but requires different calculations of loads because they use no horizontal bottom chord. These trusses provide no space for attic insulation and are less energy-efficient than styles that afford large spaces for insulation. Trusses that require large attic storage spaces or rooms require different calculations of strength.
Wood and Spacing
The type of wood and truss spacing affect truss calculations as well. Most trusses are made with 2-by-4-inch pine lumber, but some locales or building styles require 2-by-6-inch lumber. Some trusses use Douglas fir or other lumbers that have different strengths. Most trusses are spaced 24 inches apart, but spacing trusses at 16 inches can change the strength calculations. Each truss must be calculated with these elements considered.
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