Roof Trusses

In this section you will find information pertaining to roof trusses. Click the arrows on the bottom of each page to navigate.

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Terms 1

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Truss Terms


Axial Force - The number of pounds of tension or compression in a truss member acting parallel to the length of the member resulting from a load applied to the truss.

Axial Stress - The axial force in a member, divided by the cross sectional area of the member, is usually measured in pounds per square inch.

Bending Moment - A measure of the amount of bending in a member due to forces acting perpendicular to its length. The bending moment at a given point along a member equals the sum of all forces, whether to the left or right of the point, times their corresponding distances from the point. It is measured in inch-pounds.

Bending Stress - The stress in a member caused by the bending moment, measured in pounds per square inch.

Camber - A very slight arch built into a truss, or sometimes only in its bottom chord, to offset deflection in a truss caused when loads are applied.

Combined Stress Index - The combination of axial compression and bending stresses acting on a member simultaneously, such as occurs in a top chord, or the tension and bending combined in a bottom chord.

Continuous Lateral Bracing - A member placed and connected at right angles to a chord or web to prevent buckling. Required on some chords and webs, depending on their length and the forces in the member.

Concentrated Load - Additional loads applied at a given point on a truss, other than the uniform load of an assembly, such as an air conditioner on a roof or a hoist that might lift car engines and the hoist being attached to the truss assembly above.

Cricket - A portion of a roof where it is built up for the purpose of draining water towards a desired drainage point.

Compression - A force caused by loads being placed on a member that causes squeezing or shortening effect on the member as in the top chord of a truss when a load is applied.



Checking - The cracks or splits that normally occur on the truss surface of lumber during the drying process. The amount of checking allowed is controlled by the grading rules for each size, grade, and species.

Dead Load - The load of the member itself plus the weight of any permanent part of the roof or floor assembly.

Deflection - Movement of a structural member, like a truss in place, due to the application of the dead and live loads.

Diaphragm - The decking or bracing built into a roof or floor assembly to assist in offsetting horizontal forces and withstand the racking caused by wind and seismic loads.

Duration Factor - A percentage increase in the allowable stresses based on the length of time the live loads will be applied. The shorter the duration of live loads, the higher the precent of increase is allowed. A unique feature in wood is its ability to carry high loads for short periods of time.

F - Rating - The allowable stress in bending sometimes stamped on the stick of wood near the end. It is determined either visually or by machine (Machine stress rated lumber) and depends on the number, size, and location of typical characteristics such as knots.

Force Diagram - A graphical method of calculating the axial forces in a truss.

Force - A reaction caused and existing in the components of a structural member, such as a truss when external loads are applied.

Flashing - Pieces of light gauge metal or orther materials that are used to make water-tight openings or seams in a roof system, such as in the case of chimneys or vent pipes.

Kip - A term used in measuring forces that mean 1000 pounds (kilopound).

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