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

Terms 2

Terms 3

Truss Webbing

Truss Types 1

Truss Types 2


Truss Terms


Lateral Brace - See Continuous Lateral Bracing

Live Load - Any load that is not a permanent nature, such as snow, wind, temporary construction loads, etc. In some codes, the weight of partitions which are always moveable and given designated per square foot weights are added in with the live loads.

Machine Stress Rated (MSR) - Lumber graded by the process of running it through an electronic machine which determines its stiffness value (E). The allowable stresses are then determined from this value.

Modulus of Elasticity (or E value) - A measure of the stiffness of a given material. The larger the M.O.E., the stiffer the piece, the less it will deflect under load. The M.O.E. of wood used in trusses usually ranges between 1,200,000 and 2,400,000.

Nominal Dimension - An approximate dimension usually used to describe the size of an item such as a 2x4, which should actually measure 1-1/2" x 3-1/2".

Piggyback Truss - A truss supported directly on top of another truss. It is usually not intended to span any distance since it is resting on the truss below and may have only vertical webs.

Rafters - The wood members used to support the roof in conventional framing.

Rake - The inclined edge of the Roof.

Reaction - The upward force at each support which resists the total dead and live loads.

Ridge - The uppermost line of the roof where two slopped or pitched surfaces meet.

Roof Cap - The total roof assembly members located above the wall line, such as beams, trusses, plates, purlins, decking, bridging, ceiling & roofing, etc.

Saddle - A covering on the ridge of the roof so that water will drain.



Scupper - An opening in a roof usually faced with metal flashing to drain water from the roof at a given point.

Shear (Horizontal) - The force in a member which tends to cause the top half to slide horizontally in the opposite direction from the bottom half, usually near the support.

Secondary Bending - The bending stress in a member caused by the deflection of the whole truss.

Splice - A point at which a chord member is butt-jointed by use of truss connectors.

Stress - A unit force working within a member. Stress is expressed in pounds per square inch (PSI)

Stress Rated Lumber - Lumber which carries a grademark of an independent grading agency who has assigned the allowable working stresses and modulus of elasticity values based upon the physical characteristics of the particular piece and species.

Symmetrical - A truss whose loading and configuration is exactly the same on both sides of the centerline.

Square - The amount of roofing material needed to cover 100 square feet of roof.

Tension - Forces being exerted on a truss member that creates pulling apart or elongating effect.

Tributary Loads - All those loads applied to a system that contribute to a total load being brought to bear on a main structural support from a given area. Also these loads occurring in an area that contribute to the total load on a given structural member.

Uniform Loads - A total load that is equally distributed over a given length, usually expresses in pounds per lineal foot (PLF).

Valley - A depression in a roof where two roof slopes meet.

Valley Set - A group of trusses required to fill in a section of a roof to give it its required shape. They are supported on top of other trusses.

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