Truss Terminology M-R

Machine Evaluated Lumber (MEL)

Type of machine-graded lumber designated by the letter "M" followed by a number (e.g., M-10, M-19, etc.) which increases with increasing values for design properties. MEL grade design properties are listed in the NDS.

Machine-Graded Lumber

Type of lumber that has been evaluated by mechanical grading equipment approved American Lumber Standards Committee (ALSC) Board of Review. Machine-graded lumber is distinguished from visually graded lumber in that each piece is nondestructively evaluated and marked to indicate a grade classification. Machine-graded lumber is also required to meet certain visual requirements as set forth by the grading agency. Examples of machine-graded lumber include: Machine-Stress Rated Lumber (MSR) and Machine Evaluated Lumber (MEL).

Machine-Stress Rated Lumber (MSR)

Type of machine-graded lumber designated by the design bending stress, Fb, and modulus of elasticity, MOE or E, values. For example, an MSR grade of 1650f-1.5E designates the bending stress of 1650 psi and an MOE of 1.5 million psi. Other design properties are listed in the NDS.

Mean Roof Height

The elevation of the roof mid-way between the eave and the ridge.

Metal Connector Plate

Connector plate manufactured from ASTM A653/A653M, A591, A792, or A167 structural quality steel protected with zinc or zinc-aluminum alloy coatings or their stainless steel equivalent. The metal connector plate has integral teeth and shall be manufactured to various sizes (i.e., length and width) and thickness or gauges and shall be designed to laterally transmit loads in wood. Also known as truss plate, plate, metal plate, etc.

Miter Cut

Single cut made at an angle to the member length.

Modulus of Elasticity (MOE or E)

Measure of the inherent rigidity or stiffness of a material. For a given geometric configuration, a material with a larger MOE deforms less under the same stress.

Moisture Content

Amount of water contained in the wood, usually expressed as a percentage of the mass of the ovendry wood.


Force that produces rotation of a member and resulting bending stresses.

Monopitch Truss

Truss that has a single top chord, and a slope greater than 1.5/12.

Multi-Ply Truss

A Truss designed to be installed as an assembly of two or more individual Trusses fastened together to act as one. Ply-to-ply Connections of Multi-Ply Trusses are specified on the Truss Design Drawing.

Nail-On Plate

Light-gauge cold-formed steel metal connector plates with pre-punched holes or, if cut to size, without holes having identifying marks through which nails are driven by hand or pneumatic means into the lumber. They are typically used in repairs.

Net Free Ventilated Area

Area required by building codes to allow for proper ventilation in enclosed building spaces.

Nominal Size

As applied to products such as lumber, traditionally the approximate rough-sawn commercial size by which it is known and sold in the market. Actual rough-sawn sizes may vary from the nominal. Reference to standards or grade rules is required to determine nominal/actual finished size relationships.


Vertical and horizontal cut at the end of a chord, joist or rafter.

On Edge

Vertical placement of a member's wide edge.

On The Flat

Horizontal placement of a member's wide edge.


Wood member nailed to a truss to form a rake overhang beyond the wall line.

Overall Span

Outside-of-frame dimensions, not outside-of-sheathing dimensions.

Overall Truss Depth

Vertical distance between bearing and the uppermost point of the peak.


Extension of the top chord of a truss beyond the outside of the bearing support.


Person having a legal or equitable interest in the property upon which a Building is to be constructed, and: (1) either prepares, or retains the Building Designer or Registered Design Professional to prepare the Construction Documents; and (2) either constructs, or retains the Contractor to construct the Building.


Chord segment defined by two adjacent panel points.

Panel Length

Horizontal distance between the centerlines of two consecutive panel points along the top or bottom chord.

Panel Point

Location on a truss where the web members and top or bottom chords intersect and are connected by metal connector plates.

Parallel Chord Truss (PCT)

Truss with top and bottom chord slope less than 1.5/12.

Parallel Strand Lumber (PSL)

Composite of wood strand elements joined using an adhesive with wood fibers primarily that are oriented along the length of the member. The least dimension of the strands shall not exceed 0.25", and the average length shall be a minimum of 150 times the least dimension.

Particleboard Underlayment

Underlayment grade particleboard made or machined to close thickness tolerances for use as a leveling course and to provide a smooth surface under floor covering materials.


Point on the truss where the sloped chords meet.


Common nail length. Originally, nails were sold by "penny weight", or price per hundred.

Permanent Bracing

Bracing installed to provide support at right angles to the plane of the truss to hold it in its assumed design position. Permanent bracing stays in place for the life of the structure. The Building Designer may design the permanent bracing to resist lateral forces imposed on the completed building by wind load or seismic load.

Permanent Building Stability Bracing (PBSB)

Lateral force resisting system for the Building that resists forces from gravity, wind, seismic and/or other loads.

Personal Fall Arrest System

An individual worker's Fall Protection System, composed of a safety belt or full body harness, and lanyard, lifeline, and any other connecting equipment that is used to secure the worker to an individual anchor or to a horizontal lifeline system; designed to stop a worker's fall before the worker hits the surface below.

Piggyback Truss

Truss made in two pieces usually consisting of a hip type truss with a triangular cap fastened to it. Designed when shipping or manufacturing limitations are affected by overall truss height.


(1) Incline of the roof described as inches of rise over inches of run. For example, 5/12 is 5" of rise over 12" of run. (2) Resin occurring in the wood of certain conifers.


Small, soft tissue occurring in the structural center of a tree trunk, branch, twig, or log.


The cavity formed by the floor above and the ceiling below a floor truss as a supply or return air duct.

Plumb Cut

Top chord end cut to provide for vertical (plumb) installation of fascia.


One truss of a multiple-truss girder.


Usually, a cross-banded assembly made of layers of veneer or veneer in combination with a lumber core or other wood-based panel material joined with an adhesive. Plywood generally is constructed of an odd number of layers with grain of adjacent layers perpendicular to one another. Outer layers and all odd-numbered layers generally have the grain direction oriented parallel to the long dimension of the panel.

Pressure-Treated Wood

Wood treated by applying pressure to force the preservative into it.

Professional Engineer (PE)

Registered Professional Engineer holding a current license in each state in which s/he conducts business.

Profile Drawing

Sketches of truss profiles used by a mechanical engineer to determine where mechanical ducts, piping, etc., are located.

Proprietary Metal Restraint/Bracing Products

Metal products used as Diagonal Bracing, Lateral Restraint, bridging and Web Reinforcement, which are available from a number of manufacturers.

Publication of the Truss Plate Institute (TPI)

developed under the American National Standards Institute (ANSI) consensus process. Provides a state-of-the-art consensus standard for testing and evaluating wood trusses designed and manufactured in accordance with ANSI/TPI 1.

Publication of the Truss Plate Institute (TPI)

developed under the American National Standards Institute (ANSI) consensus process. Provides the analysis, design and manufacturing criteria for the metal plate connected wood truss industry.


Horizontal member attached perpendicular to the truss top chord for support of the roofing (e.g., corrugated roofing or plywood and shingles).


Coincident with the radius from the pith to the circumference of the tree or log. A radial section is a longitudinal section in a plane that passes through the pith of the tree.


Sloping roof framing member.


The procedure of building the entire roof system, or portions thereof, on the ground and lifting it into place.


Edge of a roof with the intersection of the gable.


Total load transmitted by a framing member to its bearing.

Registered Design Professional

Architect or engineer, who is licensed to practice their respective design profession as defined by the Legal Requirements of the Jurisdiction in which the Building is to be constructed.


A piece of steel section attached along the length of a Truss member as reinforcement against buckling instability.

Repair Detail

A written, graphic or pictorial depiction of the required fix to an altered or damaged component or part.


Line made by the intersection of two roof planes.

Ridge Vent

Prefabricated and formed metal strip placed along the peak of the roof to aid in ventilation.

Rim Joist

Framing member installed on edge of the exterior parameter, usually tying the ends of floor trusses together. Also called ribbon or band board.


Vertical distance from low end to the high end of a sloping member.

Roller Press

Press that embeds metal connector plates by forcing them through two opposing rollers.

Roof Scuttle

Framed opening in commercial roofs surrounded by a hinged door used for access to a commercial roof.


Horizontal distance from the low end to the high end of a sloping member.