Truss Concepts: Parts, Profiles, and How Trusses Carry Loads

Why Trusses Are Engineered Assemblies

A roof truss is not just “a bunch of 2x4s nailed together.” It is an engineered assembly: a specific arrangement of members (chords and webs) connected at defined joints so the whole unit behaves predictably under design loads. The key idea is that trusses are designed to act primarily in axial forces (tension and compression) rather than bending. That is why trusses use triangulated webbing: triangles are geometrically stable, so loads can be routed through straight members into the supports with minimal member bending.

Because a truss is engineered as a system, changing any part (cutting a member, drilling a chord, removing a brace, swapping connector plates, or altering bearing) changes how forces flow and can overload members that were never intended to carry that new demand. Field modifications should be treated as an engineering issue, not a carpentry preference.

How Triangulation Distributes Loads

In a typical residential truss, roof loads are collected by the top chords and transferred through the webs to the bearing points at the ends (and sometimes intermediate bearings). The web pattern breaks the span into triangles so the truss can “resolve” loads into axial forces. In simplified terms:

• Top chord segments often see compression under gravity loads.
• Bottom chord segments often see tension (acting like a tie) and may also carry ceiling loads if designed for it.
• Web members alternate between tension and compression depending on their orientation and where the load is applied.

Even though the roof load is distributed along the top chord, the truss is analyzed at its joints (panel points). That is why the location of webs and joints matters: loads are intended to enter and leave the truss at specific points.

Truss Anatomy: Parts You Must Be Able to Name

Primary Members

• Top chord: the sloped (or sometimes flat) outer members forming the top edges of the truss. Roof sheathing and roofing loads are carried into the top chord.
• Bottom chord: the lower member spanning between bearings. Often forms the ceiling line and may support ceiling drywall, insulation, and attic storage loads if designed.
• Webs: interior diagonal and vertical members that connect top and bottom chords, creating triangles and transferring forces to bearings.

Geometry and Reference Points

• Panel: the segment of a chord between two adjacent joints (panel points). Truss drawings often label panel lengths.
• Panel point: a joint where members meet (typically where a metal connector plate is located). Loads and reactions are commonly referenced at these points.
• Peak: the top apex joint where the two top chords meet on a symmetrical truss.
• Heel: the end joint where the top chord and bottom chord meet at the bearing. Heel height affects insulation space and eave geometry.

Connectors

• Metal plate connectors (MPCs): toothed steel plates pressed into the wood at joints. They are sized and placed based on calculated forces. Do not bend, remove, or “re-seat” plates with a hammer; damaged plates are an engineering repair issue.

Quick Identification Checklist (Jobsite)

• Find the bearing ends first (heels).
• Trace the top chords from heel to peak (or to the high end on a mono truss).
• Trace the bottom chord between bearings.
• Count the panels along the bottom chord (joints define panels).
• Identify the web pattern (diagonals/verticals) and note any special members (girder truss, valley set, etc., if shown).

Common Residential Truss Types and What They’re Used For

Truss “type” usually describes the web pattern or the roof shape it creates. The type affects how loads are distributed, where interior webs land, and how the truss is braced.

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King Post Truss

A simple triangular form with a central vertical web (the “king post”) and usually two diagonal webs. Common in shorter spans or decorative applications. Fewer webs means fewer panel points, which can increase chord forces for longer spans.

Fink Truss (Most Common in Houses)

Recognizable by its “W” web pattern. Efficient for typical residential spans because it creates multiple triangles and spreads forces through several web members. It also provides predictable panel points for bracing and for routing loads to the bearings.

Mono Truss

A single-slope truss used for shed roofs, porch roofs, or where one side ties into a taller wall/roof. Mono trusses can introduce different reactions at the high and low bearings and often require careful attention to bracing and bearing details shown on the truss design.

Hip Sets (Hip Truss Systems)

Hip roofs typically use a set of trusses: a girder truss (or multiple-ply girder) that supports jack trusses stepping down toward the corner. The “hip set” is not one truss; it is a coordinated system. The girder carries concentrated loads from the jacks, so its bearing and fastening requirements are usually more demanding.

Typical Residential Spacing

Common truss spacing is 24 inches on center, with 16 inches on center also used depending on design loads, sheathing thickness, and roof system requirements. Always follow the truss placement shown on the truss layout and the building plans; spacing is part of the engineered system.

Handling and Installation Rules (Non-Negotiables)

Most truss failures in the field come from damage, improper handling, missing bracing, or unauthorized modifications. Use these rules as a minimum standard.

Rule 1: Do Not Cut, Notch, or Drill Chords or Webs

• No cutting for ducts, skylights, attic access, or “it’s in the way.”
• No drilling holes in chords or webs unless the truss engineer provides a repair detail.
• If something conflicts (plumbing vent, HVAC, recessed lights), stop and coordinate: reroute the trade work or obtain an engineered truss repair.

Rule 2: Do Not Remove Bracing or “Temporary” Members Without Approval

Trusses typically require both temporary erection bracing (to keep them stable during installation) and permanent bracing (to restrain members against buckling). Removing braces can allow a compression web or chord to buckle even under modest loads.

• If a brace is in the way, treat it as a structural element until the plan says otherwise.
• Follow bracing notes on the truss design drawings and the building plans.

Rule 3: Store Trusses to Prevent Damage and Warping

• Store trusses on level blocking with support points aligned under panel points when possible.
• Keep them out of standing water and minimize prolonged exposure to weather.
• Do not stack heavy materials on trusses unless the truss supplier explicitly allows it.
• Reject or set aside trusses with cracked chords, split webs, or damaged connector plates for evaluation.

Rule 4: Lift and Set Trusses Without Twisting Them

Trusses are strong in their designed plane and weak out of plane. Twisting during lifting can crack members or loosen plates.

• Use proper pick points (often near panel points) and spreader bars when required.
• Keep the truss vertical during hoisting; avoid dragging across the deck.
• Set trusses in sequence and brace immediately so the first few trusses do not domino.

Step-by-Step: A Safe, Repeatable Erection Rhythm

1. Verify layout: confirm bearing lines, truss spacing marks, and any special trusses (girder, valley, hip) are identified.
2. Set the first truss: plumb it and brace it securely to a stable reference (often a gable end or a temporary strongback).
3. Set the next truss: align to layout marks, install temporary lateral bracing along top chords and/or bottom chords as required.
4. Continue setting: keep trusses plumb, parallel, and evenly spaced; add bracing as you go, not after.
5. Install permanent bracing: follow the truss design bracing notes and the building plan bracing details before loading the roof with sheathing bundles.

Reading Practice: Interpret a Simplified Truss Diagram

Use the simplified diagram below to practice identifying parts, bearing points, bracing notes, and load transfer locations. This is not a full engineering drawing; it is a learning tool to build your “truss reading” habits.

SIMPLIFIED FINK TRUSS (NOT TO SCALE)            Span: 24'-0"   Spacing: 24" o.c. (typ.)   Pitch: 6:12 (typ.)  NOTE: DO NOT CUT MEMBERS.  NOTE: CONTINUOUS LATERAL BRACING (CLB) REQUIRED ON WEBS MARKED "C" WITH DIAGONAL BRACING TO TOP CHORD.                         (PEAK)                           ▲                           ●                           |                           |  (vertical web)                           |                         /   \                       /  C  \                     /       \                   ●----●----●----●----●   (bottom chord panel points)                  ^               ^               |               |              Bearing A       Bearing B           (heels at exterior walls) Legend: ● = panel point (joint with metal plate)  C = compression web requiring CLB

Task A: Identify the Components

• Top chords: the two sloped outer members running from each heel up to the peak.
• Bottom chord: the long horizontal member along the bottom connecting Bearing A to Bearing B.
• Webs: the interior members forming the “W” pattern (including the vertical at center and diagonals).
• Panels: the bottom chord is divided into segments between the ● joints; each segment is a panel.
• Heel: the end joint at each bearing where the top chord meets the bottom chord (near Bearing A and Bearing B).
• Peak: the top center ● joint.
• Metal plate connectors: implied at every ● joint; in real drawings you may see plate sizes or joint IDs.

Task B: Locate Bearing Points and Describe the Load Transfer

In the diagram, the truss bears at two points:

• Bearing A: left heel, sitting on the left exterior wall top plate (or beam if specified).
• Bearing B: right heel, sitting on the right exterior wall top plate (or beam if specified).

Describe the load path in words (practice):

• Roof loads enter the truss through the top chords.
• Forces move through the webs at panel points (●), redirecting forces toward the ends.
• Reactions leave the truss at the heels and transfer into Bearing A and Bearing B (the supporting walls).

Task C: Interpret Bracing Notes

The diagram includes a simplified bracing note: CLB REQUIRED ON WEBS MARKED "C". Practice interpreting it:

• Webs marked C are intended to be in compression under typical gravity loading.
• Compression members can buckle sideways, so they need continuous lateral bracing (CLB) to restrain them.
• CLB must be tied into a bracing system (often diagonal bracing to a stiff line such as the top chord bracing or a designated bracing plane). CLB by itself, floating, is not enough.

Task D: Spot “No-Go” Field Changes From the Diagram

• The note DO NOT CUT MEMBERS applies to all chords and webs shown.
• Do not remove or relocate bracing indicated by notes. If a web is marked for CLB, it is not optional.
• Do not assume you can move bearing locations. Bearings are part of the engineered design; shifting a truss off its bearing can overload chords and plates.

Optional Self-Check Questions

• Which joints are panel points? (Answer: every ●.)
• Where are the heels? (Answer: at the two end joints over Bearing A and Bearing B.)
• Which members need CLB in this example? (Answer: the webs marked C.)
• Where do the truss reactions go? (Answer: into the supporting walls at Bearing A and Bearing B.)