Roof Truss: Parts, Types and Why You Never Cut One

What a roof truss is

Look up in most attics built in recent decades and you will see a row of identical triangle frames, spaced evenly, joined with flat metal plates. Those are roof trusses. A factory builds them to a computer design, a truck brings them to the site, and a crane or crew sets them on the walls.

The triangle is the key. A triangle cannot change shape without stretching or crushing one of its sides, so a truss turns the weight of the roof into simple push and pull forces in its members.

The Structural Building Components Association (SBCA) describes webs as the members that join the top and bottom chords into the triangular patterns typical of trusses, carrying mostly axial forces ².

Because trusses are engineered, they are not like the lumber you buy at a store. Each one is designed for a specific span, pitch, spacing and load.

The design drawing that comes with the truss is part of the building’s record, and the code requires the building official to approve it before the trusses go up ¹.

How a truss carries the roof

Loads flow through a truss in a fixed path. SBCA’s parts glossary describes each step ².

1. The top chord takes the roof. Sheathing, roofing, snow and wind press on the sloped top chords.
2. Webs pass the force down. Webs carry the load to the chords as push or pull along their length.
3. Connector plates hold every joint. Each panel point, where webs meet chords, is joined with metal connector plates.
4. The bottom chord holds the ends together. It usually works in tension and bending, stopping the walls from being pushed apart; it often carries the ceiling too.
5. The heel and bearings pass loads to the walls. Every truss needs at least two bearings strong enough to carry the reactions down to the foundation.

Take away any one piece and the load path breaks. That is why cutting a single web to make room for a duct or attic stair can overload the rest of the truss.

Common types of roof trusses

Example: reading a truss design drawing before an attic project

This shows where a homeowner finds key facts on a truss design drawing. The list of required items is from 2021 IRC Section R802.10.1; the project is an example ¹.

If you cannot find the drawing, the building department may have it in the permit file, or the truss manufacturer may have a copy. Without it, an engineer must assess the trusses before any change.

Where trusses show up in a remodel

In the attic. Trusses decide whether you can add storage, run ducts or install a pull-down stair. ENERGY STAR advises walking only on joists or truss chords in an attic, never on bare ceiling drywall ⁶.

Under the ceiling. Because trusses often span wall to wall, interior walls below may not carry roof load. That can open up a layout, but only a professional should confirm it. See our load-bearing wall page before planning any wall removal.

On plans and permits. For an addition, new trusses need their own approved design drawings, and the tie-in to the old roof needs an engineered detail.

On a quote. A bid for attic work, a new HVAC unit in the attic or a skylight should say how the trusses will be protected or who engineered any change.

Benefits and limitations of roof trusses

What trusses rule out

The biggest limit is attic space. Common trusses fill the attic with webs, so a bonus room, a vaulted ceiling or a dormer usually means new framing, not a simple change. That is engineering and permit work.

Trusses are also sensitive to added weight. The IRC says alterations that add load beyond the truss design, such as HVAC equipment or a water heater, are not permitted without verification that the truss can carry it ¹.

Finally, a truss is only as strong as its bracing. SBCA notes temporary and permanent bracing are both required during and after installation, and that lateral restraints need their own bracing to work ².

What drives the cost of truss work

We do not publish prices for trusses or framing. For a remodel, the cost drivers are the span, the truss type (attic and scissors trusses are more complex than common trusses), the number of trusses, crane or lift access, engineering for repairs or alterations, and the need to tie into an existing roof.

Repairing a cut or damaged truss adds an engineer’s repair design and the labor to install it.

Trusses have no fixed lifespan. Kept dry and unaltered, they last as long as the house. Water damage, cut members and overloads are what shorten their life. Budget engineering into any project that touches them; our remodeling cost guide explains how professional fees fit a budget.

How a remodel that touches trusses should be handled

This is the order of work. The engineering and structural steps belong to licensed professionals, under a permit.

1. Locate the truss design drawings, or note that they are missing.
2. Describe the change: a new opening, added equipment, attic storage or a new roof tie-in.
3. Hire a registered design professional to evaluate and, if needed, design a repair or reinforcement ¹.
4. Apply for a permit with the engineered design; our permit checker helps you start.
5. Have a licensed contractor do the work exactly as designed, with temporary bracing per the drawings and BCSI ¹⁴.
6. Pass the framing inspection before closing the ceiling.
7. Keep the repair drawings with your house records.

Looking after roof trusses

Trusses need no routine service, but a yearly attic look catches the problems that harm them.

• Check for roof leaks: stains, damp wood or mold on chords and webs.
• Look at the plates: plates should sit flat in the wood, not backed out or rusted.
• Leave bracing in place: never remove lateral braces to make room for storage or ducts.
• Keep loads light unless the drawing allows storage.
• Keep attic vents clear so moisture does not build up on the framing.

Warning signs and when to call an engineer

• A cut, notched or drilled truss member, often left by a past duct, plumbing or attic-stair job.
• Sagging roof lines or a ridge that dips between ends.
• Cracked drywall or dips in ceilings, which SBCA notes can follow when a bearing crushes under load ².
• Plates pulling out of the wood, or split chords.
• Rot or long-term water stains on truss members.

Any of these calls for a licensed structural engineer before more work is done. Do not try to repair a truss by adding lumber on your own; the fix must be designed.

What the 2021 IRC says about wood trusses

Design drawings (R802.10.1). Truss design drawings must be provided to and approved by the building official before installation, and shipped with the trusses to the job site. They must show, at a minimum, slope or depth, span and spacing, joint locations, required bearing widths and design loads ¹.

Design (R802.10.2). Wood trusses shall be designed by accepted engineering practice, and metal-plate-connected wood trusses shall comply with ANSI/TPI 1. Drawings are prepared by a registered design professional where state law requires ¹⁵.

Bracing (R802.10.3). Trusses must be braced to prevent rotation and provide lateral stability per the construction documents and truss drawings, or, without specific requirements, per accepted industry practice such as the SBCA/TPI BCSI guide ¹⁴.

Alterations (R802.10.4). Truss members shall not be cut, notched, drilled, spliced or otherwise altered without the approval of a registered design professional, and added loads beyond the design, such as HVAC equipment or a water heater, need verification first ¹.

Adoption. States and cities adopt the IRC on their own schedule and may amend it. Remodels of existing homes may also fall under the existing building code. Ask your building department which applies.

Roof truss vs. other roof and ceiling framing

Myths about roof trusses

Myth One cut web will not matter.

Reality The IRC bans cutting any truss member without a registered design professional’s approval ¹; every member is part of the design.

Myth You can floor the attic and use it for storage.

Reality Only if the truss design includes that load; attic trusses are built for it, ordinary trusses may not be ².

Myth If trusses span wall to wall, no interior wall is load-bearing.

Reality Girder trusses, interior bearings and point loads can still sit on interior walls ²; confirm before removing any wall.

Myth Bracing is only needed during construction.

Reality Permanent bracing and lateral restraints stay in place for the life of the roof ²¹.

Myth Adding a heavy unit in the attic is a plumbing or HVAC question.

Reality The IRC requires verifying the truss can carry the added load first ¹.

How snow, wind and local rules change truss design

When trusses should change your remodeling plan

Trusses matter whenever a project touches the attic or ceiling: a finished attic, a vaulted ceiling, a skylight, a dormer, a new attic HVAC unit, a whole-house fan or an addition that ties into the roof. In each case, plan for an engineer’s review before design is final.

They matter less for remodels that stay below the ceiling, though wall removal still needs a check of what the trusses bear on.

Your next step: find the truss drawings and share them with your designer or engineer at the first meeting.

Related terms

Wall Stud

A wall stud is one of the vertical boards, usually 2x4 or 2x6 lumber, that run from the bottom plate to the top plate of a framed wall and carry its load, hold its sheathing and drywall, and make room for wires, pipes and insulation.

Shear Wall

A shear wall is a wall built and fastened to resist sideways forces from wind or earthquakes, so the house does not rack, lean or slide off its foundation.

Floor Joist

A floor joist is one of a series of parallel horizontal framing members, usually lumber or engineered wood I-joists, that span between walls, beams or foundations to carry the subfloor and everything on it down to the supports at each end.

Sill Plate

A sill plate, also called a mudsill, is the lowest piece of wood framing in a house: a board laid flat on top of the concrete or masonry foundation and bolted to it.

Blown-In Insulation

Blown-in insulation is loose, fluffy insulation, usually cellulose or fiberglass, that a machine blows through a hose into attics, wall cavities and other hard-to-reach spaces, where it settles into a blanket that slows heat flow.

Building Permit

A building permit is an official document from the local building department that authorizes specific construction work, such as building, altering, repairing or demolishing a structure, and requires that work to be inspected against the adopted building code.

Existing Building Code

An existing building code is a model code, most often the International Existing Building Code (IEBC), that sets the rules for repairing, altering, adding to, changing the use of and moving buildings that already exist, so that work on them does not have to meet every new-construction rule.

Also referenced from: Load-Bearing Wall, Ceiling Joist, Wall Stud, Engineered Wood Beam, Blown-In Insulation