Steel Roof Trusses: Estimating, Fabrication and Installation

A steel roof truss is a triangulated frame of chords and web members that carries roof loads over a long span using far less steel than a solid beam. Estimating one comes down to a member list: every chord, web and plate multiplied by its weight per metre, then by the number of trusses. Installation is erector work done to an engineered lift and bracing plan, because a single truss is unstable sideways until it is braced to its neighbours.

This guide covers the common truss types, how to estimate them, what the shop drawings show, how they are fabricated, and what happens on site.

What a steel roof truss is

Every truss has the same basic parts:

• Top chord: the upper member, usually sloped, that carries the roof purlins or deck and works mostly in compression.
• Bottom chord: the lower member, usually in tension under gravity load. It can reverse into compression under wind uplift, which is why it needs bracing too.
• Web members: the verticals and diagonals between the chords.
• Panel points: where webs meet the chords. Loads are best applied here, so purlins are normally spaced to land on them.
• Gussets and connection plates: plates that join members at panel points, splices and bearings.

Chords are commonly made from WT sections, double angles, HSS or W-shapes; webs from angles, double angles or HSS. Tube trusses (all HSS) are popular where the steel is exposed, because they look clean and are easy to paint or galvanize.

Two related products are often confused with custom trusses. Open web steel joists are light, repetitive trusses designed and built by joist manufacturers, covered in our post on steel joists vs steel beams. Pre-engineered metal buildings use tapered rafters rather than trusses. A custom steel truss is usually a heavier, one-off member designed by the project engineer and built by a structural fabricator.

Common steel truss types

The engineer chooses the type based on span, roof slope, the ceiling the architect wants, and how the truss connects to columns. The truss depth and the panel spacing set the size of every member, so changing either after design means redesign.

Who designs the truss

Steel trusses in Ontario are structural members designed under Part 4 of the Ontario Building Code, which requires structural steel to be designed to CSA S16. That means a professional engineer, not a span chart.

On most projects one of two things happens:

1. The engineer of record designs the truss fully and shows member sizes, geometry, camber and connection forces on the structural drawings.
2. The engineer of record gives the loads and geometry, and the truss or its connections are a delegated design by an engineer working for the fabricator. That engineer seals their design, and the engineer of record reviews it.

Either way, the loads come from the building code: dead load of the roof, snow (including drifts beside parapets and higher roofs), wind uplift, rooftop equipment, and anything hung from the bottom chord such as sprinkler mains or ductwork. Extra hanging loads added late in a project are a common reason trusses need rechecking.

How to estimate steel trusses

A steel truss estimate is a takeoff, the same method as for beams and columns, just with more pieces. The steps:

1. Count the trusses and group identical ones. Ten identical trusses are one takeoff multiplied by ten.
2. List every member in one truss: section, length and quantity. Chords may be sloped, so measure along the member, not the plan dimension.
3. Look up mass per metre for each section and multiply by length.
4. Add plates: gussets, splice plates, bearing plates and stiffeners. Plate weight is 7.85 kg per square metre for every millimetre of thickness.
5. Add the extras as separate lines: bracing between trusses, bottom chord bracing, bridging, purlin clips, bolts and finish.
6. Note the labour drivers: number of panel points, welded versus bolted joints, field splices, camber, and whether the finish is primer, paint or galvanizing.

Worked example

An illustrative truss with HSS chords and angle webs, 12 m span, using sections from published Canadian tables. This is an estimating exercise, not a design.

Eight identical trusses come to about 4.7 tonnes before bracing, bolts and bearing plates. Our guides to the structural steel takeoff and to estimating tonnage early cover the method in more depth, including what to carry before the drawings are finished.

Tonnage is only half the estimate. Two truss packages of the same weight can take very different shop hours: a truss with 20 welded panel points and a camber takes far longer than a simple bolted one. That is why a fabricator quoting from drawings will ask about the connection details, not just the sizes.

Truss shop drawings and fabrication

Steel truss shop drawings show:

• Overall geometry: span, depth, slope, working points and camber.
• Every member with its section, length, end cuts and piece mark.
• Every connection: gusset sizes, weld types and sizes, bolt holes and bolt grades.
• Field splices where the truss is too long or too deep to ship in one piece.
• Bearing details at each end, including slotted holes if one end needs to slide.
• Finish: primer, paint system or hot-dip galvanizing.

In the shop, trusses are typically laid out and fitted in a jig or on a layout table so the working points line up, tacked, checked for dimensions and camber, then fully welded. Large trusses are often built in halves and joined on site with bolted splices, because transport limits the length and height of what can go on a trailer. Ask about shipping limits early if your trusses are long or deep.

Each truss is marked to match the erection drawings so the crew knows exactly where it goes and which way it faces. A truss installed backwards (with a sloped chord or camber reversed) is a real and avoidable mistake.

Steel truss installation

Truss installation is part of steel erection and is done by an erector following the erection drawings and a lift plan. What the process usually looks like:

1. Check the bearings first. Column caps, anchor bolts or embed plates must be at the right elevation and spacing before any truss arrives.
2. Rig and lift at the pick points set out in the lift plan. Long trusses are often lifted with a spreader beam so the chords are not bent sideways.
3. Set the first truss and brace it before the crane releases it. A single truss is very slender out of plane and can buckle or tip over under its own weight. The first truss is usually guyed or braced to the columns or the ground.
4. Set the next truss and tie the pair together with permanent or temporary bracing, creating a stable braced bay that the rest can lean on.
5. Install purlins, bottom chord bracing and bridging as trusses go up.
6. Bolt up or weld field connections, plumb and align, then get the work inspected where the project requires it.
7. Remove temporary bracing only when the roof deck and permanent bracing are complete and the engineer's sequence allows it.

In Ontario, the Construction Projects regulation (O. Reg. 213/91) governs erection safety, including fall protection and keeping the structure stable while it goes up. Our post on steel erection safety in Ontario covers those rules, and structural steel bracing types explains the permanent bracing that trusses rely on.

This is not a DIY job. Even a small truss for a garage or a canopy needs an engineered design, a permit where the building code applies, and lifting equipment and crew appropriate to its size and weight.

Getting a truss package priced

If you have structural drawings with steel trusses, we can prepare a free takeoff and quote, produce the shop drawings in-house, and fabricate in our CWB certified shop with primer or galvanizing to suit. Delivery is available across the GTA and Ontario, and we can discuss installation as part of a structural steel package. Send your drawings for a quote, or see our services for what we do from detailing to install.