Roof Truss and Joist Details: Design, Types & Structural Engineering for Steel Roof Systems
The roof is one of the most structurally critical and architecturally defining elements of any building. In steel construction, roof trusses and joists form the primary load-carrying system that spans between supports, carries roofing loads, and transfers forces to the main structural frame. A well-designed and accurately detailed roof truss or joist system directly determines the safety, economy, and durability of the entire roof structure, making correct design and detailing non-negotiable.
What are Roof Trusses and Joists?
A roof truss is a triangulated structural framework of steel members top chords, bottom chords, verticals, and diagonals designed to span large distances and carry roof loads to the supporting columns or walls. A roof joist is a lighter, closely spaced secondary beam that spans between trusses or main beams to support the roof decking, purlins, and cladding.
At BestGrid.in, we provide expert roof truss and joist design, analysis, and fabrication drawing services, delivering IS code-compliant, fabrication-ready roof structure details for industrial, commercial, and infrastructure projects across India.
Main Components of a Roof Truss
A steel roof truss is an assembly of individual members connected at nodes (panel points) to form a stable triangulated structure. Each component plays a specific structural role:
01
Top Chord (Rafter)
The sloping upper members of the truss that follow the roof pitch, carrying compression under gravity loads and directly supporting purlins and roof cladding. Top chords are typically designed as continuous compression members, with intermediate purlin connections that provide lateral restraint.
02
Bottom Chord (Tie Member)
The horizontal lower member of the truss carrying tension under gravity loads and resisting the horizontal thrust at the truss supports. The bottom chord also serves as the tie beam of the roof structure and may support ceiling loads or suspended equipment in industrial buildings.
03
Vertical Members (Posts)
Vertical web members connecting the top and bottom chords at panel points, carrying tension or compression depending on the truss configuration and load pattern. In Pratt trusses, verticals carry compression; in Howe trusses, verticals carry tension under gravity loads.
04
Diagonal Members (Web Braces)
Inclined web members between the top and bottom chords carrying shear forces within the truss panel. Diagonal orientation determines whether each member is in tension or compression under a given load case, a key consideration in member selection and connection design.
05
Gusset Plates
Steel plates at truss panel points connecting multiple members at a single node — transferring forces between the chord and web members through welds or bolts. Gusset plate design is critical: plate thickness, weld length, bolt group layout, and eccentricity must all be carefully designed per IS 800.
Types of Roof Trusses
The choice of truss type depends on span, roof pitch, load conditions, and architectural requirements. The most common roof truss configurations used in Indian construction are:
| Truss Type |
Web Configuration |
Best Application |
| Pratt Truss | Verticals in compression, diagonals in tension | Most efficient for medium spans 10–30 m |
| Howe Truss | Verticals in tension, diagonals in compression | Short to medium spans, timber-steel hybrid |
| Warren Truss | Alternating diagonals, no verticals | Long spans, bridges, industrial roofs |
| Fink Truss | W-shaped web members in tension | Residential and light commercial roofs |
| North Light Truss | Asymmetric profile vertical glazing face | Industrial buildings needing natural lighting |
| Scissor Truss | Crossed bottom chords, vaulted ceiling | Architectural buildings, auditoriums |
| Flat / Parallel Chord Truss | Horizontal top and bottom chord | Flat roofs, mezzanine floors, bridges |
Roof Joist: Types and Details
Roof joists are secondary structural members that span between main trusses or beams supporting the roof deck, purlins, and cladding. They are designed as simply supported or continuous beams depending on the structural system:
01
Open Web Steel Joists (OWSJ)
Lightweight trussed joists with open web configuration made from steel angles or rods for top chord, bottom chord, and web members. OWSJs are highly economical for spans of 6–18 m, allow mechanical services to run through the open web, and are faster to fabricate than solid section beams. Widely used in industrial sheds, warehouses, and commercial buildings.
02
Cold-Formed Z & C Section Purlins as Joists
Cold-formed Z-section and C-section purlins are the most widely used joist type in PEB and industrial steel buildings, spanning between primary rafters at 1.5 m to 2.5 m spacing to support roof sheeting. Designed per IS 811 (Cold-Formed Steel), they offer light weight and fast installation.
03
Hot-Rolled Section Joists (ISMB / ISMC)
Hot-rolled I-beams (ISMB) or channel sections (ISMC) used as secondary roof joists for heavier loading conditions such as industrial roofs with heavy equipment, walkways, or large snow loads. Designed per IS 800 for bending, shear, and deflection.
04
Castellated & Cellular Beams as Joists
Castellated (hexagonal web openings) and cellular (circular web openings) beams derived from standard I-sections by cutting and re-welding the web, producing a deeper beam with the same steel weight. Used as roof joists where long spans, reduced deflection, and service integration through web openings are required.
Loads on Roof Trusses and Joists
Roof trusses and joists must be designed for all relevant load combinations per IS 875 and IS 1893:
| Load Type |
Description |
IS Code |
| Dead Load | Self-weight of truss + purlins + cladding + services | IS 875 Part 1 |
| Live Load | Access and maintenance load on roof min 0.75 kN/m² | IS 875 Part 2 |
| Wind Load | Pressure and suction on roof surfaces govern uplift design | IS 875 Part 3 |
| Seismic Load | Horizontal inertia forces from roof mass during earthquake | IS 1893 |
| Equipment Load | Concentrated loads from rooftop equipment, HVAC, solar panels | Vendor data |
| Crane Surge Load | Horizontal forces from overhead crane on roof truss bottom chord | IS 807 |
Key Connection Details in Roof Trusses
Connection detailing is where roof truss design is most often underestimated. Every node, support, and purlin connection must be carefully detailed to ensure force transfer without eccentricity, buckling, or fatigue:
Detail 1
Panel Point (Node) Connection
At each panel point, chord members and web members meet at a gusset plate. The centroidal axes of all members must pass through a common point to avoid eccentric moments, a critical detailing requirement per IS 800 Clause 10.11. Gusset plate thickness, weld length, and bolt group are all designed for the maximum member force at that node.
Detail 2
Truss Support Connection (Bearing)
The truss end bearing connection transfers vertical reaction and horizontal thrust to the supporting column or wall. One end is typically a fixed bearing (restrained horizontally) and the other a sliding bearing (free to expand), accommodating thermal expansion and deflection without inducing secondary forces in the supporting structure.
Detail 3
Purlin-to-Top-Chord Connection
Purlins are connected to the top chord of the truss at panel points or at intermediate positions using cleat angles or purlin cleats bolted to the top chord. Purlin connections must also provide lateral restraint to the top chord, preventing lateral-torsional buckling of the compression chord between panel points.
Detail 4
Wind Uplift Connections Holding Down Details
Under wind uplift, particularly critical in IS 875 Part 3 wind zones, the roof truss must be held down against uplift at its supports. Hold-down connections using anchor bolts, cleat plates, or clevis connections must be designed for the net uplift reaction, including the appropriate wind pressure coefficients for the specific roof geometry.
Roof Truss vs Roof Joist Key Differences
| Feature |
Roof Truss |
Roof Joist |
| Span | 10 m to 60+ m | 2 m to 12 m typically |
| Role | Primary load-carrying member | Secondary member spans between trusses |
| Structural Form | Triangulated frame of multiple members | Single beam or light truss |
| Spacing | 4 m to 9 m centres typically | 1.2 m to 3 m centres |
| Steel Weight | Heavier major structural element | Light small section sizes |
| Fabrication | Complex multiple members, gussets | Simple standard sections |
| IS Code | IS 800 | IS 800 / IS 811 (cold-formed) |
What Is Included in Our Roof Truss Design Package?
BestGrid.in delivers a complete roof truss and joist design and detailing package from load analysis to fabrication-ready drawings:
| # |
Deliverable |
Purpose |
| 1 | Truss Analysis & Member Design | IS 800-based member force analysis and section sizing |
| 2 | General Arrangement Drawing | Overall truss layout, spacing, and roof geometry |
| 3 | Truss Fabrication Drawing | Dimensioned member, gusset, and connection details |
| 4 | Purlin & Joist Layout Drawing | Secondary member spacing, cleat details, and fixing |
| 5 | Connection Detail Drawings | Panel point, bearing, purlin, and uplift hold-down details |
| 6 | Bill of Materials (BOM) | Complete material list for procurement and cost control |
Frequently Asked Questions
What is the difference between a roof truss and a roof joist?
A roof truss is a primary structural element, a triangulated frame spanning between column supports, typically spaced 4–9 m apart. A roof joist is a secondary structural element, a lighter beam or open-web member, spanning between trusses at closer spacing (1.2–3 m) to support the roof deck and cladding.
Which is the most efficient roof truss type for industrial buildings in India?
The Pratt truss is the most commonly used and structurally efficient truss for industrial buildings in India — with diagonals in tension and verticals in compression under gravity loads. It minimises steel weight for spans of 10-30 m and is straightforward to fabricate and erect.
Why must truss member axes meet at a common point at each node?
If the centroidal axes of truss members do not pass through a common point at a node, bending moments are induced in the members due to eccentricity in addition to the intended axial forces. These secondary bending moments increase member stresses and can lead to premature failure. IS 800 Clause 10.11 requires that eccentricity at connections be accounted for in design.
What is wind uplift and how does it affect roof truss design?
Wind uplift is the net upward suction force on a roof surface caused by wind pressure differences. Under strong wind, the uplift force can exceed the dead weight of the roof, causing the truss to reverse its reactions from compression to tension at the supports. IS 875 Part 3 requires uplift forces to be calculated using external and internal pressure coefficients, and hold-down connections must be designed for the net uplift.
Which IS codes apply to roof truss design in India?
Roof trusses in India are designed in accordance with IS 800 (Steel Structures), IS 875 Parts 1–3 (Dead, Live & Wind Loads), IS 1893 (Seismic Design), and IS 811 for cold-formed purlins and joists. Connection welds follow IS 816 and bolts follow IS 3757 and IS 4000.
Does BestGrid.in design roof trusses for all building types?
Yes.
BestGrid designs and details roof trusses and joists for industrial sheds, warehouses, PEB structures, commercial buildings, sports facilities, and infrastructure projects across India, delivering IS code-compliant, fabrication-ready drawings for all truss types and spans.
© 2026 BestGrid.in — Structural Design & Detailing Services India
