When you look up at a finished roof you see tiles, sheets, ridges and gutters. Hidden beneath that outer skin is the real workhorse: the roof frame truss. In modern construction, the roof frame truss is the pre-engineered, prefabricated skeleton that defines the roof’s shape, carries the covering, and safely channels wind, rain and maintenance loads into the walls and foundations. Get the truss right and everything above (and below) it works better—water sheds cleanly, ceilings stay crack-free, and the building feels solid for decades.
What exactly is a roof frame truss?
A roof frame truss is a rigid, triangular framework made from straight members (chords and webs) connected at engineered joints. Because triangles are inherently stable, a truss achieves excellent stiffness with minimal material. In practice, trusses arrive on site as labelled, ready-to-install components sized for your span, pitch, covering, wind zone and local climate. They support battens or purlins, set the roof geometry, and create a predictable load path to the supporting structure.
The purpose of a roof frame truss (and why it beats guesswork)
- Define shape and pitch
Trusses lock in the angles that control drainage rate, overhangs and the roof’s look. This geometric accuracy keeps sheet ribs aligned and tile courses straight. - Carry and transfer loads
Dead loads (self-weight), live loads (maintenance traffic), and environmental loads (wind uplift, hail, occasional snow at altitude) are converted into axial forces in the members, then delivered as safe reactions to walls or beams. - Stabilise the roof plane
With correct bracing and tie-downs, trusses resist racking and uplift—especially at edges and corners where suction is highest—maintaining the roof’s integrity through storms. - Enable speed and repeatability
Prefabricated trusses cut programme time. The first truss and the last are identical, reducing site errors and call-backs. - Future-proof the building
A properly engineered roof frame truss accommodates skylights, walk-ways, solar arrays and services without compromising structure or waterproofing.
Materials for roof frame trusses
Treated timber trusses
Kiln-dried, strength-graded South African pine remains a mainstay in housing. It offers a strong strength-to-weight ratio, excellent workability and good thermal performance. Treatment (typically UC2 for enclosed roof spaces, higher for humid/coastal sites) protects against insects and decay. Timber trusses pair naturally with tile and slate coverings and can be left exposed for architectural effect in double-volume spaces.
Lightweight steel trusses
Cold-formed (light-gauge) steel trusses excel where spans are long, programmes are tight or fire/pest risk is a concern. They are dimensionally stable, immune to rot and termites, and deliver superb strength for their mass. Correct coating and fasteners are essential in coastal/industrial air, and acoustic/condensation detailing should be included under metal sheeting over occupied spaces.
Hybrid systems
Many projects mix materials for performance and cost—for example, steel trusses with timber purlins, or timber trusses with steel ridge/girders. Hybrids let designers fine-tune weight, span, cost and buildability.
Common roof frame truss designs (and where they shine)
- King Post / Queen Post: Simple, economical options for shorter spans in residential roofs.
- Fink: The “go-to” for many houses—efficient web layout for moderate spans with minimal timber or steel.
- Howe / Pratt / Warren: Popular in steel; different diagonal orientations suit specific span/load scenarios and fabrication preferences.
- Mono-pitch (Mono): Ideal for contemporary roofs, saw-tooth industrial bays or when headroom is needed on one side.
- Scissor: Creates vaulted ceilings while still providing triangulated strength; great for feature spaces.
- Girder truss: A beefed-up truss that carries other trusses framing into it at right angles—common at hips, valleys and large openings.
Dezzo models these options in design software so you can compare weight, cost and performance for your span and covering.
Key engineering considerations for a reliable roof frame truss
- Span, pitch and spacing
Span drives member sizes; pitch affects drainage and appearance; spacing influences loads on battens/purlins and coverings. Most residential trusses are set at regular centres, tuned to the covering and wind zone. (See the FAQ for typical South African spacing guidance.) - Load combinations and deflection
Trusses are checked for strength (axial capacity, plate/joint capacity, bearing) and for serviceability (deflection and vibration). Tight deflection control keeps sheet lines straight and prevents cracked tiles or plasterboard. - Edge and corner zones
Wind suction peaks at perimeters. Expect denser fixings, stronger bracing and sometimes upsized members or closer centres in these zones. - Connections and hangers
Correct nailplate sizes (timber) or gusset/bolt patterns (steel) are critical. Where secondary trusses frame into a girder, use rated truss hangers and follow the full fastener schedule—capacity assumes every specified hole is filled. - Corrosion and compatibility
Match coatings and fasteners to the environment and, for timber, to preservative chemistry (e.g., copper-based treatments can attack mild steel). Stainless or class-rated screws and washers are a wise upgrade near the coast. - Thermal, acoustic and moisture control
Insulation must fit the truss depth while maintaining ventilation pathways. Include vapour control layers or breathable membranes to manage condensation; add acoustic blankets under metal roofing where noise matters. - Buildability and QA
Factory-cut, labelled components reduce site error. A numbered layout, fixing schedule and photo sign-offs create a clean audit trail for warranties and approvals.
How Dezzo approaches your roof frame truss
- System thinking: We design the roof frame truss, purlins/battens, underlay, fixings and covering as one system, under the right wind zone, exposure and occupancy.
- Optimisation: Software iterates truss types and web layouts to minimise weight without compromising strength or serviceability.
- Precision manufacture: CNC cutting, accurate jigging and consistent nail-pressing (for timber) or roll-forming (for steel) ensure every truss matches the model.
- Documentation & support: Engineer’s calculations, PI-backed drawings, fixing schedules and on-site QA keep projects compliant and on programme—nationwide.
Typical pitfalls (and how to avoid them)
- Under-specifying perimeters: Edge/corner failures usually trace back to light fixings or missing bracing. Follow the enhanced fixing schedule.
- Late penetrations: Unplanned skylights/ducts can undermine strength and waterproofing. Coordinate early; Dezzo will model openings and reinforce as needed.
- Ignoring condensation: “Sealed” roof spaces invite moisture problems. Include balanced intake/exhaust ventilation and the right underlay.
- Mix-and-match hardware: Incompatible fasteners/coatings accelerate corrosion. Keep the system consistent, especially in coastal SA.
Conclusion
A roof frame truss is the quiet hero of a robust roof: it sets geometry, carries loads, resists storms and speeds construction—while creating a platform for insulation, ventilation and future upgrades like solar. When the truss is designed as part of a complete roof system and manufactured with precision, the whole building benefits: straighter lines, drier interiors, lower maintenance and a smoother handover.
Planning a new roof or replacement? Share your drawings and site details with Dezzo Roofing. We’ll model an optimised roof frame truss system—timber, steel or hybrid—then deliver a PI-backed specification and a quote that keeps your project on time, on budget and watertight for decades.
FAQs
What is a truss in roof framing?
A truss is a triangulated structural frame—made from straight members joined at engineered nodes—that supports the roof covering, sets the roof geometry and safely transfers loads to the building’s structure. In a roof frame truss, top and bottom chords are tied together by webs to create a stiff, predictable system.
How much do roof trusses cost in South Africa?
Costs vary widely with span, pitch, material (timber vs. steel), coating/treatment, wind zone and roof complexity. Small residential roofs and long-span commercial bays price very differently. The most accurate route is a take-off from drawings—Dezzo will provide an engineer-signed design and itemised quote for your site.
What is the difference between roof trusses and roof framing?
“Roof framing” is the broader term for all structural elements that form the roof (trusses or rafters, purlins/battens, bracing). A roof frame truss is a specific, prefabricated component within that framing—an engineered triangle (or series of triangles) that replaces piece-by-piece stick framing with a stronger, faster, more predictable unit.
How far apart should roof trusses be in South Africa?
Typical centres for residential work often fall between 600 mm and 900 mm, adjusted for covering weight, span, wind zone and the purlin/batten layout. Heavier coverings or high-suction sites may require closer centres; some metal roofs allow wider centres with deeper purlins. Final spacing must come from the engineer’s design and the relevant SANS requirements.