Selecting the correct timber truss sizes is one of the most important decisions in a roofing project. The truss is the structural skeleton that sets your roof’s shape, carries the covering, and transfers loads safely into the walls and foundations. Size it correctly and your roof looks straight, drains well, resists wind uplift, and lasts for decades. Get it wrong and you risk deflection, cracked finishes, noisy roofs, and costly remedial work.
This guide explains what timber trusses are, the structural types you will encounter, the factors that drive timber truss sizes, and a clear process for deciding what you need on a South African home or light commercial project.
What is a timber truss?
A timber truss is a prefabricated triangular frame. It uses top and bottom chords joined by internal webs to create a stiff structure that spans from wall to wall or beam to beam. Because triangles are inherently stable, trusses achieve long spans with less material than stick framing.
Modern South African trusses are factory-made from kiln-dried, strength-graded members. Joints are pressed with steel nail plates under controlled pressure. Each truss is engineered for its span, roof pitch, covering weight, wind zone, and exposure. When delivered to site they are labelled, set out by bay, and installed with a prescribed bracing and tie down schedule.
Structural types that influence timber truss sizes
Different layouts use material differently. Knowing the main types helps you understand why depth or spacing changes from one area of a roof to another.
King Post
A classic triangle with a central post. Good for short spans with simple gables. Compact depth and economical material use.
Queen Post
Two vertical posts and a horizontal straining beam. Efficient for slightly longer spans than a King Post without a large size increase.
Fink
The familiar W shaped web arrangement. The workhorse for suburban homes because it delivers excellent stiffness with modest member sizes.
Howe and Pratt
Similar to Fink but with diagonals oriented to suit specific load paths. Useful where spans increase or where girder trusses pick up intersecting roofs.
Warren
Alternating V webs without verticals. A clean force path that can be efficient for certain spans and architectural lines.
Mono Pitch (skillion)
A single slope using asymmetric geometry. Often used for modern designs and saw tooth roofscapes.
Scissor
Creates a vaulted ceiling by pitching the bottom chord upward. Needs additional depth to control deflection.
Attic
Deeper trusses that create usable room volume inside the roof. Increased depth and careful web layout are required to keep the floor and roof stiff.
Girder truss
A heavier truss that carries other trusses at right angles at hips and valleys. Member sizes and plates are larger to deal with concentrated reactions.
Each layout affects the final timber truss sizes because member forces and deflection limits change with geometry.
The sizing triangle: span, spacing, and depth
Think of truss selection as a three-sided problem where span, spacing, and depth work together.
Span
Longer spans raise bending and axial forces. Designers respond by increasing truss depth, using stronger grades, or tightening spacing.
Spacing
Centres are chosen to balance loads on purlins or battens and to match roof coverings. Closer centres reduce load per truss but increase truss count.
Depth
Deeper trusses control deflection and vibration. Depth is usually the most efficient lever for performance, though it must still fit the architecture.
A good design solves all three at once for the selected covering and the site wind zone.
What really drives timber truss sizes on a project
1) Roof covering and minimum pitch
Tiles, slate, and sheeting each have minimum pitches and weight ranges. Heavier coverings and steeper pitches usually push sizes upward. Metal sheeting is light and can allow for smaller members at the same span.
2) Wind zone and uplift
Edges and corners experience the highest suction. Truss sizes often remain the same, but tie downs, bracing, and purlin centres are strengthened. In very high wind areas, spacing may be reduced and web layouts adjusted.
3) Deflection limits
Roofs that deflect too much develop visible waviness under sheeting and cracked plaster at ceilings. Engineers set deflection limits relative to span. When the limit is tight, truss depth or member sizes increase.
4) Load combinations
In addition to self weight, roofs carry live load from maintenance, occasional hail, and environmental actions. Where solar arrays or walkways are planned, local loads increase and member sizes or layouts may change.
5) Timber grade and treatment
Higher strength grades allow smaller members for the same span. Treatment level does not directly change capacity but affects detailing at plates and cut ends. Always re treat cut ends on site.
6) Openings and penetrations
Skylights, dormers, and chimneys introduce discontinuities. Girder trusses and trimmed openings concentrate loads, which can drive local size increases.
7) Aesthetic or functional requirements
Vaulted interiors, attic rooms, or feature rafters demand specific geometries. These choices often require additional depth to keep performance in check.
How to determine the right timber truss sizes
Step 1: Fix the brief
Confirm span between supports, roof pitch, plan shape, and the covering. Note any skylights, PV arrays, or walkways.
Step 2: Establish the site data
Identify wind zone, exposure class, and environmental constraints such as coastal air.
Step 3: Select a structural type
Choose Fink or Howe for standard spans, Scissor for vaulted spaces, or Attic where room volume is required. Pick a layout that fits the architecture first, then optimise.
Step 4: Choose preliminary spacing
Match truss centres to the covering and the purlin or batten design. Many residential roofs start at 600 to 900 mm centres and adjust from there.
Step 5: Run the engineering
Your truss designer will calculate member forces, plate capacities, and deflection. Expect to see depth or web patterns change slightly as the model is refined.
Step 6: Detail edges and bracing
Perimeter zones get denser fixings, strap hold downs, and specific bracing. These details protect the roof in high winds and are part of the final sizing outcome.
Step 7: Finalise documentation
Accept the layout drawings, bracing plan, and fixing schedule. These define the approved timber truss sizes and are essential for municipal approvals and the A19 completion certificate.
Advantages of timber trusses
Efficient strength to weight
Triangulation places wood where it is most efficient, allowing long spans with low mass.
Thermal and acoustic comfort
Timber conducts less heat than steel and helps dampen rain noise, which improves comfort under metal sheeting.
Speed and predictability
Prefabricated trusses arrive labelled and ready to install. Earlier dry in reduces weather risk and prelim costs.
Design flexibility
There is a truss for almost any roof shape. Vaulted ceilings, room-in-roof, and complex hips and valleys are all achievable.
Sustainability
Responsibly sourced timber is renewable. Factory fabrication reduces waste, and offcuts are easier to reuse than mixed site scrap.
Common pitfalls to avoid
Assuming one size suits all
Changing from metal sheeting to concrete tiles without revisiting design can overload members. Always recheck timber truss sizes when the covering changes.
Ignoring edge and corner zones
Skipping the denser fixing and bracing schedule at perimeters is a leading cause of wind damage.
Underestimating deflection
Shallow trusses might pass strength checks but still deflect too much. Insist on serviceability checks.
Late penetrations
Cutting for skylights or vent stacks after fabrication weakens load paths. Coordinate all openings before the trusses are made.
Skipping treatment of cut ends
Untreated cuts invite decay and reduce capacity over time. Re treat every cut.
How Dezzo Roofing helps you choose correctly
We design the roof as a system, not as isolated pieces. Our engineers model spans, wind zones, coverings, and penetrations, then propose the most efficient timber truss sizes and layouts for each area of the roof. We coordinate battens or purlins, underlays, insulation, flashings, and gutters so every decision supports the next. Trusses are manufactured to tight tolerances, delivered labelled by bay, and supported with a fixing schedule, QA checklist, and certificate pack for smooth approvals.
Conclusion
The right timber truss sizes are the outcome of good data, sound engineering, and careful coordination. Start with your roof shape and covering, confirm the site conditions, and let the calculations set depth and spacing. With Dezzo Roofing, you get a system design, precision manufacture, and a clean documentation trail, which means straighter lines, quieter interiors, and a roof that stays strong through South Africa’s toughest seasons.
FAQs
What size are standard timber trusses?
There is no single standard. In South African housing, truss centres are often 600 to 900 mm, with member depths selected to suit span, pitch, covering, and wind zone. A typical Fink truss for a modest span might use 38 mm thick members with depths set by engineering.
What are standard truss sizes?
“Standard” refers more to common configurations than fixed sizes. King Post and Fink trusses are common for short to moderate spans. Howe, Pratt, Scissor, Attic, and Girder trusses are used as spans increase or where special geometry is required. Actual sizes follow from calculations on your drawings.
What are standard timber sizes in South Africa?
Common structural sizes include 38 mm thick members in depths such as 76, 114, 152, 228 mm and up, along with larger sections for girders or special spans. Availability varies by supplier. Your truss design will select from available sections that meet strength and deflection limits.
Are trusses 2×4 or 2×6?
Both. Many residential trusses use members similar to 2×4 in thickness and depth equivalents, while longer spans or high loads may need 2×6 equivalents or larger. The engineer chooses member sizes based on span, spacing, covering weight, and wind zone.