Steel Truss Calculator (Pratt / Howe / Fink / KingPost)
Estimate member lengths and total steel weight for common roof truss types (Pratt, Howe, Fink, KingPost) from span, pitch, and load.
Member breakdown
| member | length_m | kg |
|---|---|---|
| Bottom Chord | 8.00 | 200.00 |
| Top Chord (Rafters) | 8.25 | 206.16 |
| Diagonals | 8.25 | 206.16 |
How this works
We compute the rafter from the half-span and pitch, then assemble the per-type member breakdown. Total weight uses an average 25 kg/m blended across chord and web members.
rise = (span / 2) × (pitchPct / 100)
rafter = √((span/2)² + rise²)
Pratt / Howe (4 panels per half-span):
bottom chord = span
top chord = 2 × rafter
verticals = 8 × rise × 0.5
diagonals = 8 × √(panelLen² + rise²)
Fink:
bottom chord = span
top chord = 2 × rafter
diagonals = 4 × √((span/4)² + (rise/2)²)
KingPost:
bottom chord = span
rafters = 2 × rafter
king post = rise
struts = 2 × (rafter / 2)
totalKg = totalLength × 25 kg/mWorked example
An 8 m span Fink truss with a 25% pitch (rise = 1 m):
- rafter = √(4² + 1²) ≈ 4.12 m
- top chord = 8.25 m
- diagonals = 4 × √(2² + 0.5²) ≈ 8.25 m
- bottom chord = 8.00 m
- Total length ≈ 24.50 m → weight ≈ 612 kg
Switch the type to KingPost and the total length drops to about 16.6 m (≈ 415 kg) because the king-post layout uses fewer web members — at the cost of less efficient load distribution above 8 m span.
Sources
- Standard pitched-roof truss geometry (Pratt, Howe, Fink, KingPost)
- IS 800 — General Construction in Steel (Code of Practice)
FAQ
What is the difference between Pratt, Howe, Fink, and KingPost trusses?
All four are common pitched-roof trusses. A Pratt truss has vertical web members in compression and diagonal web members in tension under gravity load. A Howe truss is the inverse — verticals in tension, diagonals in compression — and is preferred for timber. A Fink truss has W-shaped diagonals and no verticals, giving the shortest member lengths for spans of 8 to 14 m. A KingPost truss has a single vertical king post plus two struts, suitable for short spans up to about 8 m.
How does pitch affect the result?
Pitch is the rise divided by half-span, expressed as a percentage. A 25 percent pitch (rise 1 m for half-span 4 m) gives a steeper roof and longer rafters, increasing top-chord length by roughly 3 percent for every 10 percent of additional pitch. Steeper pitches shed water and snow more easily but use slightly more steel; flatter pitches use less steel but need higher-grade waterproofing.
What weight factor is used per metre of member?
An average of 25 kg/m, blended across the heavier chord members (typically ISA 75x75x6 or ISMC 100) and the lighter web members. Real trusses might range from 18 kg/m (light shed truss with ISA 50x50x5) to 35 kg/m (long-span truss with ISMC 150 chords), so treat the figure as a planning estimate. For a precise weight, size each member with the Steel Beam / Column calculators and multiply by length.
Does the calculator size individual members?
No. It computes geometric lengths and applies a constant kg/m factor to give a total weight. Individual member sizing under the design load is a separate engineering step using IS 800 — chords work in axial compression and tension, web members work in axial loads with possible buckling. For shed and warehouse spans above 12 m, member sizing must be done by a structural engineer.
What span range does this support?
The calculator is geometrically valid for any span, but the simplified composition rules (4 panels per half-span for Pratt / Howe / Fink, single king post for KingPost) work best for the typical residential and small-shed range of 4 to 16 m. For spans above 16 m, professional truss designs use 6 to 8 panels per half-span and additional sub-bracing — the simplified model under-counts the diagonal length by 5 to 15 percent.
What is the load input used for?
The load (kN per metre of horizontal span) is echoed in the report so you have a record of the design assumption alongside the geometry. The geometric breakdown does not depend on load — that comes in only when sizing individual members. Use 1.0 to 1.5 kN/m for sheet-clad sheds without snow load and 2.0 to 3.0 kN/m for tile-clad roofs with snow load in the Himalayan foothills.