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The basics
What is a Flat Truss?
The parallel-chord truss is what makes a flat roof structurally sensible, and the same shape turns up under floors for exactly the same reasons.
Open webs are the reason it dominates commercial low-slope roofs and modern basements. Ducts, plumbing and wiring pass straight through, which no solid joist (opens in a new tab) allows without drilling and losing capacity.
The same geometry is used as a floor truss. What changes is the load case: occupancy live load rather than roof loads, and a much tighter deflection limit, because people notice a bouncy floor long before it becomes unsafe.
- Services pass through Open web space takes full-size ductwork without notching or drilling anything structural.
- Long spans for the depth With 2x6 chords and 36 to 48 in of depth these routinely span 50 to 60 ft.
- Flat ceiling, sloping roof Sloping the top chord while keeping the bottom chord level gives drainage and a level ceiling in one truss.
- Ponding is the real risk Depth, positive slope and drains that actually drain are the three defences. Skimping on depth is how flat roofs fail.
Build the slope into the truss
Sloping the top chord about a quarter inch per foot while keeping the bottom chord dead level gives you a flat ceiling and a draining roof in one component, far cheaper than tapered insulation over a level deck.
The maths
Flat Truss Formula
Flat truss geometry
Doubling the depth roughly quadruples bending capacity for the same chord lumber. Nothing else you can change has that leverage.
Method
How to Calculate a Flat Truss
A parallel-chord truss is sized on depth rather than pitch, so the calculation starts from a depth-to-span ratio and works outward.
- 1 Set the span Bearing to bearing. Flat trusses routinely outspan solid joists by a factor of two for the same depth.
- 2 Set the depth This is the vertical distance between the chords. Start at span divided by 20 and adjust from there.
- 3 Add a drainage slope A truly flat roof ponds. Even a quarter inch per foot built into the truss gives positive drainage without tapered insulation.
- 4 Check the ratio The calculator reports depth-to-span. Below roughly 1:24 you are in deflection trouble regardless of what the strength numbers say.
A parallel-chord truss carries load as a couple: the top chord in compression, the bottom in tension, and the lever arm between them is the depth. That is why depth is the variable worth spending on.
In practice deflection, not strength (opens in a new tab), sets the depth on a flat roof. A truss can be strong enough and still visibly sag, and on a flat roof a sag becomes a puddle, a puddle becomes more load, and more load becomes more sag. That feedback is called ponding instability and it is the specific failure mode flat roofs are known for.
Worked example
A 24 ft span starting at span over 20 gives a 14.4 in depth, which rounds up to a 16 in truss in practice. Sloping the top chord a quarter inch per foot across that span produces 6 in of fall while the bottom chord stays level. Confirm the depth against your actual load and deflection limit.
Practical starting depths for parallel-chord roof trusses under standard residential loading.
| Span | Minimum depth | Comfortable depth | Depth : span |
|---|---|---|---|
| 16 ft | 12 in | 16 in | 1:16 to 1:12 |
| 20 ft | 14 in | 20 in | 1:17 to 1:12 |
| 24 ft | 16 in | 24 in | 1:18 to 1:12 |
| 30 ft | 20 in | 30 in | 1:18 to 1:12 |
| 40 ft | 28 in | 40 in | 1:17 to 1:12 |
| 50 ft | 36 in | 50 in | 1:17 to 1:12 |
Note: Minimum depths assume light roofing and no ponding. Add depth for heavy membranes, ballast or rooftop mechanical loads.
The tool
Why Choose Our Flat Truss Calculator
Flat roofs are sized on depth, not pitch, so this calculator is built around the depth-to-span ratio rather than a pitch box.
- Depth as the main input Set the structural depth directly and see the ratio the design will be judged on.
- Drainage fall calculated A small top-chord slope over a level bottom chord, with the actual fall in inches.
- Ratio flagged Shallow depth-to-span combinations are called out, because deflection governs long before strength does.
- Works for floor trusses too The same parallel-chord geometry, with your own load figures entered in the advanced panel.
More tools
Explore Related Truss Calculators
Calculators that pair with the flat tool, each running the same geometry engine.
- Roof Truss Span Calculator Check any span against chord size, species, pitch and load in one pass. Open calculator
- Roof Truss Load Calculator Snow, dead and live load combined into per-truss forces and bearing reactions. Open calculator
- Mono Roof Truss Calculator Single-slope geometry, wall height difference and full take-off, lean-to ready. Open calculator
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Answers
Frequently Asked Questions
Common questions about flat roof truss calculator.
How far can a flat roof truss span?
With 2x4 chords and a 16 in depth, roughly 20 ft. At 24 in depth, around 30 ft. Parallel-chord trusses with 2x6 chords and 36 to 48 in of depth routinely span 50 to 60 ft, which is why they dominate commercial low-slope roofs.
How deep should a flat roof truss be?
Start at span divided by 20 and go deeper if the space allows. Depth is far cheaper than heavier chords: more depth costs a few inches of web lumber, while a bigger chord costs across every lineal foot of the truss.
Is a flat roof truss really flat?
Almost never. Codes require a minimum fall on any roof, so a well-designed flat truss has a slightly sloped top chord and a level bottom chord. The result reads as flat and still drains.
What is the difference between a flat truss and a floor truss?
Geometrically nothing, both are parallel-chord trusses. The difference is the design loads and the deflection limit. A floor truss is designed for occupancy live load at a tighter deflection limit than a roof truss.
What causes flat roofs to fail?
Ponding, overwhelmingly. Water collects in a low spot, the extra weight deflects the structure further, which deepens the low spot. Adequate depth, positive slope and working drains are the three defences.
Can I put a rooftop unit on a flat truss roof?
Only where the truss was designed for that point load. Mechanical units concentrate weight, and they also create snow drift on the lee side. Both need designing in before the trusses are made.
References
Flat Roof Truss References
Related calculators on this site, plus the standards and reference works the figures on this page are based on. External links open in a new tab.
Authoritative external references
- WIKI Flat roof, Wikipedia (opens in a new tab)
- WIKI Truss, Wikipedia (opens in a new tab)
- STD American Wood Council, NDS and span tables (opens in a new tab)
- ORG SBCA, truss handling and bracing guidance (opens in a new tab)
- CODE International Code Council, model building codes (opens in a new tab)
Codes and standards are revised on their own cycles. Confirm the current edition adopted in your jurisdiction with your local building department before relying on any figure.