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The basics
What is an Attic Truss?
Attic trusses are the cheapest habitable floor area you can build, provided the span and pitch cooperate.
An ordinary roof truss bottom chord is designed for a ceiling and nothing else. An attic truss bottom chord is a floor, designed for real live load and held to floor deflection limits (opens in a new tab) rather than ceiling ones.
That is why attic trusses cost 60 to 80% more than a standard truss of the same span, and why you cannot convert a standard truss roof into an attic room after the fact. The trusses were never designed to carry it, and the webs run straight through the space you want to occupy.
- Span under 24 ft Storage only, realistically. Not enough width remains once the sloped ceilings take their share.
- Span 26 to 30 ft A real room at 10:12 or steeper. This is the sweet spot for most houses.
- Span over 32 ft Generous rooms, but the bottom chord is now a serious floor member and the truss gets heavy.
- Pitch is the cheap lever Every step up in pitch buys width at the headroom you care about, for a modest increase in material.
Access and egress come first
A habitable attic room needs a permanent staircase rather than a loft ladder, and usually an egress window or dormer. Plan both before the trusses are ordered, since a dormer opening needs a girder and headers designed into the layout.
The maths
Attic Truss Formula
Attic room geometry
Subtract the floor build-up and both chord depths from the headroom figure to get finished ceiling height.
Method
How to Calculate an Attic Truss
An attic truss is calculated backwards from the room: fix the headroom you need, then see what span and pitch deliver it.
- 1 Enter span and pitch These two set the ceiling of what is possible. Under 26 ft of span you need a steep pitch to get a real room.
- 2 Set the room width The distance between the two vertical posts. Wider rooms leave less web space and need heavier chords.
- 3 Set the room height 7 ft 6 in is the usual target. Many codes require 7 ft over at least half the floor area for a habitable room, but confirm locally.
- 4 Check the floor load An attic room floor needs real live load designed into the bottom chord. Confirm this before you order.
There is a hard geometric floor here. To get 7 ft 6 in of headroom you need at least that much rise between the bottom chord and the ceiling chord, plus the depth of both chords, plus the floor build-up.
The lever that costs almost nothing is pitch. Going from 8:12 to 12:12 on a 28 ft span takes the usable room from about 8 ft wide to about 14 ft wide. Same footprint, same walls, roughly a 20% heavier truss, nearly double the room.
Worked example
A 28 ft span at 10:12 gives a 14 ft run and an 11 ft 8 in total rise. Targeting 7 ft 6 in of headroom leaves 4 ft 2 in of rise to spend on width, which works out to roughly 10 ft of full-height room between the posts. Floor loading is a design question, not a table lookup.
Approximate room width achievable with 7 ft 6 in of headroom, using standard attic truss proportions.
| Span | 8:12 | 10:12 | 12:12 |
|---|---|---|---|
| 24 ft | No usable room | 7 ft wide | 10 ft wide |
| 26 ft | 6 ft wide | 9 ft wide | 12 ft wide |
| 28 ft | 8 ft wide | 11 ft wide | 14 ft wide |
| 32 ft | 12 ft wide | 15 ft wide | 18 ft wide |
| 36 ft | 16 ft wide | 19 ft wide | 22 ft wide |
Note: Room width is measured between the vertical posts. Sloped ceiling areas outside the posts add floor area but not full headroom.
The tool
Why Choose Our Attic Truss Calculator
The only question that matters for an attic truss is whether you get a usable room, so this calculator answers that first.
- Room width and headroom Set the room you want and see whether your span and pitch can actually deliver it.
- Posts drawn in place The two vertical posts and the ceiling chord appear on the drawing, so the room is visible rather than implied.
- Pitch as the cheap lever Step the pitch up and watch the usable width grow for a modest increase in material.
- Floor load flagged The tool is explicit that an attic bottom chord is a designed floor, not a standard ceiling chord.
More tools
Explore Related Truss Calculators
Calculators that pair with the attic tool, each running the same geometry engine.
- Gambrel Roof Truss Calculator Two slopes, one cut list. Works for 12 ft, 16 ft, 24 ft and 40 ft gambrel spans. Open calculator
- Scissor Roof Truss Calculator Dual-pitch geometry with vault gain, clearance heights and deflection advisories. Open calculator
- Roof Truss Load Calculator Snow, dead and live load combined into per-truss forces and bearing reactions. Open calculator
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Answers
Frequently Asked Questions
Common questions about attic roof truss calculator.
What span do I need for an attic truss room?
26 ft is the practical minimum for a genuinely usable room, and 28 to 32 ft is where attic trusses really work. Below 26 ft you need a very steep pitch and still end up with a narrow space better suited to storage.
What pitch is needed for an attic truss?
8:12 is the minimum that produces any usable room, 10:12 is the common choice, and 12:12 gives generous rooms. Every step up in pitch buys width at the ceiling height you care about.
How much weight can an attic truss hold?
It depends entirely on what was specified. A storage-rated attic truss and a habitable-room truss carry different live loads, and neither figure applies unless it was designed in before fabrication. The truss drawing is the only reliable source.
Are attic trusses more expensive?
Yes, typically 60 to 80% more than a standard truss of the same span, because the bottom chord is a designed floor member and the whole truss carries far more load. Compared with building the same floor area at ground level, it is still inexpensive space.
Can I convert existing trusses into an attic room?
No, not without a full structural redesign. Standard trusses have web members running straight through the space you want to occupy, and cutting any of them destroys the truss. Retrofitting means removing the roof structure and starting again.
How wide is the room in an attic truss?
Roughly 40 to 50% of the span at a 10:12 pitch. A 28 ft span at 10:12 gives about 11 ft of full-height width, with sloped ceiling areas either side adding floor area at reduced headroom.
References
Attic Truss Sources & Standards
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 Attic, Wikipedia (opens in a new tab)
- WIKI Truss, Wikipedia (opens in a new tab)
- STD ANSI/TPI 1, Truss Plate Institute (opens in a new tab)
- STD American Wood Council, NDS and span tables (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.