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The basics
What is a Mono Truss?
Mono trusses solve one problem no other shape can: adding a roof to an existing wall without touching the existing roof.
They are also the cheapest truss per square foot of roof you can buy. One top chord instead of two, no peak joint, and a simple N-pattern web. For a carport, a storage lean-to or a covered walkway, nothing beats it on price.
The catch is stability. A mono roof has no opposing slope to brace against, so permanent lateral web bracing (opens in a new tab) and properly sheathed end walls are not optional. They are what stops the whole roof racking.
- Lean-to additions High end on a ledger bolted to the existing wall, low end on a new post-and-beam or stud wall.
- Carports and canopies Both ends on posts. Lateral bracing needs designing, since four posts alone give no rigidity.
- Clerestory glazing Two mono roofs sloping away from a raised centre wall let you glaze the height difference between them.
- Shorter practical span Less structural depth than a gable of the same width, so a mono always spans less for the same lumber.
Bracing matters more here
A mono truss roof has no opposing slope to lean against. Permanent lateral web bracing and a properly sheathed end wall are part of the structure, not an optional extra.
The maths
Mono Truss Formula
Mono truss geometry
Because the run is the full span, a mono truss gains height twice as fast per degree of pitch as a gable of the same width.
Method
How to Calculate a Mono Truss
A mono truss uses the full span as its run, not half, which is the single most common mistake people make with this shape.
- 1 Span the opening For a mono truss, the span is the full horizontal distance from the low wall to the high wall, not half of it.
- 2 Pick the pitch The pitch sets how much taller the high wall must be. This is the number that decides whether your lean-to clears the existing windows.
- 3 Check the height difference Height difference equals span times pitch divided by 12. On a 16 ft mono at 3:12 that is exactly 4 ft of extra wall.
- 4 Confirm the low end Mono trusses fail their headroom check at the low end more often than anywhere else. Check the finished ceiling clearance there first.
Because a mono roof drains in a single direction, the entire roof's water arrives at one gutter. On a 24 ft by 40 ft mono roof that is 960 ft² feeding one gutter line, so size it and its downspouts (opens in a new tab) for the full area rather than half as you would on a gable.
The other trap is pitch. Mono trusses tempt you toward very low pitches because every degree of pitch costs wall height. Below 3:12 you are out of asphalt shingle (opens in a new tab) territory and into membrane or standing-seam metal, which changes both cost and detailing.
Worked example
A 16 ft mono at 3:12 raises the high wall exactly 4 ft above the low wall and produces a 16 ft 5⅞ in top chord. Add the heel depth at the low end to get the true wall-to-wall difference, and size the gutter for the full roof area rather than half.
How much taller the high wall has to be, by span and pitch. This is the number that stops most lean-to plans.
| Span | 2:12 | 3:12 | 4:12 | 6:12 |
|---|---|---|---|---|
| 8 ft | 1' 4" | 2' 0" | 2' 8" | 4' 0" |
| 12 ft | 2' 0" | 3' 0" | 4' 0" | 6' 0" |
| 16 ft | 2' 8" | 4' 0" | 5' 4" | 8' 0" |
| 20 ft | 3' 4" | 5' 0" | 6' 8" | 10' 0" |
| 24 ft | 4' 0" | 6' 0" | 8' 0" | 12' 0" |
Note: Add the heel height at the low end and the truss depth at the high end to get the true wall-to-wall difference.
The tool
Why Choose Our Mono Truss Calculator
The single most common mono truss mistake is halving the span. This calculator uses the full span as the run, which is what a single slope actually does.
- Full span as the run No hidden halving, so the height difference it reports is the height difference you will build.
- Wall height difference up front The number that decides whether a lean-to clears the existing windows, reported directly.
- Both end heights Low end and high end, with the heel depth included, so the headroom check is easy.
- Roof area for drainage A mono roof drains one way, so the full area figure is there for sizing the gutter.
More tools
Explore Related Truss Calculators
Calculators that pair with the mono tool, each running the same geometry engine.
- Shed Roof Truss Design Calculator Built for 8 to 24 ft shed spans, with overhang and gusset take-off included. Open calculator
- Flat Roof Truss Calculator Parallel-chord depth, drainage fall and load capacity for low-slope roofs. Open calculator
- Roof Truss Angle Calculator All three notations plus cut angles and the slope factor, live. Open calculator
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Answers
Frequently Asked Questions
Common questions about mono roof truss calculator.
What is a mono truss?
A mono truss is a single-slope truss: one top chord, one bottom chord, and webs between them, with the high end and low end at different heights. It is also called a shed truss, single-slope truss or lean-to truss depending on the region and application.
How far can a mono truss span?
A 2x6 mono truss comfortably spans 16 to 20 ft. With 2x8 chords and reasonable depth at the high end you can reach 30 ft. Because a mono truss has less structural depth than a gable of the same span, it always spans less for the same lumber.
What is the minimum pitch for a mono truss?
Structurally you can go very shallow, but roofing dictates the practical floor. 3:12 is the minimum for asphalt shingles, 2:12 with a full underlayment for metal, and much lower for a fully adhered membrane. Confirm with the roofing manufacturer.
Is a mono truss the same as a lean-to truss?
Yes. Lean-to describes the application, a roof leaning against an existing structure, while mono describes the shape. The same truss is sold under both names, along with single-slope and shed truss.
How do I calculate mono truss height?
Multiply the span by the pitch and divide by 12. A 20 ft mono truss at 4:12 rises 6 ft 8 in from the low end to the high end, before you add heel height at the low end.
Can mono trusses be used back to back?
Yes. Two mono trusses meeting at a raised ridge beam is a common way to build a gable-like roof where the two slopes need different pitches or spans. It also opens the option of clerestory glazing between them.
References
Mono Truss References & Further Reading
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 Roof pitch, 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.