Mono Truss Calculator
A mono truss (also called a mono-pitch truss or shed truss) has one sloped top chord from a tall wall to a short wall, with no ridge peak. It is the standard truss for lean-tos, sheds, carports, and porch additions. This mono truss calculator returns height difference, top chord length, truss count, and engineered truss cost for any mono truss calculator span up to 40 ft.
How a Mono Truss Differs from a Gable Truss
A gable (common) truss has two sloped chords that meet at a ridge. A mono truss has only one sloped chord running from a high wall to a low wall. Use a mono truss when:
- You are adding a lean-to or shed off an existing wall
- You want a flat-looking roof with minimum pitch (shed / modern style)
- You are building a carport, porch, or pavilion tied to a main building
- You need clear ceiling span with minimal headroom at the low side
Mono Truss Calculator Span Guidance
A mono truss calculator span check confirms whether standard lumber works:
| Span | Top Chord | Pitch Range | Typical Use |
|---|---|---|---|
| Under 12 ft | 2x4 | 2:12 - 4:12 | Garden shed, small lean-to |
| 12-20 ft | 2x4 or 2x6 | 2:12 - 4:12 | Carport, porch roof, standard shed |
| 20-30 ft | 2x6 | 2:12 - 3:12 | Large shed, workshop, pole barn addition |
| 30-40 ft | 2x8 or 2x10 | 1:12 - 2:12 | Equipment bay, commercial shed |
| Over 40 ft | Engineered only | varies | Commercial / sealed drawing required |
Always verify with local code (IRC R802.5 span tables or WFCM) and truss plant drawings.
Mono Truss Geometry
The math is simpler than a gable truss:
- Height difference (rise across the span) = span * (pitch / 12)
- Top chord length = sqrt(span^2 + height^2)
- Truss count = building length / spacing + 1 (round up)
Example: a 20 ft span with 3:12 pitch:
- Height difference = 20 * (3/12) = 5 ft
- Top chord = sqrt(20^2 + 5^2) = sqrt(425) = 20.62 ft
For a 40 ft long building at 24 in on-center spacing: 40 / 2 + 1 = 21 trusses.
Mono Truss Pitch Recommendations
Mono trusses work best at low pitches (1:12 to 4:12):
- 1:12 to 2:12: minimum slope for shingle drainage; common for modern shed-style buildings
- 2:12 to 3:12: standard for residential lean-tos and carports
- 3:12 to 4:12: typical for porches and covered patios with more headroom at the low side
- Over 4:12: uncommon; the height difference gets awkward and the high wall gets very tall
Mono Truss Cost (2025)
Engineered mono trusses run $3-6 per linear foot of span in 2025 (Builders FirstSource, 84 Lumber, local truss plants). A 20 ft mono truss costs about $60-120 each delivered. For a 40 ft long shed at 24 in OC with 21 trusses: $1,260-2,520 in trusses, plus labor to install ($2-4 per sq ft of roof).
Lean-To vs Freestanding Mono Truss
A lean-to mono truss bears on one existing wall plus one new low wall. The high chord attaches to a ledger board on the existing wall. A freestanding mono truss bears on two new walls (tall post wall + short wall) and is common for detached sheds and carports.
When Not to Use a Mono Truss
- Span over 40 ft: use engineered parallel-chord or scissor truss
- Heavy snow or tile roof at over 3:12 pitch: engineered only
- Structural uncertainty: mono trusses put lateral thrust on the high wall; tall high walls may need a beam or bracing
Mono Truss vs Gable Truss Cost
A mono truss uses roughly 60-70% of the lumber of a gable truss covering the same span (one chord plus simple webs vs two chords plus peak plates). Expect 20-30% less in engineered truss cost per truss when going mono.