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 Member Sizing
This calculator does not size chords or webs. Wood trusses must be engineered: under IRC R802.10 they are built to truss design drawings prepared for your span, spacing, pitch, and local snow and wind loads. The IRC R802.5 rafter span tables do not apply to trusses. Send your span, pitch, spacing, and overhang to a truss manufacturer or a licensed engineer and build from their sealed 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: common for modern shed-style buildings, but too flat for asphalt shingles (IRC R905.2.2 requires at least 2:12), so use metal panels or a low-slope membrane
- 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: make sure the truss design drawings cover those loads
- 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.