# Mono Truss Calculator

Mono truss calculator for shed roofs and lean-tos. Get height difference, top chord length, truss count, lumber estimate, and engineered truss cost across any mono truss calculator span up to 40 ft.

## What this calculates

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.

## Inputs

- **Building Span** (ft) — min 4, max 60 — Horizontal distance between the high wall and the low wall
- **Roof Pitch** (:12) — min 0.5, max 12 — Rise per 12 inches of run (2:12 to 4:12 typical for mono trusses)
- **Building Length** (ft) — min 4, max 200 — Length of the building for total truss count
- **Truss Spacing** — options: 12 in O.C., 16 in O.C., 24 in O.C. (standard), 48 in O.C. (with purlins) — 24 in is standard residential spacing
- **Eave Overhang** (in) — min 0, max 48 — Horizontal projection past each wall (12-24 in typical)
- **Engineered Truss Price per Foot of Span** ($) — min 0, max 20 — 2025 residential mono truss: $3-6 per ft of span. Use 0 to skip cost.

## Outputs

- **Height Difference (low to high wall)** (ft) — How much taller the high wall is than the low wall
- **Top Chord Length** (ft) — Sloped top member from low wall to high wall (no overhang)
- **Total Truss Length (with overhangs)** (ft) — Top chord including eave overhangs on both ends
- **Number of Trusses**
- **Total Lumber (rough estimate)** (linear ft) — Rough linear footage (top chord + bottom chord + 2 webs) for budgeting only; actual members come from the engineered truss design
- **Estimated Engineered Truss Cost** — formatted as currency — Span x price per ft x truss count
- **Member Sizing** — formatted as text — Trusses must be engineered per IRC R802.10; this calculator does not size members.

## Details

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.

## Frequently Asked Questions

**Q: What is a mono truss?**

A: A mono truss (mono-pitch truss or shed truss) is a triangular roof truss with a single sloped top chord from a high wall to a low wall. Unlike a gable truss, there is no peak in the middle. Mono trusses are used for sheds, lean-tos, carports, and porch roofs.

**Q: What is the maximum mono truss calculator span?**

A: There is no single maximum span from a lumber chart. The span a mono truss can carry comes from its engineered design, which accounts for span, spacing, pitch, and local snow and wind loads (IRC R802.10). This calculator handles geometry up to 40 ft; ask the truss manufacturer or a licensed engineer to size the chords and webs for your building.

**Q: What pitch is best for a mono truss?**

A: Most mono trusses are 2:12 to 4:12 pitch. At 1:12 you get the minimum slope for asphalt shingle drainage. At 3:12 you get a comfortable shed style. Above 4:12 the high wall gets unreasonably tall relative to the low wall and the look becomes awkward. For very low-slope membrane roofing, 1/4:12 to 1:12 is possible.

**Q: How many mono trusses do I need?**

A: Number of trusses = building length / spacing + 1, rounded up. For a 40 ft long building at 24 in (2 ft) on-center, you need 40 / 2 + 1 = 21 trusses. At 16 in OC, you need 31 trusses. Always add one gable truss at each end (two total).

**Q: How much does a mono truss cost in 2025?**

A: Engineered mono trusses cost $3-6 per linear foot of span in 2025. A 20 ft mono truss runs $60-120 each. For a 40 ft long shed at 24 in OC with 21 trusses, that is $1,260-2,520 in trusses plus $2-4 per sq ft to install. A 40 ft x 20 ft shed roof totals roughly $2,900-5,700 for trusses plus install.

**Q: Can I use a mono truss as a lean-to on an existing building?**

A: Yes. A lean-to mono truss attaches the high chord to a ledger board bolted into the existing wall framing, and the low chord bears on a new short wall (typically 6-10 ft tall). You must verify the existing wall can carry the additional load (lateral thrust plus uplift). A structural engineer should check the ledger attachment on any lean-to over 10 ft span.

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Source: https://vastcalc.com/calculators/construction/mono-truss
Category: Construction
Last updated: 2026-04-21