Roof Truss Calculator

Roof Truss Calculator: How to Design Your Truss in 2026

A roof truss calculator takes your building span, roof pitch, and truss type, then returns the peak height, member lengths, and the number of trusses you need for your structure. Roof trusses have replaced traditional rafter framing on the vast majority of new residential construction because they are engineered, factory-built, and significantly faster to install on site — a typical home roof can be “set” (trusses craned into place) in a single day versus a week or more of stick-framing.

This advanced truss calculator supports six of the most common residential truss profiles: common (king/fink), attic, scissor, fink, king post, and shed/mono trusses. Each truss type solves a different design problem, from maximizing attic storage to creating a vaulted cathedral ceiling, and each carries different span limitations and lumber requirements.

Roof Truss Design: Understanding the 6 Truss Types

Choosing the right truss profile is the first and most important decision in any roof truss design. Use the truss type selector above as a free attic truss calculator, scissor truss calculator, fink truss calculator, or king post truss calculator — just pick the matching card and every result below updates for that profile. The breakdown below covers when each type is used, sourced from current residential framing standards and truss manufacturer association (SBCA/WTCA) guidance.

Common (King Post) Truss

The simplest truss profile: a single vertical king post running from the peak down to the center of the bottom chord, with two diagonal web members bracing each side. Best suited to short-to-medium spans up to about 24-28 feet. Extremely economical because it uses the fewest lumber pieces of any triangulated truss design — select “King Post” above to use this tool as a dedicated king post truss calculator.

Attic Truss (Room-in-Roof Truss)

An attic truss design raises the bottom chord partway up and adds vertical webs to frame a usable rectangular room inside the roof cavity — essentially building a room and a roof structure in one engineered piece. Run the numbers through the attic truss design calculator above (select “Attic” as your truss type) to see your projected attic truss room size before you order. The attic truss room size you get depends directly on the span, pitch, and how wide you set the vertical webs. Attic trusses require deeper top chords (often 2×6 or 2×8) and heavier connector plates than a standard truss because they carry both roof load and floor load for the room below.

Scissor Truss

A scissor truss design uses a shallower-pitched bottom chord than the top chord, creating a sloped (vaulted) ceiling instead of a flat one. Popular for great rooms, primary bedrooms, and any space where a cathedral ceiling is desired without full stick-framing. Our scissor truss design calculator mode computes the reduced bottom-chord pitch and the resulting rise differential automatically once you select “Scissor” above. The “scissor” angle (the difference between top and bottom chord pitch) typically ranges from 1/12 to 4/12 less than the roof pitch — a steeper differential creates a more dramatic vault but increases lumber and connector plate costs.

Fink Truss (W-Truss)

The most widely used residential truss in North America. The web members form a W-shape between the top and bottom chords, which is why it is also called a W-truss. Fink trusses are the most material-efficient design for typical home spans (20-36 feet) and are the default truss type most manufacturers quote unless you specifically request an alternative profile.

Shed/Mono Truss

A shed roof truss design calculator handles this profile differently than the others above, since a shed (mono) truss has only one sloped top chord running the full span with no ridge — it is essentially half of a common truss. Used for lean-to additions, porch roofs, and modern single-slope architectural roofs. Because the entire run carries the rise instead of just the half-span, shed trusses generally need deeper lumber for the same pitch and span compared to a symmetrical truss.

Shed / Mono-Pitch Truss

A single-slope truss with no ridge — one top chord runs from a high point down to a low point. Common on home additions, lean-to structures, porches, and modern single-slope roof designs. Shed trusses are calculated differently from symmetrical trusses because the rise is measured across the full run rather than half the span.

Roof Truss Span Calculator: How Span Limits Work

Span is the single biggest factor in truss design. The “span” is the horizontal distance the truss must bridge, measured wall-to-wall (not including overhangs). Every truss type, lumber grade, and spacing combination has a maximum allowable span before deflection, chord stress, or web buckling becomes a structural problem. A roof truss span calculator like this one estimates geometry (height, member lengths) for your span, but final span/load capacity is always verified by a truss engineer using software certified to ANSI/TPI 1.

* Spans shown are typical planning ranges for standard residential loads (20-30 psf live load). Actual allowable span always depends on lumber grade (No. 2 SPF vs SYP), spacing, snow load zone, and engineered plate design — always confirm with a stamped truss drawing.

Roof Truss Height and Spacing Explained

Two numbers drive almost every downstream decision in a truss order: height and spacing. Get familiar with both before you call a truss plant for a quote.

Using a Truss Height Calculator

Truss height (also called total rise) is the vertical distance from the top of the bottom chord to the peak of the top chord. It is calculated from half the span and the pitch using the same rise-over-run math as standard roof framing: height equals half the span multiplied by the pitch ratio (rise divided by 12). A wider span or steeper pitch both increase the truss height, which directly affects whether your design clears local height restrictions, gable-end window placement, and the size of the gable vent.

Roof Truss Spacing: 16″ vs 19.2″ vs 24″ On-Center

Roof truss spacing determines how many trusses you need for a given building length and directly affects both material cost and roof sheathing span ratings. The vast majority of residential roofs use 24-inch on-center spacing because it minimizes the truss count while still meeting standard sheathing span ratings (most 7/16″ OSB is rated for 24″ o.c. roof spans). Heavier roof coverings like clay tile, slate, or high snow-load regions often require tighter 16-inch spacing to keep deflection within code limits.

Build Your Own Trusses: What This Calculator Tells You

While the vast majority of residential trusses are factory-built and delivered pre-assembled, some owner-builders, barn projects, and small outbuildings use a build your own trusses calculator approach to frame trusses on the ground and lift them into place. If you are framing your own trusses, this calculator’s lumber cut list gives you a starting bill of materials — but you still need an engineer or truss table from your local building code (most jurisdictions reference span tables in the IRC for site-built trusses) to confirm member sizing for your specific load.

Residential Roof Trusses: Why They Replaced Rafters

Residential roof trusses dominate new home construction for several measurable reasons. They are engineered as a triangulated system, which distributes load far more efficiently than a stick-framed rafter and ceiling joist combination, allowing for smaller lumber sizes (commonly 2×4) across longer spans than solid-sawn rafters could achieve. They are factory-built under quality control, arrive ready to set, and a typical residential roof can be fully trussed in a single day with a crane crew, compared to a week or more of on-site rafter cutting and ceiling joist installation.

Roof Truss Design: Anatomy of a Truss

Every truss, regardless of profile, is built from the same three categories of structural members connected at engineered joints. Understanding this anatomy makes it much easier to read a truss drawing or discuss design changes with a manufacturer.

• Top chord: The sloped outer members that form the roof surface line and carry roof sheathing, shingles, and live loads like snow or workers.
• Bottom chord: The horizontal member that ties the two bottom ends of the top chord together and typically supports the ceiling drywall below.
• Web members: The interior diagonal and vertical pieces that triangulate the truss, transferring load from the top chord down to the bearing points at the bottom chord ends.
• Connector plates: Galvanized steel gusset plates pressed into both faces of the lumber at every joint, engineered to transfer tension and compression forces between members without requiring nails or bolts.
• Bearing points: The locations (usually at the bottom chord ends) where the truss rests on and transfers load into the building’s top wall plate.

Related Roofing & Framing Calculators

Use these companion tools from roofpitch.net to plan the rest of your roof project, from pitch and sheathing through final material cost.

Frequently Asked Questions

Sources & Data

• American National Standards Institute / Truss Plate Institute (ANSI/TPI 1) – National Design Standard for Metal Plate Connected Wood Truss Construction
• Structural Building Components Association (SBCA) – Residential Truss Design Guidance, 2026
• International Residential Code (IRC) – Span Tables for Site-Built Roof Framing