# Attic Truss Calculator

Attic truss calculator for roofs with usable living space. Enter span, pitch, and knee wall height to get room width, headroom, rafter length, and lumber count.

## What this calculates

An attic truss (also called an attic storage truss or a room-in-attic truss) is engineered to create a usable room inside the roof structure, not just dead storage above a flat ceiling. This attic truss calculator takes your building span, roof pitch, and knee wall height and returns the clear room width, peak headroom, rafter length, and total lumber needed. Use it to sanity check a design before ordering from a truss plant.

## Inputs

- **Building Span** (ft) — min 18, max 60 — Exterior wall to exterior wall. 24-32 ft is typical for usable attic rooms.
- **Roof Pitch** (:12) — min 6, max 18 — Attic trusses need 8:12 minimum, 10:12 or 12:12 preferred for usable headroom
- **Knee Wall Height** (ft) — min 3, max 8 — Height of vertical side wall inside the attic room. 4-5 ft typical.
- **Building Length** (ft) — min 8, max 200 — Length of the building for total truss count
- **Truss Spacing** (in) — min 16, max 24 — Attic trusses usually at 24 in on-center
- **Eave Overhang** (in) — min 0, max 36 — Rafter projection past the wall

## Outputs

- **Peak Height** (ft) — Height from top plate to peak of truss
- **Attic Room Width** (ft) — Clear width between knee walls (usable floor width)
- **Headroom at Peak** (ft) — Peak height above knee wall top
- **Rafter Length (with overhang)** (ft)
- **Number of Trusses**
- **Usable Floor Area** (sq ft) — Attic room width x building length
- **Total Lumber** (linear ft) — Rough estimate across all trusses

## Details

## How an Attic Truss Is Different

A conventional roof truss has a triangular interior filled with webs and is not walkable. An **attic truss** replaces that web pattern with a rectangular opening: vertical knee walls on each side, a flat ceiling across the top, and sloped cathedral sections above the knee walls. The result is a room inside the roof.

Three inputs drive the geometry:

1. **Span** (building width): sets the maximum possible room width
2. **Roof pitch** (rise:run): steeper pitch = more headroom and narrower room
3. **Knee wall height** (interior vertical wall): sets how tall the side walls are inside the attic

## Attic Room Width Formula

The usable attic floor width between the knee walls is:

**Attic Room Width = Span - 2 x (Knee Wall Height / Pitch Ratio)**

Where Pitch Ratio = pitch / 12 (so an 8:12 pitch is 0.667, a 12:12 pitch is 1.0).

Example: a 28 ft span with 10:12 pitch and 5 ft knee walls:

- Inset from each exterior wall = 5 / (10/12) = 6 ft
- Attic room width = 28 - 2(6) = 16 ft

## Headroom at the Peak

**Peak Height = (Span / 2) x (Pitch / 12)**
**Headroom = Peak Height - Knee Wall Height**

For the same 28 ft span, 10:12 pitch, 5 ft knee wall:

- Peak height = 14 x (10/12) = 11.67 ft
- Headroom = 11.67 - 5 = 6.67 ft at the peak

US building codes (IRC) require **at least 7 ft ceiling height** over at least 50% of the usable floor area for a habitable room. Conditioned space also requires at least **7 ft clear at 5 ft off the floor line**, which means the knee wall + pitch combination must give you that clearance partway in from each knee wall, not just at the peak.

## Common Design Rules of Thumb

- **24 ft span, 8:12 pitch, 4 ft knee walls:** marginal, tight headroom, storage only
- **28 ft span, 10:12 pitch, 5 ft knee walls:** typical for a second-story living space
- **32 ft span, 12:12 pitch, 5 ft knee walls:** spacious, comfortable, higher material cost
- **Below 8:12 pitch:** not enough headroom unless span is very wide

## Lumber and Truss Count

For a 40 ft long building at 24 in on-center spacing, you need 21 trusses (40 / 2 + 1). Each attic truss contains about 85-120 linear feet of lumber depending on span and pitch. Factory-built attic trusses are far stronger and cheaper than site-built framing for this geometry; never frame a usable attic with stick rafters without an engineer's design.

## When to Use This Attic Truss Calculator

- Planning a 1.5-story or "Cape Cod" style roof
- Adding conditioned living space inside a garage roof
- Estimating lumber before getting quotes from a truss plant
- Deciding whether your current span and pitch support a room-in-attic design

Always confirm final dimensions with a licensed structural engineer. Attic trusses carry floor loads in addition to roof loads and require specific lumber grades and steel connector plates.

## Frequently Asked Questions

**Q: What is the minimum roof pitch for an attic truss?**

A: Most truss plants will build attic trusses with 8:12 pitch or steeper. Below 8:12 you do not get enough headroom for a usable room. 10:12 is the sweet spot for comfort, 12:12 gives the most room but requires more lumber and steeper roofing work. A 6:12 roof will only give you dead storage space, not a living room.

**Q: How wide a building do I need for an attic truss?**

A: You need at least 24 ft of exterior span to get a meaningful attic room. A 24 ft span with 10:12 pitch and 4 ft knee walls gives about 14 ft 5 in of room width, which is fine for a bedroom or office. Below 24 ft, the usable room becomes too narrow (under 10 ft) for comfortable use. 28-32 ft is the sweet spot for residential construction.

**Q: How tall should the knee walls be?**

A: 4-5 ft is standard. A 4 ft knee wall gives maximum room width but feels cramped; a 5 ft knee wall is a better balance. Taller knee walls (6+ ft) narrow the room significantly and push walls inward where they interfere with furniture. Remember that plumbing and HVAC runs often live inside the knee wall cavity, so 5 ft gives you more working space.

**Q: Do attic trusses cost more than regular trusses?**

A: Yes, typically 40-60% more than a standard roof truss of the same span. An attic truss has to carry both roof loads and floor loads (typically 40 PSF live load on the attic floor), requires heavier lumber (often 2x10 or 2x12 bottom chord), and uses more steel connector plates. However, they are still much cheaper than site-built framing for the same room-in-attic result.

**Q: Can I add insulation and HVAC inside an attic truss?**

A: Yes, and you must if the attic room is conditioned space. The sloped ceiling portion (above the knee walls) is typically spray-foamed or filled with high-density batt insulation between the rafters. The flat ceiling gets standard blown-in or batt insulation. Knee wall cavities need insulation on the cold side (the dead storage triangle). HVAC ductwork runs either in the knee wall dead space or through the flat ceiling cavity.

**Q: How many attic trusses do I need for a 40 ft long building?**

A: At 24 inch on-center spacing, a 40 ft long building needs 21 attic trusses (40 / 2 + 1 for the closing truss). At 16 inch on-center (used for heavy snow loads or long spans), you need 31 trusses. Always add one gable truss on each end; these are sometimes engineered as panels rather than conventional trusses and are priced separately.

---

Source: https://vastcalc.com/calculators/construction/attic-truss
Category: Construction
Last updated: 2026-04-08