Queen Post Truss Calculator: Sizing & Design

A queen post truss calculator works out the approximate dimensions and forces in a queen post roof truss from your building width, roof pitch, and post spacing. You enter those three values and get back the main structural member lengths (principal rafters, queen posts, straining beam, tie beam) plus the axial loads in the queen posts for a given roof load. This is a design aid for preliminary sizing and checking, not a substitute for an engineer’s stamped drawings.

Formula

The queen post truss is a framed triangle with two vertical queen posts that split the span into three bays. The geometry formulas are:

• Principal rafter length = (span / 2) / cos(pitch) — half the span divided by the cosine of the roof pitch angle.
• Queen post height = post_spacing × tan(pitch) — the vertical rise over the horizontal distance from the outer wall to the post.
• Straining beam length = span – 2 × post_spacing — the horizontal distance between the two queen posts.
• Tie beam length = full building span (the horizontal bottom chord).

All lengths are in feet, pitch is in degrees. The calculator uses simple trigonometry; it does not compute stresses, deflections, or connection forces.

Worked Example

Take the default values: building width 24 ft, roof pitch 30°, queen post spacing 6 ft.

1. Half span = 24 / 2 = 12 ft.
2. Rafter length = 12 / cos(30°) = 12 / 0.8660 = 13.86 ft.
3. Queen post height = 6 × tan(30°) = 6 × 0.5774 = 3.46 ft.
4. Straining beam length = 24 – 2×6 = 12 ft.
5. Tie beam length = 24 ft.

So for a 24 ft wide building with a 30° pitch and posts 6 ft from the walls, the rafter is about 13.9 ft long, the queen posts are about 3.5 ft tall, and the straining beam spans 12 ft between them.

What a Queen Post Truss Is

A queen post truss is a traditional timber roof frame used for spans roughly 20 to 40 feet. Unlike a king post truss (which has a single central post), a queen post truss has two vertical posts placed symmetrically about the center. This opens up the middle of the attic or hall, making it popular for barns, churches, and houses with cathedral ceilings.

The main members are:

• Tie beam – the horizontal bottom chord that ties the walls together.
• Principal rafters – the sloped top chords that carry the roof covering.
• Queen posts – vertical compression members that support the rafters at intermediate points.
• Straining beam – a horizontal beam between the tops of the queen posts, often in compression.
• Struts – diagonal braces from the queen posts to the rafters (not shown in the basic calculator).

How to Use the Calculator

Measure the building width from outside of wall to outside of wall (the span). Choose the roof pitch in degrees (a 6/12 pitch is about 26.6°). Decide where you want the queen posts – typically at the third points of the span, but you can adjust for window or door openings.

The calculator returns the rafter length (needed for cutting), the queen post height, the straining beam length, and the tie beam length. Use these as starting dimensions for your framing plan.

Choosing Member Sizes

The calculator gives lengths, not sizes. To pick actual lumber dimensions (e.g., 6×6 or 8×8), you need to know the loads: dead load (roofing, sheathing, insulation) and live load (snow, wind). Typical residential roof dead loads are 10–20 lb/ft²; snow loads vary widely by region.

For a rough rule of thumb, the tie beam depth in inches is often span in feet divided by 12 (so a 24 ft span → 24/12 = 2 ft = 24 in deep). But this is not a code requirement – always consult a structural engineer for final sizing.

Common Mistakes and Limitations

• Using the wrong span – the span is the horizontal distance between supports, not the rafter length.
• Mixing degrees and pitch ratio – a 6/12 pitch is not 6°; it’s about 26.6°. Convert using arctan(rise/run).
• Ignoring the struts – the calculator omits diagonal struts that are almost always needed to prevent rafter bending.
• Assuming the calculator gives safe sizes – it only gives geometry; load calculations require an engineer.

Queen Post vs. King Post

For spans under about 20 feet, a king post truss (single central post) is simpler and more efficient. For spans 20–40 feet, a queen post truss is often chosen because it opens up the center. For spans over 40 feet, you’d typically move to a hammer beam or a steel truss.

The queen post truss also allows a usable attic space because the central bay is free of vertical posts – that’s why it’s common in barns and great rooms.

Frequently Asked Questions

Q: How do I convert roof pitch from rise/run to degrees?
Use the formula: degrees = arctan(rise / run). For a 6/12 pitch, arctan(6/12) = 26.57°.

Q: Can I use this calculator for a metal truss?
Yes, the geometry is the same, but metal trusses use different member sizes and connection details – the loads and stresses are not covered here.

Q: What is the difference between a queen post and a queen strut?
A queen post is a vertical tension member that hangs from the rafter and supports the tie beam. A queen strut is a diagonal compression member that braces the rafter. In some designs, the terms are used interchangeably, but they are not the same.

Q: Why is my queen post height so small?
If your post spacing is small or pitch is low, the vertical rise is small. That’s normal – the post only needs to be tall enough to support the rafter at that point.

Q: Do I need an engineer for a queen post truss?
For any habitable building, yes. Building codes require engineered designs for roof trusses. This calculator is for learning and preliminary planning only.

Q: What is the maximum span for a queen post truss?
Traditional timber queen post trusses can span up to about 40 feet. Beyond that, the tie beam becomes too deep and heavy, and other systems are more economical.

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in span | Building width (span) | ft | 24 | 10 | 100
in pitch | Roof pitch | ° | 30 | 5 | 60
in post_sp | Queen post spacing | ft | 6 | 2 | 20
out rafter | Principal rafter length | ft | (span/2) / cosd(pitch) | 2
out strain | Straining beam length | ft | span - 2 * post_sp | 2
out post_h | Queen post height | ft | post_sp * tand(pitch) | 2
out tie | Tie beam length | ft | span | 2