Truss Design

Part of Woodworking

A truss lets you span wide openings without interior supports. By arranging timbers into triangles, you turn bending forces into compression and tension — forces that wood handles far better. Understanding truss design is essential for building any structure wider than a single beam can span.

Why Trusses Work

A single beam across a wide opening will sag under load. The longer the span, the worse the sag. A truss solves this by breaking the span into triangles.

The triangle is the strongest geometric shape in structural engineering. A rectangle can be pushed into a parallelogram — it deforms. A triangle cannot change shape without breaking one of its sides. Every truss design, no matter how complex, is built from triangles.

The Triangle Rule

If you look at any truss and cannot identify the triangles, something is wrong with the design. Every member should be part of at least one triangle.

Truss Components

Every truss has these parts:

Component	Location	Function
Top chord (rafters)	Upper edges, angled	Carry the roof load down to the joints
Bottom chord (tie beam)	Horizontal base	Prevents the rafters from spreading outward
King post / queen posts	Vertical, center	Transfer load from the peak down to the tie beam
Struts (webs)	Diagonal, interior	Brace against shear and distribute loads
Ridge	Very top, where rafters meet	Connection point (not always a separate piece)

Truss Types

King Post Truss

The simplest truss. One vertical post runs from the center of the tie beam up to the peak where the two rafters meet.

• Span: Up to about 20 feet (6 m)
• Best for: Small buildings, sheds, workshops
• Parts: 2 rafters, 1 tie beam, 1 king post, 2 struts (optional)

The king post is in tension — it hangs from the peak and holds the tie beam up, preventing sag. This is counterintuitive. The post pulls up, not pushes down.

Queen Post Truss

Two vertical posts instead of one, connected by a horizontal straining beam between their tops. This opens up the center for usable attic space.

• Span: 20 to 30 feet (6-9 m)
• Best for: Barns, larger workshops, homes with loft space
• Parts: 2 rafters, 1 tie beam, 2 queen posts, 1 straining beam, 2 struts

Scissor Truss

The bottom chords cross each other like an open pair of scissors, creating a vaulted ceiling inside.

• Span: 15 to 25 feet (4.5-7.5 m)
• Best for: Churches, great halls, any space where you want a high interior ceiling
• Trade-off: Less rigid than king or queen post — requires heavier members

Hammer Beam Truss

Short horizontal beams (hammer beams) project from the walls partway into the span. Curved braces and vertical hammer posts support the rafters from these projections.

• Span: 25 to 50+ feet (7.5-15 m)
• Best for: Large halls, ambitious builds
• Difficulty: Highest — complex joinery, heavy lifting, critical load paths
• Note: This is a medieval masterwork. Attempt only after mastering simpler designs.

Sizing Members

For a survival context, precise engineering tables may not be available. Use these rules of thumb:

Tie beam depth = Span / 12

For a 24-foot span, the tie beam should be at least 2 inches thick and 24 inches deep — or equivalent. In practice, a solid timber 8” x 8” works for spans up to about 16 feet with moderate loads.

Rafter sizing:

Span (feet)	Minimum Rafter Size	Spacing
10-14	4” x 6”	24” on center
14-20	6” x 6”	24” on center
20-28	6” x 8”	24-36” on center

These Are Minimums

These sizes assume moderate roof loads (basic thatch or light boards). Heavy tile, thick sod roofs, or areas with significant snow load require larger members. When in doubt, go bigger. An overbuilt truss wastes some wood. An underbuilt truss kills people.

King post / queen posts: Same depth as the tie beam, at least 4” x 4” for small trusses, 6” x 6” for larger ones.

Calculating Loads

Every truss must support three categories of load:

Dead Load

The weight of the roof itself — rafters, sheathing, roofing material. Estimate 10-15 pounds per square foot (psf) for thatch, 15-25 psf for heavy tile or sod.

Live Load

Temporary loads — workers on the roof during construction, maintenance. Use 20 psf minimum.

Environmental Load

Snow and wind. Snow can add 20-40 psf or more. Wind creates uplift on the leeward side and pressure on the windward side.

Total load per truss = (Dead + Live + Environmental) x Tributary area

Tributary area = truss spacing x half the span (each side). For a truss spaced 4 feet apart spanning 20 feet: 4 x 10 = 40 square feet per side, 80 total.

At 50 psf total load: 80 x 50 = 4,000 pounds on one truss.

Joint Design

Truss joints are where failures happen. Every connection must transfer load through bearing surfaces, not through fasteners alone.

Principles

1. Maximize bearing surface: The wood should press against wood over the largest area possible
2. Use housed joints: Cut a shallow seat (housing) so the member sits in a pocket, not just against a flat face
3. Peg through the joint: Oak pegs (treenails), minimum 1” diameter, through drilled holes
4. Offset peg holes: Drill the peg hole in the tenon slightly closer to the shoulder than the hole in the mortise — when you drive the peg, it pulls the joint tight (draw-boring)

Critical Connections

Rafter foot to tie beam: A birdsmouth cut (notch in the rafter) sits over the tie beam. The horizontal cut bears the weight. The vertical cut prevents sliding. Peg through both.

King post to tie beam: The king post hangs from a mortise and tenon at the peak and connects to the tie beam below. The bottom connection must resist tension (pulling apart). Use a through-tenon with a wedge or a heavy iron strap if available.

Strut to king post: Angled mortise and tenon. The strut is in compression, so the joint needs a good bearing surface. Cut the mortise perpendicular to the strut’s angle, not perpendicular to the post.

Test Every Joint

Before raising, hang weight from the truss on the ground. Load it to at least 50% of design load. Watch for joints that open up, pegs that bend, or members that deflect excessively.

Raising and Installing

Preparation

1. Build the truss flat on the ground, on a level surface
2. Assemble dry — check all joints for fit
3. Number every joint with chisel marks (marriage marks) so you can disassemble, transport, and reassemble
4. Pre-drill all peg holes

The Raise

1. Position the truss base on the wall plates where it will sit
2. Attach a temporary prop rope to the peak
3. Using pike poles (long poles with a fork or spike), push the truss upright while the rope team controls the lean
4. A crew of 6-10 people can raise a truss for a 20-foot span
5. Brace immediately with temporary diagonal boards once upright

Securing

1. Peg the truss to the wall plate with through-tenons
2. Install purlins (horizontal members connecting trusses) before removing temporary braces
3. Add wind bracing — diagonal members in the plane of the roof that prevent racking

Common Failures

Failure	Cause	Prevention
Tie beam sag	Undersized member, excessive span	Size to span/12 rule, add queen posts
Rafter spreading	Tie beam connection failure	Birdsmouth + peg + strap at rafter foot
King post pulling out	Tension joint failure	Through-tenon with wedge, draw-bored peg
Racking (sideways collapse)	No wind bracing	Diagonal bracing in the roof plane
Joint crushing	Bearing surface too small	Increase tenon width, add bearing plates
Wood splitting at peg	Peg too close to edge	Minimum 2.5x peg diameter from edge

Inspect After Storms

After any major wind or snow event, inspect every joint for separation, cracking, or deflection. A small gap at a joint today is a collapse next winter.

Practical Design Process

1. Determine your span (wall to wall, outside to outside)
2. Choose your truss type based on span and intended use
3. Set your roof pitch (steeper = more snow shedding but more wind exposure; 8:12 to 12:12 is typical)
4. Size your members using the rules of thumb above
5. Lay out the truss full-scale on a flat surface (a “lofting floor”)
6. Cut joints, starting with the tie beam
7. Dry-assemble and check fit
8. Disassemble, raise walls, then raise trusses

Truss Design — At a Glance

Trusses use triangles to span wide openings without interior posts. The king post truss handles spans to 20 feet with just five members. Size the tie beam to span/12 depth as a minimum. All joints must transfer load through bearing surfaces — never rely on pegs alone. Calculate total load (dead + live + environmental) to verify member sizes. Build flat, test under load, raise with a crew, and brace immediately. Inspect joints regularly — connection failure is the primary cause of truss collapse.