Roof trusses do two jobs at once. They carry the weight of the roof to the walls and foundation, and they shape the rooms below them. A timber truss can frame a view, open up a great room, or divide a floor plan, so the choice of truss type is a design decision as much as a structural one. Every building leans on support components that stay out of sight until they matter, from the pipe hanger and support systems that hold plumbing in place to the structural members that transfer roof loads to the ground. Span, load capacity, window placement, and room layout determine which style fits, and most homes can combine two or three styles for a custom look.
- King post, in suspended and full versions
- Queen post
- Hammerbeam
How a Truss System Carries the Roof
A truss is a triangulated frame. The top chords take compression from the roof surface, the bottom chord carries tension, and the web members between them keep the triangle rigid. Because a triangle cannot change shape without changing the length of its sides, a well-built truss resists bending far better than a single beam of the same timber size. The same discipline that governs pipe hanger and support installations applies here: each element must carry its share of the load without overstressing the member it connects to, and a failure at any point in the chain shows up as movement somewhere else.
Load paths and span limits
Every truss system must carry the same family of loads:
- Dead load: the roof covering, sheathing, and the truss itself
- Live load: snow, rain, and temporary construction loads, often 20 to 70 pounds per square foot in snow country
- Wind uplift acting on the windward and leeward sides
- Seismic forces in active regions
Load travels from the roof surface to the top chord, down through the web members, and out through the bearing points at each end. Span is the distance between bearing walls, and it drives the design: longer spans need deeper trusses, larger members, or both. Trusses are typically spaced 24 inches on center, so a 20-foot room might carry ten trusses, each one doing its share of the work.
Geometry beats raw timber size
A deeper truss with modest members can out-span a shallow truss built from heavy timbers, because the internal triangle does the work. When a design feels heavy on timber but short on span, the geometry, not the wood, is usually the limiting factor.
King Post Trusses: Simple Spans and Clear Centers
King post trusses use a single vertical post running from the peak down to the bottom chord. This is the simplest truss form, and it suits great rooms and spaces where trusses span moderate distances. King posts come in two varieties, suspended and full, and the difference changes how the room below behaves.
Suspended versus full king posts
A suspended king post stops at the bottom chord, leaving the space below open. This version handles rooms about 16 feet wide, a comfortable size that leaves room for centered windows or furniture. The full-length king post continues to the floor and transfers the load of the ridge beam to the foundation. That extra load path lets it span farther, but it puts a post in the middle of the room.
Where a full king post belongs
Use the full king post between bays or on an end wall, where windows and wall space can be balanced on either side of the post. Treated as a deliberate divider rather than an obstacle, it can separate living and dining zones in a long room while the truss overhead keeps the roof simple.
Support conditions change where loads go, a lesson that shows up in everything from retaining walls to roof framing. Engineers compare the difference between free earth support and fixed earth support methods when designing earth-retaining structures, because fixing the base of a member redirects the entire load path. A full king post behaves like a fixed support, anchoring the ridge beam load to the floor; a suspended post leaves the load path free to pass around it. The same reasoning applies at the scale of a single room.
Queen Post Trusses: Open Entryways and Framed Views
A queen post truss is a rectangle inside a triangle. Two vertical posts stand a few feet apart, connected by a horizontal beam near the top, and the assembly sits inside the triangular outline of the roof. The opening at the center changes what the room can do: it frees up entryways and creates a place for a window or architectural feature directly under the ridge.
Windows under the ridge
The gap between the two posts leaves room for a window or an eye-catching architectural element under the ridge, while the lower part of the wall stays open for picture windows or large furnishings. Light drops into the center of the room, and the truss reads as a frame rather than a barrier. Because the posts sit in from the walls, the truss also breaks up a long ceiling plane without blocking the view.
Coordinating with fireplace and hearth framing
A queen post often frames a fireplace or entry, so check the floor framing around fireplaces, headers, and hearth support before finalizing the layout. The truss posts and the hearth both carry load, and aligning them on the same bearing points keeps the structure clean. If a post lands on a floor joist that was cut for a chimney chase, the engineer needs to know before framing starts.
Hammerbeam Trusses: Wide-Open Spaces
The hammerbeam is the showpiece of the truss family. Short horizontal beams project inward from the walls, braced with curved timbers, and the assembly spans roughly 24 feet with no vertical support posts. That clear span makes it the classic choice for great halls and cathedral rooms, and it opens the possibility of a whole wall of uninterrupted glass.
Engineering a 24-foot clear span
The hammerbeam works because the projecting beam acts like a lever anchored into the wall. The roof thrust is resisted at the wall line, and hidden steel straps or tie rods handle the tension that timber alone cannot. The same logic shows up below grade: ground support systems used in tunneling and underground construction remove intermediate supports so openings stay clear while the surrounding structure carries the load. Above grade, the hammerbeam does the same for a room.
A wall of uninterrupted glass
With no posts interrupting the span, a hammerbeam allows a wall of glass held together only by the mullions needed to support the panes. If the view is the point of the room, this is the truss that gets out of the way. Pair it with low-profile framing and the glass becomes the wall.
Combining Truss Styles and Setting Priorities
Most homes do not need one truss type repeated everywhere. A king post can frame the great room, a queen post can open the dining area, and a hammerbeam can cover the entry. Mixing styles gives each space its own character while the roof structure stays unified, and the transitions between styles become design features of their own.
| Truss type | Typical clear span | Vertical posts | Best suited for |
|---|---|---|---|
| King post, suspended | Up to about 16 ft | One, stops at the chord | Centered windows and furniture |
| King post, full | 16 ft and up | One, runs to the floor | End walls and between bays |
| Queen post | 16–24 ft | Two, spaced apart | Windows under the ridge |
| Hammerbeam | About 24 ft | None | Uninterrupted glass and great halls |
A five-step selection routine
- Measure the clear span between bearing walls.
- List what must stay open: windows, doors, furniture, and traffic.
- Have an engineer confirm the roof load for your region.
- Match each room to a truss type from the table above.
- Verify that every post lands on a wall, beam, or the foundation.
Cost and buildability
Custom timber trusses cost more than standard roof trusses, with joinery and labor driving the difference. During erection, construction site support equipment like temporary shoring and bracing holds members in position until the permanent connections are made. Budget for both the trusses and the staging that keeps them safe while they go up.
Supporting the Truss From Ridge to Foundation
A truss is only as good as what it bears on. Each end sits on a top plate, a beam, or a post, and the load continues down through the wall framing to the foundation. Connection hardware, bearing plates, and strapping tie the pieces together, and a structural review catches mismatches before framing starts.
Connections and bearing details
Check every bearing point for a solid, continuous support. A truss end resting on an unsupported span transfers its load to nothing, and the sag shows up in the finished ceiling. On weak soils or heavy roofs, foundation and piling equipment installs the deep supports that anchor the entire load path to the ground, so the truss, the walls, and the ground work as one system.