The roof truss structure and design are integral to the roof’s structural integrity and shape. There are many types of roof trusses (as well as many types of roofs)… more types than there are parts.
This article and series of illustrated diagrams show you all the parts of a basic roof truss, king truss, and queen truss. See our detailed diagram showing the parts of a roof here.
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1. Anatomy of a basic roof truss
Roof truss parts explained
Rafter: One of a series of diagonal members of the truss that meet at the apex in order to support the roof deck and its loads.
Underpurlin: Horizontal beams supported by posts and used to support the mid-span of rafters to cover longer spans. These are used in large buildings like the traditional large old barns in the US.
Strut: A diagonal member of the truss that has a compressive force acting on it and is used to maintain rigidity and add strength to the structure.
Ridge Beam: So-called because it is the horizontal beam that supports the ends of the rafters at the ridge.
Collar Tie: A horizontal beam that connects two rafters near the ridge of a roof. It is often used to prevent the separation of the rafters during high winds but can also be used simply to frame the ceiling.
Plumb Cut: The outward-facing vertical cut of a rafter designed to ram against the ridge board.
Strutting Beam: The horizontal beam that supports roof loads and transfers them directly to load-bearing walls.
Birdsmouth Cut: Also birds mouth joint or bird’s beak cut, it’s so-called because the joint’s indention cut resembles that of a bird’s mouth. It is used to connect the rafter to the supporting wall.
Cantilevered Top Plate: A projecting beam that transports the load it is carrying to load-bearing support.
Ceiling Joist: The horizontal base that supports the rafters.
Top Plate: The horizontal beam that supports the roof by supporting the rafters to the wall studs.
2. Anatomy of a traditional king post roof truss
a. What is a king post roof truss?
The King Post Roof Truss is the simplest of trusses because of its simple composition. In a nutshell, it’s made up of a central vertical post called the king post, two rafters meeting at the apex, and a tie beam or the horizontal base. It is often used in simple roof trusses such as in the shed, porch, and garage.
b. King post truss parts explained:
Ridge Board: A non-structural member of the truss where rafters can lean against and connect to.
Common Rafter: Also called minor rafter because it is smaller than Principal Rafter/Major Rafter.
Principal Rafter: The larger rafter that sits directly on a tie beam and is used to carry a purlin.
Wall Plate: A piece of lumber laid horizontally on a wall to support the rafter.
Supporting Wall: A chief structural member as it is a load-bearing wall.
King Post: The central vertical post in a King Post Roof Truss.
Strut: A diagonal member of the truss that has a compressive force acting on it and is used to maintain rigidity and add strength to the structure.
Tie Beam: The horizontal beam connecting two rafters.
Purlin Cleat: The strip of iron attached to the purlin.
Purlin: The longitudinal horizontal beam that is sitting on a post or the principal rafter of a truss and used to support common rafters. There are three types of purlins in wood construction: Purlin plates (Under purlin), Principal purlins, and Common purlins.
3. Parts of a traditional queen post roof truss
a. What is a queen post roof truss?
A Queen Post Roof Truss is essentially a modification of the King Post Truss. Unlike the King Post, the Queen Post has two vertical wooden posts called the queen post. It’s also more lightweight and can be used to cover larger areas.
b. Queen truss parts explained:
Ridge Board: A non-structural member of the truss where rafters can lean against and connect to.
Straining Beam: A short piece of timber that keeps the ends of struts and rafters in place.
Common Rafter: Also called minor rafter because it is smaller than Principal Rafter/Major Rafter.
Purlin Cleat: The strip of iron attached to the purlin.
Wall Plate: A piece of lumber laid horizontally on a wall to support the rafter.
Supporting Wall: A chief structural member as it is a load-bearing wall.
Straining Sill: The horizontal compression member resting on the tie beam and keeping the ends of the stats apart.
Queen Post: The two vertical posts in a Queen Post Roof Truss.
Tie Beam: The horizontal beam connecting two rafters.
Strut: A diagonal member of the truss that has a compressive force acting on it and is used to maintain rigidity and add strength to the structure.
Purlin: The longitudinal horizontal beam that is sitting on a post or the principal rafter of a truss and used to support common rafters. There are three types of purlins in wood construction: Purlin plates (Under purlin), Principal purlins, and Common purlins.
Related:
- Homes with Gambrel Roofs
- Parts of a Roof Gutter
