A roof truss is a structural assembly, typically made from timber, that forms the framework of a roof. Rather than being built piece by piece on site from individual rafters and purlins in the traditional cut rafter method, trusses are manufactured in a factory to precise engineering specifications and delivered to site ready to install. They are the dominant form of roof construction in UK new-build housing today.

How a Roof Truss Works

A truss is a triangulated structure. It achieves its strength and rigidity by the geometric principle that a triangle is a stable form that cannot be distorted without changing the length of one of its sides. The triangulated web of a roof truss distributes the load from the roof covering and any snow or wind loads through the members of the truss to the supporting walls at each end. The specific arrangement of the web members varies between truss types, but all rely on the triangulation principle.

The Fink Truss

The most common roof truss type in UK residential construction is the fink truss, also called a W-truss. It has two rafters meeting at the apex, a horizontal ceiling tie at the base, and a W-shaped pattern of web members connecting the apex and the ceiling tie. The fink truss is efficient in its use of timber and spans the full width of the house in a single unit, typically 6 to 12 metres, without requiring intermediate support.

Advantages of Trussed Roofs

Trussed roofs have become dominant in new construction because they offer several advantages over traditional cut rafter roofs. They are faster to erect: a gang of two or three operatives can install all the trusses for a house in a single day using a crane. They use less timber than a cut rafter roof because the engineered geometry uses material efficiently. They are manufactured under factory quality control to precise dimensions, reducing on-site variation. And they are designed by structural engineers to carry the specific loads for the building, providing reliable structural performance.

The Drawback: Loft Conversions

The main disadvantage of trussed roofs is that the dense pattern of web members in the truss void makes the loft space unusable for storage and makes loft conversion significantly more complex and expensive than in a cut rafter roof. To convert a trussed loft, the trusses must be modified or replaced with new structural elements to create a clear usable space, typically involving the installation of a new ridge beam and new rafters with structural engineering design. This is technically feasible but adds cost compared with converting a cut rafter loft.

Summary

A roof truss is a factory-made triangulated structural frame that supports the roof covering. Fink trusses are the most common type in UK new-build housing. They offer speed, efficiency, and reliable structural performance but fill the loft void with web members, making loft conversion more complex than in cut rafter roofs. Never alter a roof truss without structural engineering advice.