Precast Roof Truss

What is a precast roof truss?

A precast roof truss, also known as a roof girder or double-pitched roof beam, is a precast roof member that spans the main bay between two columns in an industrial building. Its top edge is usually highest at mid-span and slopes down toward the ends, giving the roof the fall it needs to drain. Both double-pitched and single-pitched types are used in the industry.

Its role in an industrial building

The roof truss is the primary load-bearing member of a precast roof. The weight of the cladding, together with snow and wind loads, passes first to the purlins, then to the truss and finally to the columns. Because the truss span determines how much of the interior is free of columns, it has a direct effect on production line layout and on forklift and crane movement. The truss-to-column connection is detailed to stop the truss from sliding off the column head or overturning during an earthquake.

Truss spacing and purlins

The distance between trusses (the bay spacing) sets the length and section of the purlins. Wider spacing means fewer trusses, but longer purlins that need to be stronger. Because this balance affects the total number of elements, erection time and cost, it is weighed up together at the design stage.

Section and design

Roof trusses are most often designed with an I-section whose depth reduces from mid-span toward the supports. For long spans, the industry generally favors prestressed production. Length, depth, pitch, prestressing and reinforcement are set in the structural design based on the span, roof cladding type, snow load and any services to be hung from the roof.

Production

Trusses are produced in long molds adjusted to the length and pitch of each project. Dowels, sleeves and cast-in parts for the purlin connections are placed before casting. As long, slender members, they must be lifted from the correct points during demolding, storage and loading; these lifting points are defined at the design stage.

Delivery and erection

Roof trusses are delivered on transport suited to their length. On site, they are lifted by crane onto the column heads and secured with dowel connections. Once the trusses are in place, the purlins follow, so the roof is quickly ready for cladding. For particularly long trusses, the delivery route and crane capacity are planned together at the start of the project.

Choosing the right truss

Selecting a truss is not only about span. If solar panels, ventilation equipment or suspended services may be added later, allowing for these loads from the outset keeps the building flexible throughout its life. Sharing the column-free span you need and how you plan to use the roof is the first step toward the right truss design.