# What is a Belgian truss?
**A Belgian truss is a type of traditional timber roof truss characterized by its use of double diagonal struts that extend from the base of a central king post (or the tie beam) outward to the principal rafters. This configuration creates a strong, efficient triangulated structure capable of spanning moderate to large distances. The Belgian truss is sometimes referred to as a “truss with double struts” and has been a common roof framing solution in European construction for centuries.**
## Application
The Belgian truss is designed to carry the weight of the roof covering, resist wind forces, and transfer all loads down through the supporting walls to the foundations. Its geometry makes it particularly effective for spans typically ranging from 8 to 14 metres, though the exact span depends on the timber section sizes and the applied loads.
The key components of a Belgian truss include:
– **Tie beam** — the horizontal bottom member that spans between the supporting walls. It resists the outward thrust of the rafters and keeps the walls from spreading apart.
– **Principal rafters** — the inclined top members that follow the roof slope and carry the purlins and roof covering.
– **King post** — a vertical central member connecting the ridge of the rafters to the tie beam. It carries the weight of the tie beam in tension and provides a central connection point for the struts.
– **Double struts** — the defining feature of the Belgian truss. These two diagonal members run from the lower portion of the king post (or from the tie beam near the king post) outward and upward to the mid-span of each principal rafter. They reduce the unsupported length of the rafters, preventing excessive bending.
Compared to a simple king post truss, the Belgian truss can handle longer spans and heavier loads because the double struts provide intermediate support to the rafters. This makes it a practical choice for agricultural buildings, warehouses, churches, and larger residential roofs.
When designing or restoring a Belgian truss, proper timber joinery is essential. Traditional connections use mortise and tenon joints, often secured with hardwood pegs. In modern applications, steel connector plates and bolts may supplement or replace traditional joinery.
## Related terms
– King post truss
– Queen post truss
– Rafter
– Tie beam
– Purlin
– Roof loads
– Timber framing
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