What is a Philibert truss?
A Philibert truss is a curved roof truss made not from a single bent beam but from short, shaped boards set on edge in two or three layers with staggered joints. It is named after the French architect Philibert de l’Orme (1512-1570), who described the principle in the sixteenth century. In the Low Countries the technique only came into wider use from around 1800.
How it works
The boards stand on edge, flat faces against each other, fastened with nails or wooden dowels. Each board is sawn to a slight curve; together they form a smooth arch. The ingenuity lies in the staggering: a joint in one layer is always bridged by continuous timber in the layer beside it. The result is an arch far longer than the longest piece of timber required to build it.
Why it was used
- Long spans from short timber. No heavy, long beams are needed — boards a few metres long will do. At the time that was a major economic advantage.
- Clear space below the roof. Because the arch carries the load out to the walls, no posts or struts stand in the room. That made it popular for riding schools, granaries, churches, barns and later factory halls.
- Light structure. A Philibert roof is considerably lighter than a traditional purlin roof of the same span.
Recognising one
Philibert trusses turn up mostly in nineteenth-century farmhouses, mills, warehouses and churches. The giveaways are the visible layers of boards in the arch and the rows of nails or dowels along the joints. Its successor is the glued laminated arch, where the laminations are turned a quarter turn and bent into shape on assembly.
Points to watch in restoration
- Check the fasteners first. The strength lies in the nailed and dowelled connections. Corroding nails or torn-out dowel holes are the first sign of decay, often before the timber itself is affected.
- Leaks at ridge and foot. Water coming through the roof covering attacks the end-grain joints; inspect the board ends.
- Do not simply drill through. New services, lighting or a roof window cut through the arch boards directly undermine the layered action.
- Added load needs calculation. Solar panels or a heavier roof covering on a Philibert roof should always be checked by a structural engineer; these roofs were designed for a light covering.
Related terms
- Roof structure
- Three-hinged arch
- Glued laminated timber
- Purlin roof
- Arched truss
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