Roof truss design
A roof truss turns a triangle of light timbers into a member that spans wall to wall without internal support. We design and check roof trusses, their loads and bracing to the Eurocodes, for clients across the UK.
Why can a roof truss span so far on such light timber?
Because its triangulated shape makes each member carry load in tension or compression rather than bending, which timber does far more efficiently. That lets a truss span wall to wall on light members where a solid beam would have to be very deep and heavy.
How a truss carries the roof
A truss works because a triangle cannot change shape without stretching or crushing its sides. Arrange timbers into a network of triangles and each member carries load in pure tension or compression rather than bending, which is why a truss can span a house from wall to wall on light timbers where a solid beam would be enormous. The rafters form the top chords in compression, the bottom chord acts as a tie in tension holding the feet together, and the internal webs carry the loads between them. We analyse the frame to find the force in every member for each load case, then check each one, the compression members for buckling and the tension members for their capacity, to BS EN 1995-1-1.
The standard domestic roof uses fink or fan trussed rafters, prefabricated from stress-graded timber joined by punched metal nail plates, spaced at close centres so no member is heavily loaded. An attic or room-in-roof truss is a different animal: its bottom chord has to carry a floor and the web layout opens up a habitable space, so the members are larger and the analysis heavier. We design and check both, and coordinate with truss fabricators where a manufactured system is used, or design a purpose-made truss where the geometry is non-standard.
Loads, joints and stability
The load on a truss is more than the roof weight. Snow to BS EN 1991-1-3, including drift and unbalanced patterns, wind to BS EN 1991-1-4, which can reverse the forces and put the bottom chord into compression under uplift, and any water tank or plant supported off the frame all have to be considered. We run the truss for the governing combinations, because the case that sizes the top chord is rarely the case that sizes the webs, and a truss checked for gravity alone can fail when the wind lifts the roof.
The joints are where trusses succeed or fail. In a nail-plated truss the plate has to transfer the member forces across the joint without pulling out or tearing, and the plate size and position are part of the design, not a detail left to the press. In a bolted or connectored timber truss we design each joint to BS EN 1995-1-1 for the force it carries. Just as important is out-of-plane stability: a tall thin truss is stable in its own plane but will topple sideways without bracing, so the bracing that ties the trusses together into a stable roof, longitudinal, diagonal and chord bracing, is designed as part of the roof, not left to chance.
Alterations, bearings and coordination
Trussed rafters are designed as a complete system, which is why cutting one on site to fit a loft, a rooflight or a water tank is a frequent cause of trouble. Removing a web or the bottom chord breaks the triangulation the truss relies on, and the remaining members overload. Where an existing trussed roof has to be altered, for a loft conversion, a new opening or plant, we assess the effect and design the strengthening or the replacement structure, often a pair of beams to carry the cut trusses, so the roof still works.
At the base the truss reaction lands on the wall plate and runs down through the walls to the foundation, and wind uplift means the connection often has to hold the roof down as well as up, so we specify the truss clips, straps and holding down details. The output is a set of calculations, member and joint checks, a bracing layout and bearing details, ready for the fabricator, the builder and Building Control. We deliver these designs across the UK, with site visits arranged by region where an existing roof needs surveying before it is altered.
What we check.
The points our calculations resolve for a project like this.
- Member forces found for each load case, chords and webs checked to BS EN 1995-1-1
- Snow drift and wind uplift, which can reverse member forces, both considered
- Nail plate or connectored joints designed for the force they transfer
- Out-of-plane bracing designed to stabilise the trusses into a whole roof
- Effect of cutting or altering trusses assessed, with strengthening designed
- Bearings, clips and holding down straps for reaction and wind uplift
Issued drawings, redacted.
From enquiry to sign-off.
Enquiry
Send drawings or describe the problem. We confirm the scope, the deliverables and a target timescale.
Information
We agree the survey, drawings or data we need and any site access required.
Engineering
Design, calculation or assessment to the relevant Eurocodes and UK National Annex.
Issue
A clear, defensible report or set of calculations, with assumptions and limitations stated.
Common questions
Why can a roof truss span so far on such light timber?
Because its triangulated shape makes each member carry load in tension or compression rather than bending, which timber does far more efficiently. That lets a truss span wall to wall on light members where a solid beam would have to be very deep and heavy.
Can I cut a roof truss to make space in my loft?
Not without engineering. Trussed rafters are designed as a complete system, and cutting a web or the bottom chord breaks the triangulation and overloads the rest. If you need to open up a trussed roof we design the beams or new structure to carry the cut trusses safely.
Do you design attic and room-in-roof trusses?
Yes. Attic trusses carry a floor on the bottom chord and open up a habitable space, so the members are larger and the analysis heavier than a standard fink truss. We design and check both standard and attic trusses, and coordinate with fabricators where a manufactured system is used.
Why does the bracing matter on a truss roof?
A truss is stable in its own plane but will topple sideways without bracing. The longitudinal, diagonal and chord bracing ties the trusses together into a stable roof and carries wind forces along the building. We design the bracing as part of the roof rather than leaving it to chance.
Does the roof need holding down against wind?
Often yes. Wind can lift a lightweight roof, reversing the forces and trying to pull the trusses off the walls. We design the truss clips, straps and holding down details so the roof is secured against uplift as well as carrying its downward load.
Related in structural design & elements.
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