Purlin and rafter design
Rafters carry the roof down its slope and purlins prop the rafters at mid span so they can reach across the roof. We design purlins and rafters to the Eurocodes for cut and steel roofs, for clients across the UK.
What does a purlin do in a roof?
A purlin props the rafters part way up the slope so they can span the roof in shorter, shallower lengths. It gathers the load from the rafters bearing on it and carries it along to supports at the gables or on struts, which lets the rafters be a sensible size.
How rafters and purlins work together
In a traditional cut roof the rafters run up the slope from the wall plate to the ridge, carrying the roof covering, the battens and the imposed loads down to the walls. If a rafter had to span the full slope unaided it would need to be very deep, so purlins are introduced part way up to prop the rafters, breaking the rafter span into shorter, shallower lengths. The purlin runs horizontally along the roof, gathers the load from all the rafters bearing on it, and carries it along to supports at the gables or on struts and internal walls. The rafter is then designed for the reduced span between wall plate, purlin and ridge.
The rafter itself is a member on a slope carrying combined bending and axial load, and it is sized for the span the purlin gives it, checked for bending and deflection to BS EN 1995-1-1 for timber. Purlin position is a design choice: put it too high and the lower rafter span governs, too low and the upper span does, so we place it to balance the spans and keep both the rafters and the purlin economical. Struts from a loadbearing wall or a binder often support the purlin at intervals, and we trace those strut loads down to something that can carry them.
Loads, restraint and steel purlins
A pitched roof in the UK carries snow to BS EN 1991-1-3, with drift and unbalanced cases that load one slope more than the other, and wind to BS EN 1991-1-4, which on a roof produces suction that can reverse the load and try to lift the covering off. We check purlins and rafters for these combinations, because the design case for downward snow is not the case for wind uplift, and a member sized for one can be caught out by the other. On the slope, rafters and purlins also carry a component of load along the pitch, which we resolve rather than ignore.
Longer or more heavily loaded roofs move from timber purlins to steel. Cold formed steel purlins, the zed and cee sections used on portal frame and steel roofs, are light, efficient and designed to the manufacturer's data or to BS EN 1993-1-3 for cold formed steel. Their thin sections are prone to buckling, so the restraint the roof sheeting and any anti-sag ties give the purlin is part of the design; an unrestrained cold formed purlin buckles well below its apparent capacity. We check the restraint honestly and specify the purlin, its cleats to the frame and any sag bars.
Connections, bearings and coordination
Where a rafter meets a purlin the load has to pass cleanly, so we detail how the rafter sits on or is birdsmouthed over the purlin, and how the purlin bears at its supports, on the gable, on a strut, or cleated to a steel rafter. On steel roofs the purlin cleat to the rafter is designed for the reaction and any restraint force it provides to the main frame, because purlins often double as restraint to the rafters of a portal frame. We specify the fixings, cleats and bolt sizes so the connections carry what the analysis assumes.
Wind uplift means many rafter and purlin connections have to hold the roof down as well as up, so we allow for the reversal in the fixings and straps. The output is a calculation package with the rafter and purlin sizes, their spans and positions, connections, restraint and holding down, ready for the builder or steel fabricator 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 or extended.
What we check.
The points our calculations resolve for a project like this.
- Purlin position chosen to balance the upper and lower rafter spans
- Rafters checked for combined bending and axial load to BS EN 1995-1-1
- Snow drift and wind uplift cases, with load resolved along the slope
- Strut and binder loads supporting the purlin traced down to loadbearing structure
- Restraint to cold formed steel purlins from sheeting and anti-sag ties
- Connections and holding down designed for reaction and wind uplift reversal
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
What does a purlin do in a roof?
A purlin props the rafters part way up the slope so they can span the roof in shorter, shallower lengths. It gathers the load from the rafters bearing on it and carries it along to supports at the gables or on struts, which lets the rafters be a sensible size.
Where should the purlin go on the roof?
It is placed to balance the rafter spans above and below it, so neither the upper nor the lower length governs and both the rafters and the purlin stay economical. We calculate the best position for your roof rather than using a fixed rule.
Do you design steel purlins for portal frames?
Yes. Cold formed zed and cee purlins on steel roofs are designed to the manufacturer's data or to the cold formed steel Eurocode, including the restraint the sheeting and anti-sag ties provide, which is essential because these thin sections buckle if left unrestrained. We specify the purlin, cleats and sag bars.
Does wind affect rafter and purlin design?
Very much. Wind produces suction on a roof that can reverse the load and try to lift the covering, so the design case for wind uplift is often different from the case for snow. We check both and design the fixings and straps to hold the roof down as well as carry its weight.
Can you check an existing cut roof I want to alter?
Yes. We survey the existing rafters, purlins and struts, assess how your alteration changes the load paths, and design any new members or strengthening needed. Site visits are arranged by region, and the calculations are delivered anywhere in the UK or Europe.
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