When using cladding which does not provide restraint it is necessary to provide permanent lateral restraint to the purlins. The MetSPEC purlin design software will provide a comprehensive design solution to any non-restraining cladding application.
Note: bolted apex struts and 45 x 45mm lateral support angles must be used throughout.
Lateral Restraint for Non-restraining Cladding
The sheeting and fixings are deemed adequate in carrying the component of load in the plane of the roof slope on pitches up to 25˚. For roof pitches greater than 25˚, the purlins are designed for the downslope roof loads. The MetSPEC purlin design software will quickly determine the purlin section and bracing requirements for any application.
Note: bolted apex struts and 45 x 45mm lateral support angles must be used throughout in lieu of round lok anti-sag rods.
For mono-pitch roofs which are discontinuous at the high eaves level, diagonal tie wires and braces must always be used as shown on Long roof slopes, length > 20m.
However, if a structural member of adequate strength is available at the high eaves level, a threaded rod or eaves brace may be used, thereby avoiding the use of diagonal tie wires.
The relevant bracing requirements shown below will apply to the corresponding curved roof applications, provided that the cladding or liner tray offers adequate lateral support by virtue of its screw fixings at 600mm maximum spacings to the purlins.
The Metsec lateral support angle, as shown, should be used in the roof plane and wherever possible, should be tied across the apex, or fixed to a laterally rigid component. In addition to this, diagonal tie wires must be installed as shown.
Bolted apex struts and 45 x 45mm lateral support angles must be used throughout.
The MetSPEC purlin design software can be used to quickly determine the purlin section and bracing requirements for any application.
Lateral Supports for Tiled Roof
Section through Ridge Detail
Typical Purlin Arrangement (apex detail as above)
Illustration shows purlins at 1.2m centres, however the maximum purlin centres should not exceed 1.8m.
The recent and more onerous requirements of Approved Document L of the Building Regulations relating to energy conservation have shifted the emphasis for thermal efficiency from the building structure to the building envelope.
Cladding contractors now demand tighter tolerances on purlins and side rails to ensure that the building is airtight and weathertight.
The National Federation of Roofing Contractors and the Metal Cladding and Roofing Manufacturers Association recommend a maximum allowable displacement of 10mm and 20mm for composite panels and built-up systems respectively, to ensure adequate fixings to the top flange of the purlin.
For this reason, the lateral displacement of the purlin due to its own weight after erection, should be considered for low pitched roofs <5°, where restraints (struts) are not provided.
Whilst this may be relatively small, when added to the manufacturing tolerance of L/500, the resultant displacement may be outside the recommendations.
A check facility is provided on MetSPEC once the preferred purlin system has been selected.