Cantilever Details

St Pancras International

If a canopy is required, it can be achieved by cantilevering the purlins over the gable as necessary. Simple rules enabling engineers to meet the appropriate design criteria are set out below.

  • Performance

    In the approach given, it is assumed the sections are installed in one continuous length i.e. backing span and overhang, and that the sheeting offers full lateral restraint to the section.

  • Deflection criteria

    Purlins selected from this technical manual obey a deflection criteria of span/180 for conventional steel decking. The end deflection of a cantilever should be compatible with this criteria and we recommend that the cantilever should not exceed 28% of the backing span to meet this requirement.

  • Stability

    With cantilevers, it is recommended that the ends are braced together to provide stability and prevent rotation. Various details are used for this purpose with a typical example shown.

    A cleader angle fixed to the top and the bottom of the purlin will offer sufficient restraint as well as providing a base for fixing the sheeting and flashing. Cleader angles should be positively fixed across the apex to prevent downslope movement.

  • Mono pitched roofs and roof slopes exceeding 25°

    Diagonal tie wire systems are to be included. Alternatively, a suitable fixing to a structural member must be included at the top of the slope to resist downslope movement.

    Diagonal tie wires must be fixed back to the bottom holes in the purlin cleat.

    Note: for other specialist cases or heavier cladding systems (i.e. tiles and battens), consult our design office for advice.

  • Typical Cantilever Detail

    Typical Cantilever Detail