Technical Details for the Cantilevered Staircase

Engineering

Australian regulations concerning cantilevered treads staircases specify a maximum gap between treads of 125mm, and assuming a fully open flight and a typical rise of 190mm, this requires that the treads be at least 65mm thick. Arden often specifies composite cantilevered treads incorporating a structural frame core of 40mm mild steel SHS members. This conveniently satisfies engineering requirements whilst simultaneously bringing the total tread thickness to about 70mm.

Arden has several methods for supporting cantilevered treads staircases. The first method applies to installations to a heavy structural wall, where sufficient space is allowed to the plaster line. Tread supports are welded to mild steel SHS frame that is fixed to wall. Consultation with the structural engineer responsible for the main structure is advisable to ensure it is structural adequate to take the cantilever load that will be transferred to the wall. When sufficient space in the stair cavity has not been allowed for locating the plaster sheet off the wall, an alternative method is to sandwich the wall with plate steel stringers with through-wall connections. A degree of construction expertise is required to ensure that the loads do not transfer to the wall structure in shear, thereby compromising its material integrity. Where light-weight framing is used for walls, Arden advises that a double-framed wall be included in the stair construction component, in steel or potentially timber (in the case of full timber staircase). The double framed wall may therefore be constructed from structural plywood on either side of a timber frame, or alternatively as a complete structural steel frame.

Tips for the architect, client or builder

Arden suggests the following take-home points for those considering a cantilevered staircase.

• Select a reputable staircase provider with a demonstrated track-record in engineered staircases – this is not your run-of-the-mill cover-grade flight!
• Consult with your preferred supplier early in the design phase, to ensure that the space requirements of the flight are catered for in the floor plans
• Because standard wall-framing methods are often neither strong nor precise enough to meet the requirements of this staircase, the most efficient method is for your staircase supplier to include the supporting wall in their own scope of works.
• Don’t worry! Cantilevered flights are beautiful, economical and safe if approached correctly.
• Your architect, designer or staircase contractor should assist you in selecting a suitable balustrade style that does not detract from the clean lines of the flight.