Lift overrun: what it is and why it matters in design and modernisation
Lift overrun is the clear space at the top of the lift shaft, above the highest landing. It is also called headroom or the shaft head. When the car is at the top floor, the overrun is the space between the car roof and the underside of the shaft ceiling, and it has to hold more than most people expect.
What the overrun has to accommodate
The space above the car is not empty air. Depending on the type of lift, the overrun has to allow for:
- the car travelling past the top landing before it is brought to rest, for example when the counterweight settles on its buffer;
- equipment mounted on the car roof, such as the door operator, the balustrade and the inspection controls;
- a refuge space, so that a technician working on the car roof cannot be trapped against the ceiling;
- in a machine-room-less lift, the machine and other equipment that would otherwise sit in a machine room;
- anything fixed to the shaft ceiling, such as lifting beams, lighting and sprinkler heads.
The dimension a lift needs depends on its type, speed and arrangement and on the equipment selected. Manufacturers publish minimums for their own products, and the applicable standard sets the safety spaces that must be kept clear.
Why overrun matters in lift design
Overrun is one of the decisions that is set early and is very hard to change once the structure is built. It sets the height of the structure above the lift shaft, which can affect building height limits, roof plant layout and how the building looks. A shaft drawn around one manufacturer’s minimum can also narrow the field when the lift is tendered, because another supplier’s equipment may need more room. That is why we coordinate shaft, pit and overrun dimensions early in lift design, and why we prefer performance specifications, which allow fairer competition.
Why overrun matters in modernisation
Older buildings often have less overrun than current equipment and current standards expect. When a lift is modernised or replaced, the existing headroom has to be checked against the new equipment and the safety spaces that will apply to it. Where it falls short, the usual options are equipment designed for reduced headroom, a change to the speed or arrangement of the lift, an alternative solution where that is permitted, or structural work to raise the shaft head. Each one carries cost and programme implications, so the question needs answering before the work is specified and priced.
Depending on what changes, the work may also trigger a new design registration in some states. Read when a lift modernisation becomes a new design registration before you settle on scope.
What to check
- The overrun as measured on site, from the top landing floor level to the lowest point of the shaft ceiling or anything fixed beneath it, rather than the figure on the original drawings.
- Obstructions in the shaft head: beams, hooks, lighting, cable trays and sprinkler heads. See sprinklers in the lift shaft head.
- The minimum overrun each tenderer’s equipment needs, confirmed in writing.
- Whether the refuge space on the car roof will still be adequate once new car-top equipment is fitted.
- The pit at the other end of the shaft, which raises the same questions about safety spaces and is best checked at the same time.
- Safe access to the shaft head for maintenance, and whether there are suitable lifting points for installation.
Getting independent advice
Overrun problems are cheapest to solve on paper. If you are designing a new building or planning to modernise or replace a lift, an independent consultant can measure the existing shaft, check it against the equipment on offer and make sure every tenderer is pricing a solution that fits. For scope and cost, read lift and elevator modernisation: cost, scope and when to do it. Contact us or call (07) 5509 0100 to talk it through.