Elevator overhead: which dimension is used?

The term "overhead" can refer to different dimensions on an elevator project. For the RopeFactor calculations, overhead means the vertical distance from the top landing floor level to the centreline of the drive sheave. This is different from the clear architectural overhead, measured from the top landing to the overhead structure.

Overhead machine with a machine room above the hoistway. Both dimensions start at the top landing floor; only the rope overhead carries on past the floor slab to the sheave centreline.

The two dimensions

Same word, different datum
Sense	Measured	Used for
Rope overhead	Top landing floor level → centreline of the drive sheave	Rope order length, elongation, rope dynamics
Clear overhead	Top landing floor level → underside of the slab or machine-room floor	Car-top refuge space, counterweight run-by, buffer and clearance checks

Which of the two is larger depends on where the machine sits. With a machine room above the hoistway — the usual arrangement for an overhead machine — the drive sheave stands on the machine-room floor and the ropes pass down through slots in it, so the rope overhead runs past the clear overhead and is the larger figure. Where the machine is mounted inside the hoistway below the slab, the sheave centreline falls under the clear-overhead datum and the relationship reverses. Either way the two are not interchangeable: rope overhead is used for rope length, elongation and rope dynamics; clear overhead is used for elevator clearance requirements.

Where the rope overhead is consumed

Rope order length. It appears twice per rope on an overhead 1:1 job, four times at 2:1, and six times on a basement 2:1 — see the four reeving formulas.
Elongation. The car-side rope length at the lowest landing is travel + overhead, and elastic stretch is proportional to that length.
Rope dynamics. At the top landing the rope is shortest, so the overhead sets the upper end of the frequency sweep — the end where rising rope frequencies meet sheave excitation.

Effect of a 1 ft change in overhead

Job: Basement machine, 2:1, rise 200 ft, pit 8 ft, 6 hoist ropes
Base: 4·Rise + 2·Pit + 6·OH

Overhead 8 ft → base 864 ft → 890 ft per rope → 5,340 ft ordered

Overhead 9 ft → base 870 ft → 900 ft per rope → 5,400 ft ordered

1 ft of overhead changes the order by 60 ft. Overhead appears six times in the base length, so the base rises 6 ft; that crosses the 10 ft ordering increment, adding 10 ft to each of the 6 ropes.

The error runs in both directions. An overhead taken to the slab rather than the sheave centreline produces ropes longer than required, which are then cut back. An overhead taken from the wrong floor datum on a job with a raised machine-room floor can produce ropes that are short.

State the datum on the drawing. Identify the dimension explicitly — for example, Rope OH = 8'-0" from top landing FFL to drive sheave CL. This avoids confusion with architectural or code-clearance overhead dimensions.

Run a rope calculation