Posts and Footings for a Cantilever
Cantilever posts take a load pattern that no other gate produces: one pushing down and one pulling up, only feet apart. The footing guidance at Mount Pleasant Sliding Gates Narcoculture is sized from that couple rather than from leaf weight, because sliding installations on Mount Pleasant sand fail at the footing far more often than at any piece of hardware.
The Force Pattern
The front truck carries the weight of the steel frame plus the download from the overhung section. The rear truck resists an upward pull from the counterbalance. Those two forces acting in opposite directions across a short spacing create a rotating couple that the footings have to resist, and it is much larger than the gate weight alone suggests.
Why Spacing Matters So Much
Widening the distance between the two posts reduces both forces proportionally. Bringing them together to fit a tight run of side room raises them just as fast. On a constrained site the temptation is always to bunch the posts, and that single decision explains most of the alignment complaints on a cantilever gate.
Sandy Coastal Ground
Sand has low lateral resistance and a high water table reduces it further. A footing that would be adequate in clay can be undersized here by a wide margin, and the failure is gradual rather than sudden: the post rotates a degree at a time and the leading edge of the gate noses down over a season or two.
Getting It Right
Diameter matters more than depth for resisting rotation, since the resistance grows with the width of the bearing face against the soil. A concrete footing needs full cure before the gate is hung, and hanging early is a common cause of a post that never sat plumb. Galvanized post stock is worth the difference where the water table is high. Confirm the footing dimensions in writing before the hole is dug.
Checking an Existing Installation
Put a level on both posts in both directions at every service and record the readings. A cantilever post that has moved half a degree is reporting a footing problem while it is still cheap to address, and a leaf that has started grounding at its leading edge is reporting the same problem much later.