How eave height gets decided, and what it costs you later

By Johnny · 23 October 2026 · 6 min read

Eave height arrives in most enquiries as a round number on a layout drawing: six metres, eight metres, sometimes ten. Nobody in the chain can say what it came from. It was probably copied from a similar project, or set by whoever drew the first sketch, and it will not be questioned until the equipment arrives.

That is the wrong order. Height is cheap to decide and expensive to change, because it is not a property of the cladding — it is a property of the frame.

First, be clear which height you are quoting

Three different numbers get called “height”, and mixing them up is common:

• Eave height — where the wall meets the roof, measured from the floor.
• Ridge height — the highest point, at the apex of the roof.
• Clear internal height — the usable height inside, under the lowest obstruction: a rafter, a sprinkler pipe, a crane rail, a light fitting.

What your operation cares about is the third one. What the supplier prices is the first one. The gap between them is where expensive surprises live, especially under a pitched roof where the underside of the rafter is below the apex. If you write “8 metres” in an enquiry, ask yourself which of the three you meant — and state it.

Start from the object that has to fit

Work backwards from the physical constraint. The usual candidates:

Two of these are habitually underestimated: a raised tipper body is much taller than the truck that carries it, and a machine needs clearance above it for maintenance, not just for its own envelope.

Then add what the use implies

Above the object, add the layers that the operation will inevitably want:

• Services layer. Sprinklers, cable trays, ventilation ducts and light fittings all hang below the structure. On a tall building this is easy; on a marginal one it silently eats the clearance you thought you had.
• Door and opening allowance. A roller door needs track and a head unit above the opening. A large door in a low eave wall forces either a taller building or a smaller door.
• Ventilation and heat. In a hot climate, an uninsulated hall with no forced ventilation stratifies: hot air collects at the top and the usable cool zone is lower than the geometric height suggests. Height without ventilation does not automatically mean comfort.
• Future mezzanine. If a floor may come later, the height has to accommodate the floor structure plus headroom above it. Adding a mezzanine under an existing roof usually means either a low floor or a new roof.

What height actually does to the price

Raising the eave does two things to the structure at the same time, and they compound:

The column gets longer and weaker in buckling. A slender column resists less for the same section, so the section has to grow. On a tall, narrow building this effect is not marginal.

The wind area grows. Taller walls catch more wind, and that load has to travel through the frame into the foundation. So the foundation gets bigger too — which is not a steel cost and is frequently overlooked when someone compares two heights informally.

There is also a tipping point in method: past a certain height, the erection crew needs different access equipment, and the cladding installation becomes slower and more weather-sensitive. None of that appears in a per-square-metre figure.

Why raising it later is expensive

Owners often accept a low eave with the idea of “raising it if we need to”. In practice that is close to rebuilding:

• Columns change. Extending a column means either a splice designed for it — rarely the case — or new columns.
• Cladding is consumed. Wall sheets are cut to the existing height. They cannot be stretched, and re-using them on a taller wall leaves you short.
• Bracing is invalidated. The stability system was designed for the original geometry.
• The building has to be empty. The work happens above whatever is stored or running inside.
• The foundation may not be adequate for the taller frame's overturning demand.

By contrast, adding a door, re-cladding a wall, or changing the roof insulation later are all reasonable jobs. That is the test: if the change touches the columns, do it now.

The mistake I see most

Setting height from the drawing and never checking it against the door. A building can be tall enough inside and still have the wrong wall height for the opening someone needs, because the door head, its track and the wall above it were never added up. The result is either a smaller door than the operation needs, or a late change to the frame.

Write the height as a statement, not a number: “clear height of X metres under the rafter, to allow a tipper body of Y metres to discharge, with a Z-metre wide door.” A supplier can price that. A bare number invites five different interpretations.