Span, bay and eaves height: the three numbers that decide the steel.

Span

Span drives the bending moment in the arch, and bending moment drives the section. Going from 6.2 m to 8 m at the same load and covering can often be done with the same 30×60 section; going to 9.6 m usually cannot, and 12 m needs either a heavier oval or a truss. That is the single most common cause of a quotation that looks cheap and turns out to be undersized.

Indicative; the governing factor is the load case, not the span alone.

Bay

Bay affects how many arches, how many purlin runs and how long each gutter section is. A 4.0 m bay is the export standard because it fits container and transport practice; a 4.5 m bay is common in glasshouses where fewer columns mean less obstruction.

Bay also sets the purlin span — the distance a purlin has to bridge between arches. Increase the bay and the purlin either needs a deeper profile or closer arch spacing. Buyers who widen the bay to save arches often pay it back in purlins.

Eaves height

Eaves height is a climate decision disguised as a structural one. A taller house has more air volume above the crop, which buffers temperature and improves ventilation; it also has longer posts, more bracing and more film. In tomato and cucumber houses 4.0–6.0 m is normal; in leafy crops 2.5–3.5 m is enough.