Bay spacing: the quiet variable in every quote

By Johnny · 23 October 2026 · 7 min read

Ask an owner what their building's bay spacing is and most will not know. It is not in the enquiry, not in the layout drawing, and not in the comparison spreadsheet. It appears in the drawings as a dimension and then simply exists, chosen by whoever detailed the building, using a habit that was sensible for a different project.

That is a wasted opportunity, because bay spacing is a dial with a genuinely useful middle range — and because it constrains things the operation cares about, from door positions to future racking.

What a bay is

A steel building is a series of identical frames, repeated along its length. The distance between one frame and the next is the bay. The number of bays is set by the total length divided by the spacing, with the end bays often treated slightly differently.

Two numbers describe the building plan and they behave in opposite ways:

• Span is across the width — the distance the rafter has to cross. Widening it makes members much heavier.
• Bay spacing is along the length. Widening it makes the main frame slightly heavier but reduces how many frames there are.

So bay spacing is the one dimension where going bigger can save material overall. That is why it deserves a conversation.

What wider bays buy and cost

What you gain:

• Fewer frames — fewer columns, fewer base plates, fewer anchor bolt sets, and a smaller foundation count.
• Fewer crane picks on the main frame, which matters on a site with limited lifting capacity.
• More uninterrupted length inside each bay, which suits long goods, vehicle circulation and racking runs.

What you pay:

• The rafter must carry more. A wider bay means each frame picks up load from a longer strip of roof, so the rafter and column sections grow.
• Purlins and side rails span further. The secondary members between frames have to cross the bay, so their sections grow too, and at some point they need additional support.
• Cladding spans further between supports, which affects sheet thickness and how it behaves in wind and during installation.
• Lifting gets heavier per piece. Fewer, bigger picks. On a site with a small crane this is the constraint that decides the answer.

What narrower bays buy and cost

The mirror image: more frames, more foundations, more erection work, but lighter individual members and easier handling. Narrower bays are also more forgiving when the site has poor access or the available crane is small.

There is a practical floor here too. Below a certain spacing the frame count becomes the dominant cost and the building is paying for columns it does not need.

Three constraints that are more important than the theoretical optimum

1. Doors and openings must sit in a bay

Doors are framed between columns. If your layout needs a specific opening position — a loading dock at a fixed point, a machine access door aligned with a production line — the bay grid has to accommodate it. Discovering this after the grid is fixed means either moving the door or adding a secondary frame.

2. A crane bay has its own rhythm

If an overhead crane is involved, the crane runway columns interact with the bay grid. Bracket positions, rail joints and the crane's own wheelbase all want to align with the structure. This is where a crane added later becomes expensive: the grid was never designed for it.

3. Racking and storage modules

If the building will hold racking, the racking layout has a module of its own. A column line that lands on a rack line costs nothing; one that lands in an aisle costs usable area. Aligning the two is free at design stage.

Length, and the last bay

Total length rarely divides evenly by the chosen spacing. When it does not, the remainder is absorbed into one bay, which becomes a different size from the rest. That is normal, but it has consequences: a short final bay can be awkward for pallets or vehicles, and it produces a frame that is not identical to its neighbours, which slightly complicates fabrication and erection.

If the length is still negotiable, choosing it as a multiple of the intended bay spacing removes the problem entirely. If it is fixed by the site boundary, plan where the odd bay should fall — usually at an end where it will not obstruct circulation.

The mistake I see most

Letting the bay spacing be decided by the detailer's default and then discovering, at the layout stage, that the doors do not line up with the operations. The building is structurally fine and operationally wrong.

The fix is cheap: state the bay spacing in the enquiry. It forces the supplier to price the same grid, which makes quotes comparable, and it makes you check the door positions before the drawings are final rather than after.

Ask for it as a line in the quote: “bay spacing: X metres, Y bays, total length Z”. If a supplier cannot tell you what grid they priced, they have not yet priced your building — they have priced their habit.