"At what spacing should the joists go?" is one of the first questions a framing layout has to answer. On-centre spacing — how far apart the joists sit — shapes how the floor performs and how the plan reads. This article explains what spacing means in the context of a preliminary framing plan and how to represent it well. It is orientation, not prescriptive sizing: actual spacing and member sizes are an engineering decision.

If you are new to the drawing, what a floor framing plan is sets the scene, and the complete floor framing plan guide puts spacing in context.

What on-centre spacing actually measures

On-centre (often written o.c.) is the distance from the centreline of one joist to the centreline of the next — not the clear gap between them. So joists at "400 o.c." have their centres 400 mm apart. This matters because sheet materials and finishes are sized to land on joist centres, so the spacing has to be consistent across the floor.

Common spacings you will see:

  • Metric: 300, 400, 450, or 600 mm o.c.
  • Imperial: 16" or 24" o.c.

How spacing interacts with span

Spacing never works alone — it is always in tension with span (how far each joist reaches between supports) and member size. The general relationships, all else equal:

  • Closer spacing shares the floor load among more joists, so the floor is stiffer and can carry more for the same size and span. The cost is more material and labour.
  • Wider spacing uses fewer joists but asks more of each one, so you may need a deeper or stronger member, or a shorter span.
  • Shorter spans make almost everything easier — which is why running joists across the shorter dimension of a room is a common starting move.

In Framing Plan, changing the spacing updates the span status live, giving you a quick preliminary read on whether the layout sits in a sensible range. Treat that as a sketching aid, not a code check.

Representing spacing on the plan

Good spacing communication on a framing plan comes down to a few habits:

  • Draw the joist field at a consistent spacing across each bay.
  • Label it with a clear callout, e.g. J1 @ 400, beside the span arrow.
  • Keep spacing uniform within a bay; change it deliberately and label the change where the structure genuinely differs.
  • Make sure the spacing you draw matches the spacing in the schedule and notes.

For where this fits in the overall sequence, see how to draft a preliminary floor framing plan.

Choosing a starting spacing

For a preliminary layout, pick a standard spacing typical for the floor product and span you have in mind, run the joists the efficient direction, and check the live span status. Iterate: if the layout looks marginal, try closer spacing or a shorter span. The goal at this stage is a sensible, buildable starting point — not a final answer.

Leave the numbers to the engineer

Span tables and spacing rules exist, but the authoritative spacing and member sizing for a real floor depend on loads, materials, and local code, and must be confirmed by a qualified engineer. Use the preliminary plan to explore and communicate spacing intent — then have it reviewed.

Try it

Set a spacing, flip the span direction, and watch the status update as you go. Open the framing studio to experiment with joist spacing in the browser.