What is on centre (o.c.)?
On centre (o.c.) is the distance measured from the centre (heart) of one structural element to the centre of the next. It is the standard measurement in construction for the spacing of rafters, studs, joists, purlins and other repeating elements.
Why on centre?
When you measure the distance between two beams, you can measure from edge to edge (clear space between them) or from centre to centre. In construction, o.c. is the standard, because:
- Sheet materials fit — Standard sheets (plasterboard, insulation, OSB) are 60 or 120 cm wide. At 60 cm o.c., each sheet spans exactly three studs.
- Insulation fits — Insulation batts are designed for 60 cm o.c. (they are 58-59 cm wide and friction-fit between the studs).
- Easy to set out — You always measure the same distance, regardless of the element width.
Standard o.c. spacings
| Element | Standard o.c. |
|---|---|
| Studs (timber frame) | 60 cm |
| Framing (partition wall) | 60 cm |
| Rafters | 60 cm |
| Purlins | 60-100 cm |
| Trusses | 80-120 cm |
| Deck joists | 40-50 cm |
| Gutter brackets | 60-80 cm |
How do you measure o.c.?
- Place the first element — Position a stud
- Measure 60 cm — From the centre of the first stud
- Next element — The centre of the next stud is at 60 cm
- Repeat — Always 60 cm from the previous centre
> Note: The clear space between two studs of 45 mm width at 60 cm o.c. is 60 – 4.5 = 55.5 cm (not 60 cm). This is exactly the width of standard insulation batts.
Related terms
- Framing
- Rafter
- Timber frame construction
- Truss
- Purlins