How to count rafters, battens and posts correctly with the fence-post principle — and why there is always one item more than gaps when an item sits at both ends.
Introduction
Centre-to-centre spacing (cc) is the distance between the centrelines of two adjacent members — rafters, battens, posts, joists. When counting the number along a run, it's easy to be out by one member.
The fence-post principle
If you divide a stretch into n equal gaps, there are n+1 points — just as a fence with n bays has n+1 posts. The extra item comes from there being an item at each end.
Example: a run of 6.0 m at 0.6 m centres gives 6.0 / 0.6 = 10 spacings, i.e. 11 members when there is a rafter at each end.
When there are no items at the ends
- 01Member at both ends: count = spacings + 1.
- 02Member at only one end: count = spacings.
- 03No items at the ends: quantity = gaps − 1.
Typical cc values — and what controls them
A c/c value is rarely random: it is usually tied to a board format, a roof tile's lap length or the load the structure must bear. The table shows the orders of magnitude most commonly encountered — but the final c/c comes from the laying instructions, the board supplier or the structural calculation, not from a rule of thumb.
| Subject | Typical ml | What controls it |
|---|---|---|
| Posts in light frame wall | 0,45 eller 0,60 m | Plasterboard width (900/1200 mm) and load |
| Beams in floor separation | 0,60 m (ofte) | Span, load and floor panel format |
| Floor spreader | 0,60 m (ofte) | Floor board / plate type and load |
| Roof rafter | typisk 0,60–1,0 m | Roof covering, snow load and rafter type |
| Roof batten | after the roof tile's covering length | Manufacturer's assembly instructions |
The cc values are orders of magnitude from common building practice, not standard requirements — the associated field "What controls it" is what actually determines the figure in the specific project.
cc and plate formats go together
Boards come in fixed widths, and their long edges must be supported by a stud. A 900 mm plasterboard fits c/c 450 mm (two bays per board); a 1200 mm board fits c/c 600 mm. If 900 mm boards are laid on c/c 600 mm, one long edge of the board is unsupported and must be supported with extra joists or edge profiles. Therefore you choose c/c so board joints always land on a stud.
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