A rule-of-thumb DIY materials and labour estimate for a single-span timber mid-floor — joist size, spacing, hangers or skew-nail fixings, blocking, and flooring sheets. Enter the span between your two support lines (walls or bearers) and the floor's width.
Rule-of-thumb spans by joist spacing and size — indicative SG8 radiata figures for a single span under standard residential floor loading, not a citation of the actual NZS 3604 span tables. Your current selection is highlighted; a span past every row here is outside what this tool can spec at all.
| Spacing | Joist size | Max span |
|---|---|---|
| 400mm | 90 x 45mm | 1.65 m |
| 400mm | 140 x 45mm | 2.60 m |
| 400mm | 190 x 45mm | 3.40 m |
| 400mm | 240 x 45mm | 4.00 m |
| 400mm | 290 x 45mm | 4.50 m |
| 450mm | 90 x 45mm | 1.55 m |
| 450mm | 140 x 45mm | 2.45 m |
| 450mm | 190 x 45mm | 3.20 m |
| 450mm | 240 x 45mm | 3.80 m |
| 450mm | 290 x 45mm | 4.30 m |
| 600mm | 90 x 45mm | 1.30 m |
| 600mm | 140 x 45mm | 2.10 m |
| 600mm | 190 x 45mm | 2.75 m |
| 600mm | 240 x 45mm | 3.30 m |
| 600mm | 290 x 45mm | 3.70 m |
One stock length per joist, cut to span — not spliced from offcuts the way facing boards are.
Two nails skew-nailed at each end of every joist, into the top plate or bearer below.
Solid blocking between joists so they can't roll under load — more rows as the span grows.
Includes an allowance for trimming and laps — not a literal sheet-by-sheet cutting plan.
Rule-of-thumb estimate for a straightforward single-span floor — not an engineered design. Actual joist sizing depends on grade, load width, and floor loading; if in doubt, check the full NZS 3604 span tables or get a professional opinion before you build.