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Carpentry · rafters · battens
How many rafters and battens do I need?
Rafters at centres with four trimmers added for every rooflight, battens by gauge with four spare — straight off the old roof sheet, which had that +4 written into it.
Measure it
Every box has its own unit next to it.
Change the unit beside any box and the number converts. The (i) beside each question says what the box wants and where the figure usually comes from. Anything marked per cent is a percentage of the quantity above it.
Check the rates
Rates and assumptions used
These are the figures the calculator works to. Change any of them and the order list updates — your merchant’s pack sizes and your own waste allowances beat the defaults every time.
Order list
The orange figure is the one you order to. Highlighted rows are the other numbers you order to. The small print under each row is the sum that produced it, so you can check it against your own take-off. Amber rows are warnings, not quantities.
How this one works
- Rafters per slope =
roof length ÷ centres + 1 - Trimmers =
rooflights × 4— two each side of every opening - Batten rows =
rafter length ÷ gauge, because battens run up the slope, not across the plan - Batten length =
rows × roof length × slopes
The figures the calculator works to. Every one can be changed in step 2, and your merchant’s pack sizes and your own waste allowances beat these defaults every time.
| Rate | Default | Why |
|---|---|---|
| Trimmers per rooflight | 4 no. | Two trimmers and two extra rafters either side. The old sheet’s figure. |
| Spare batten rows | 4 no. | Eaves course, ridge course and the two that always get cut wrong. |
| Batten length | 4.2 m | |
| Timber length | 4.8 m | Order length for ridge board and wall plate. |
| Breather membrane roll | 100 m² | A 1.5 m × 50 m roll before laps. |
| Membrane laps | 15 % | |
| Insulation board | 2.88 m² | 2400 × 1200. |
A worked example
With the defaults the page opens on:
- How long is the roof along the ridge? 9 metres
- How long are the rafters? 4.2 metres
- What centres are the rafters at? 600 millimetres
- How many slopes? 2 number
- How many rooflights? 2 number
- What is the batten gauge? 300 millimetres
Rafters
- Common rafters per slope: 16 no. — 9 m ÷ 600 mm + 1
- Trimmers: 8 no. — 2 rooflights × 4
- Total rafters: 40 no. — 16 a slope × 2 slopes + 8 trimmers
- Rafter timber: 168 m — at 4.2 m each
- Ridge board: 2 × 4.8 m — 9 m roof ÷ 4.8 m lengths, rounded up
- Wall plate: 4 × 4.8 m — 9 m × 2 ÷ 4.8 m lengths, rounded up
- Noggins / struts: 24 m — rafters × centres
Battens & covering
- Batten rows per slope: 18 rows — rafter length ÷ 300 mm gauge, +4 spare
- Batten length: 324 m — rows × roof length × 2 slopes
- Battens: 78 × 4.2 m — 324 m ÷ 4.2 m lengths, rounded up
- Roof area: 75.6 m² — rafter length × roof length × slopes
- Breather membrane: 1 roll — +15% laps, 100 m² a roll
- Insulation between rafters: 27 sheets — at 2.88 m² a sheet
Change any figure above and every line recalculates; the small print under each row is the sum that produced it.
Questions people ask
How many rafters do I need?
Roof length ÷ centres + 1, per slope, plus four trimmers for every rooflight. For a 9 m roof at 600 mm centres with two slopes and two Veluxes, that is 40 rafters.
Why does it add four spare batten rows?
The eaves course, the ridge course and the two that always get cut wrong. The old roof sheet had that +4 written into it and it has never been wrong.
Do battens run up the slope or along it?
Along the roof, so the number of rows comes from the rafter length ÷ the gauge, and the batten length is rows × roof length × slopes.
I only have the pitch, not the rafter length
Use the roof area from pitch calculator first - it gives the rafter length from the span and the pitch - then bring that figure here.
Calculators for the same job
Doing this for a whole job rather than one wall? SiteLedger keeps the valuations, supplier statements, invoices and variations against the project they belong to.
Read before you order. These work out quantities, not structural design — spans, lintels, rebar and foundation depths need an engineer and Building Control sign-off. Waste allowances are defaults, not promises: awkward cuts, long runs and bad weather all eat more than 5%. Where a merchant sells whole packs, the figure is rounded up to whole packs.