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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.
1
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.
2
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.
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.
Rafter timber: 168 m — at 4.2 m each
Ridge board: 2 × 4.8 m
Wall plate: 4 × 4.8 m
Noggins / struts: 24 m — rafters × centres
Battens & covering
Batten rows per slope: 18 rows — rafter length ÷ 300 mm gauge, +4 spare
Breather membrane: 1 rolls — +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.
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.
