28 free tools · 25 site calculators plus HAV, 6×6 risk & accreditation · no sign-in · guidance only
All 28 tools
Roofing · gauge · battens
How many roof tiles or slates do I need?
Gauge from headlap, tiles per square metre from gauge, battens from gauge. Slates lap over two courses below, single-lap tiles over one — the calculator switches the formula with the type.
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.
Single lap (interlocking tile): gauge = length − headlap
Per m² = 1 ÷ (gauge × cover width)
Batten length = roof area ÷ gauge
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.
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 slates per square metre?
1 ÷ (gauge × cover width). A 500 × 250 mm slate at 90 mm headlap has a 205 mm gauge and needs about 19.5 per square metre; the page shows the figure for your slate.
What is the difference between double lap and single lap?
A slate or plain tile laps the course two below it, so the gauge is (length − headlap) ÷ 2. An interlocking tile laps one course, so the gauge is length − headlap. Choose the covering and the calculator switches.
How many battens?
Roof area ÷ gauge gives the total batten length, then it is divided into 4.2 m lengths. Increase the headlap for exposure or a shallow pitch and the batten count rises with it.
Why does it flag an impossible lap?
If the headlap is bigger than the tile the gauge goes to zero or below, so the page says to check the two figures instead of printing nonsense.
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.
