Ordering slate is one of those jobs where a pallet too many is annoying and a pallet too few is a lost week. The good news is that the arithmetic is straightforward. Measure the roof properly, set the headlap from the pitch, work out the gauge, and multiply. Done in that order, you will land within a slate or two of what the job actually needs.
Measure the Slope, Not the Footprint
The most common mistake is to price the job off the building's footprint. A roof at 40 degrees has considerably more surface area than the plan view suggests, and that difference is exactly what you are buying.
You have two reliable options. The first is to measure the rafter length directly — along the slope from the eaves to the ridge — and multiply by the width of that slope. This is easiest from inside a loft with a tape, or from a ladder with a helper. The second is to measure the horizontal run and the pitch, then convert:
Rafter length = plan length ÷ cos(pitch)
A 5 metre horizontal run at 40 degrees gives 5 ÷ 0.766 = 6.53 metres of rafter. That extra metre and a half is roughly 30 per cent more slate, batten and labour than the plan view implies.
If you need to establish the pitch, measure a level 300 mm along the roof and check the vertical rise over that distance. Pitch is rise ÷ run, so a 150 mm rise over 300 mm is 0.5, which is about 27 degrees. A digital angle finder or a phone app will do the same job faster, provided the phone is actually sitting flat on the slope.
Treat every slope as a separate calculation. Gables, hipped ends, dormers, mansards and lean-to sections all have different rafter lengths, and adding them together before you have each figure invites error.
Set the Headlap From the Pitch
Slate is a double-lap covering, which means each slate overlaps the one two courses below it. The amount of that overlap — the headlap — is dictated by the pitch. Steeper roofs shed water faster and need less overlap; shallow roofs need more.
Typical minimum headlaps for natural slate are roughly:
- 20–25 degrees: around 100 mm
- 25–30 degrees: around 75 mm
- 30–45 degrees: around 65 mm
- Over 45 degrees: around 55 mm
These are common industry figures, not gospel. Confirm the requirement against BS 5534 and, more importantly, against the specific slate you are buying — thickness, surface texture and the manufacturer's own guidance all affect the minimum. Below about 20 degrees, natural slate generally is not suitable without specialist detailing, so check before you commit.
Once you have the headlap, the gauge follows:
Gauge = (slate length − headlap) ÷ 2
A 500 mm slate with a 65 mm headlap gives a gauge of 217.5 mm. Rounding down slightly to 215 mm buys you a little extra overlap and costs very little. Rounding up is the wrong direction — it eats into your headlap and can leave the roof vulnerable in driving rain.
Turn That Into a Slate Count
Each slate covers one rectangle: its width multiplied by the gauge. For a 500 × 250 mm slate at 215 mm gauge, that is 0.25 × 0.215 = 0.05375 square metres.
Divide the total roof area by that figure and you have your slate count before waste:
Slates = roof area ÷ (slate width × gauge)
It is worth cross-checking with a course count, because it catches arithmetic slips and tells you how the slates will actually sit. Slates per course is the eaves-to-eaves width divided by the slate width. Number of courses is the rafter length divided by the gauge, rounded up, plus the eaves course. Multiply the two and you have the same answer from a different direction.
A Worked Example
Take a straightforward gable roof: 6 metres of rafter per slope, 9 metres from verge to verge, 35 degrees pitch, using 500 × 250 mm slates.
- Roof area per slope: 6 × 9 = 54 square metres.
- Both slopes: 54 × 2 = 108 square metres.
- Headlap at 35 degrees: 65 mm, so gauge = (500 − 65) ÷ 2 = 217.5 mm. Use 215 mm.
- Slates per course: 9000 ÷ 250 = 36.
- Courses: 6000 ÷ 215 = 27.9, so 28 courses, plus one for the eaves = 29.
- Slates per slope: 29 × 36 = 1,044.
- Both slopes: 2,088 slates.
Check it against the area method: 108 ÷ 0.05375 = 2,009. The two figures sit within a few per cent of each other, which is what you want to see. Take the higher one and move on to waste.
Waste, Extras and the Details People Forget
Waste on slate is not padding. It covers cuts at verges, breakages during handling, and the fact that a slate split on a Friday afternoon cannot be ordered on its own.
- Simple gable or lean-to: add 5 per cent
- Hipped, valleyed or with a couple of dormers: add 7.5 to 10 per cent
- Complex roof with multiple valleys, hips and abutments: add 10 to 15 per cent
Then count the bits that are not plain slates: ridge or bonnet tiles, hip slates or hip tiles, valley lining, secret gutters, dry verge systems, and any slate-and-a-half or wider slates you plan to use at verges to avoid narrow cuts. These are usually priced and ordered separately, and they are regularly forgotten.
Battens and Fixings
Battening runs on the same gauge you have just calculated. Multiply the number of courses by the width of the slope to get linear metres of batten, then add 10 per cent for cutting and for the extra rows at the eaves and ridge. Battens are typically sold in 3 metre and 4.8 metre lengths, so round up to the nearest whole length rather than ordering by the metre.
Underlay is ordered by the roll against the same roof area, plus an allowance for laps and for dressing into valleys and gutters. Nails or hooks are usually specified per slate — check the manufacturer's recommendation, and remember that some slate types and exposure conditions call for copper or stainless fixings rather than galvanised.
Before You Order
Write the numbers down slope by slope, with the gauge and headlap stated next to each one, and keep that sheet. When the delivery arrives, you can check the count against it rather than guessing from the pallets. Add your waste allowance last, after everything else is totalled, and order the whole job in one go so the slates come from a single batch and match in colour and thickness.
Finally, if the roof is anything beyond a simple gable, or if you are adding weight to an older structure, have the loading checked before the order goes in. Slate is heavy, and the cost of a professional opinion is small next to the cost of a sagging roof.
Photo: beasternchen / Pixabay
