The job
Modelled, not recorded. Every figure here is the answer the Roof Tile and Slate Calculator (Gauge, Battens and Ridge Tiles) gives for these inputs, written down in both unit systems and re-run whenever the site is built. No job was carried out, so there is no measured overrun on this page.
The house is a semi-detached house with a plain pitched roof: two slopes meeting at a ridge, a gable at the far end and the party wall at the other, with no hips, valleys or dormers. The old covering has reached the end of its life and is being stripped to the rafters, felted, battened and tiled again, so every batten and every tile is new.
The roof measures 81 m² (871.9 sq ft) on the slope, both slopes together; the ridge is 9 m (29.53 ft), each slope is 4.5 m (14.76 ft) from eaves to ridge, and the eaves and verges add up to 18 m (59.06 ft) and 18 m (59.06 ft). Those edge lengths are what the calculator uses for the ridge tiles and for any special tiles at the eaves and verges.
The tile is Marley Ludlow Major, interlocking, 420 × 330 mm (16.5 × 13 in), one of the makers' tiles the calculator lists with its published technical figures: the gauge it can be laid at, the width it covers and the tiles per square metre at the standard headlap. Interlocking tiles are single-lap, which is why they cover so much more than a traditional plain tile, and why the variant below matters.
The inputs, exactly as entered
| Input | Value | Why |
|---|---|---|
| Roof Area, Measured on the Slope | 81 m² (871.9 sq ft) | Both slopes measured on the slope, eaves to ridge and verge to party wall, not the plan area of the house. |
| Tile or Slate | Marley Ludlow Major, interlocking, 420 × 330 mm (16.5 × 13 in) | Chosen from the calculator's list, so the maker's own gauge, cover and coverage are used rather than a typed estimate. |
| Ridge Length | 9 m (29.53 ft) | The length of the ridge from the gable verge to the party wall; it decides the number of ridge tiles. |
| Hip Length | 0 m (0 ft) | Left at the calculator's own default. |
| Eaves Length | 18 m (59.06 ft) | Both eaves together, front and back. |
| Verge Length | 18 m (59.06 ft) | The gable verge on both slopes, entered as the total length of verge; the party wall end has none. |
| Rafter Length, Eaves to Ridge | 4.5 m (14.76 ft) | Eaves to ridge along one slope, which sets how many batten courses each slope takes. |
| Roof Slopes | 2 | Left at the calculator's own default. |
| Batten Stock Length | 3.6 m (11.81 ft) | Left at the calculator's own default. |
| Cutting Allowance (%) | 5 | Left at the calculator's own default. |
The working, step by step
1. Batten gauge
345 mm (13.58 in)
The gauge is the distance between battens, and so the length of tile showing in each course: 345 mm for this tile at its standard headlap, the maximum the maker publishes. Everything else on the page follows from it, because a smaller gauge means more courses of tile and of batten.
2. Covering capacity
9.83 tiles/m² (0.91 tiles/sq ft)
At that gauge and the tile's cover width, one square metre of roof takes 9.83 tiles/m², the maker's covering capacity. It is the number that turns the roof's area into tiles, and the one to check against the data sheet before ordering.
3. Tiles for the roof area, before cutting
796 tiles
The roof area times the covering capacity gives 796 tiles. The 5 % cutting allowance is then added and the result rounded up to give the headline; on a plain two-slope roof most cutting is at the verges and the ridge.
4. Batten courses on each slope
15 courses
The calculator sets out the courses up each slope from the rafter length and the gauge, with a course at the eaves and one under the ridge: 15 courses per slope. That count, not the area, is what the battens are cut to, because every course runs the full width of the slope.
5. Battens at the maker's coverage, area over gauge
234.8 m (770.3 ft)
The maker's own estimate of batten is the area divided by the gauge: 234.8 m. The calculator shows it beside its course-by-course count so the two can be compared; the course count is the larger, because each course is a full run.
6. Battens, total run
270 m (885.8 ft)
Courses times the length of each course gives 270 m of batten to fix, which the calculator then cuts into stock lengths course by course. The offcut at the end of each course is not carried to the next, which is how battens are really cut on a roof.
7. Ridge tiles
20 tiles
The ridge length divided by the cover of one ridge tile, rounded up, gives 20 tiles. A gable roof with no hips needs no hip tiles, and an interlocking tile needs no special course at the eaves, which is why those rows are zero on this roof.
Roof tiles or slates
836 tiles
The order is 836 tiles, 90 lengths of batten at the stock length entered, and 20 tiles. Tiles are delivered by the pack and loaded out across the roof before laying so the weight is spread. The count is a quantity of tile, not a specification: the batten size and the fixing of every tile against wind uplift are set by the maker's instructions for the roof's pitch and exposure, and by the fixing specification, which the calculator does not decide.
What to buy
| Quantity | Figure | What it is for |
|---|---|---|
| Roof tiles or slates | 836 tiles | Interlocking concrete tiles of the chosen pattern, with the cutting allowance included. |
| Battens in stock lengths, cut course by course | 90 lengths | Treated roofing battens in the stock length entered, counted course by course. |
| Ridge tiles | 20 tiles | Ridge tiles to suit the tile, for the one ridge. |
The allowance, and what it does here
The 5 % cutting allowance goes on the tile count once. On a simple gable roof most of it goes at the verge, where tiles are cut or a half tile is needed to finish the course, and along the top course under the ridge. A roof with hips and valleys cuts far more and wants a larger allowance. The battens carry no percentage of their own: the course-by-course count already includes the offcut at the end of every course.
The common mistake
Ordering battens from the area alone
The quick estimate for battens is the area divided by the gauge, which is how a maker's data sheet states it, and the calculator prints that figure too. It assumes the battens are used end to end with no offcut. On a real roof each course runs from verge to party wall and the last length of every course is cut, so the course-by-course count is larger, and on a roof of this size the difference is many lengths. Order from the course count, and keep the maker's figure as a cross-check.
Change the job, and see what moves
The same roof, in a clay plain tile
Tile or Slate: Marley Hawkins clay plain tile, double lap, 265 × 165 mm (10.4 × 6.5 in) → 5,155 tiles
Re-cover the same roof in Marley Hawkins clay plain tile, double lap, 265 × 165 mm (10.4 × 6.5 in) instead and the count jumps to 5,155 tiles. A plain tile is double-lap: every part of the roof is covered by two thicknesses, so the gauge falls to 100 mm and the coverage rises to 60.61 tiles/m², and the roof needs eaves and verge specials as well. It is a different job, and usually a heavier covering per square metre, which is a question for a surveyor before a lighter roof is replaced with a heavier one.
The same roof, with battens bought in shorter lengths
Batten Stock Length: 2.4 m (7.87 ft) → 836 tiles
Buy the battens in 2.4 m lengths instead and the tiles are unchanged, but the battens rise to 120 lengths. More, shorter lengths mean more joints, which must land on rafters; the total run is the same, so the stock length is a matter of handling and joints, not of tile count.
What to run next
- Roof Covering Fixing Calculator
Each tile is fixed against wind uplift to a specification for the roof's exposure, and the clips and nails are counted from the same tile count.
- Roof Batten Spacing Calculator (Tile Roofing)
Sets out the gauge on the actual rafter length so the top course lands correctly under the ridge.
- Roofing Underlayment Calculator
The felt or membrane under the battens covers the same roof area and is bought in rolls.
Sources
The method and every constant behind these figures belong to the Roof Tile and Slate Calculator (Gauge, Battens and Ridge Tiles), version 1.0.0, last verified 2026-10-05. It rests on:
- Marley, Modern interlocking tile technical specification: 420 × 330 mm; minimum headlap 75 mm (22.5° and above); maximum gauge 345 mm; cover width 292 mm (nominal); covering capacity 9.9 tiles/m² at 75 mm headlap and 10.7 at 100 mm; battens required 2.9 lin.m/m² at 75 mm headlap and 3.1 at 100 mm; 38 × 25 mm battens to 450 mm rafter centres, 50 × 25 mm to 600 mm
- Marley technical specifications for Ludlow Major (420 × 330 mm, maximum gauge 345 mm, cover width 295 mm, 9.8 tiles/m² at 75 mm headlap), Edgemere (420 × 330 mm, 345 mm, 298 mm, 9.7 slates/m²) and Ludlow Plus (387 × 230 mm, maximum gauge 312 mm, cover width 204 mm, 15.7 tiles/m², 3.2 lin.m of batten/m²)
- Marley plain tile technical specifications: concrete plain tile 267 × 168 mm and Hawkins clay plain tile 265 × 165 mm; minimum headlap 65 mm on a roof; maximum gauge 100 mm on a roof; 60 tiles/m² and 10.0 lin.m of batten/m² at 100 mm gauge
- Marley (Kevin Taylor, Training and Technical Support Manager), setting out tiling battens: the maximum gauge is the full length of the tile less the minimum headlap (420 − 75 = 345 mm), never to be exceeded and usually eased so the gauge is even; marked up from the eaves course datum to the top batten under the ridge; plain tiles need an under-eaves batten, and room at the ridge for the top course, an additional batten and an eaves/tops tile; battens to BS 5534 are supplied in lengths of at least 3 m, typically 3.6 m or more, at least 1.2 m long and over three rafters when fixed
- Marley (Kevin Taylor), roof tile sizes, how many roof tiles per square metre: for a rough estimate multiply the size of the roof in square metres by the roof covering capacity, for example 100 m² of Marley Acme plain tiles at 60 a square metre, 100 × 60 = 6,000 tiles, with nothing added; a different gauge or headlap changes the figure
- Marley concrete plain eaves/top tile and tile-and-a-half: eaves have a double course throughout their length using purpose-made eaves tiles for the undercourse; the tile-and-a-half is required to achieve the broken bond laying pattern
- Marley ridge and hip tiles: angled hip and ridge tiles typically 245 or 260 mm across, in lengths of 450 mm for clay and 457 mm for concrete
- Bristile Roofing, Concrete Roof Tiles Technical Manual, Classic product specification: 440 × 335 × 35 mm; set-out 360 mm maximum; 9.26 tiles per m²; 2.5 ridges per lineal metre; 2.9 m of batten per m²; ridge capping is a tile used for both the ridge and hips; overlapping ridge tiles laid away from the prevailing wind; installation to AS 2050
- BS 5534:2014+A2:2018, Slating and tiling for pitched roofs and vertical cladding — Code of practice: the UK standard behind the headlaps, gauges and battens the makers' specifications set; its tables are not reproduced here
Frequently asked questions
- How many roof tiles do I need for a 81 m² roof?
- In Marley Ludlow Major, interlocking, 420 × 330 mm (16.5 × 13 in), at 9.83 tiles/m², the roof takes 796 tiles before cutting and 836 tiles with the 5 % allowance.
- How many battens for re-tiling a roof?
- Counted course by course, 15 courses on each slope, the roof takes 270 m of batten, or 90 lengths at the stock length entered. The maker's area-over-gauge estimate is 234.8 m.
- Why do plain tiles need so many more tiles than interlocking ones?
- Because they are double-lap and smaller. On this roof a plain tile needs 5,155 tiles against 836 tiles for the interlocking tile.