Summary
Ordering roof tiles is a two-part calculation that's easy to get wrong in either direction. The first part — field tiles for the main roof slopes — is a straightforward area-and-gauge calculation once the tile's coursing dimensions are known. The second part — ridge tiles, verge tiles or dry-verge units, and hip tiles — is calculated separately by running length along the roof's edges and junctions, not by area, and is the part most commonly forgotten or under-ordered on a first-pass materials list.
The single biggest variable in this calculation is the batten gauge, which is set by the specific tile's length and the minimum headlap required for the roof's pitch, exposure and wind zone under BS 5534:2014+A2:2018 — see roof tile types for the full standard and how it affects fixing specification. A large-format concrete interlocking tile might have a gauge of 330–345mm, needing roughly 10 tiles per m². A traditional clay plain tile, laid double-lap with a much shorter gauge, needs considerably more tiles per m² — commonly cited around 60, though this varies by manufacturer and should always be checked against the specific product's data sheet rather than assumed.
This article works through the calculation method step by step, with a full worked example for a simple gable roof, and gives typical (not universal) tile-per-m² figures for the main tile categories as a sense check — never a substitute for the manufacturer's own gauge and cover width figures for the exact product being ordered.
Key Facts
- Roof area is sloped area, not plan area — the horizontal footprint of a roof is always smaller than its actual covered (sloped) area once the pitch is accounted for; using the plan area under-orders tiles on every pitched roof
- Slope length formula — slope length = horizontal rafter run (eaves to ridge, measured on plan) ÷ cos(pitch angle); alternatively, measure the actual slope length directly on site with a tape along the roof face, which avoids any trigonometry error
- Batten gauge (single-lap tiles) — gauge = tile length − minimum headlap; single-lap tiles (most concrete interlocking and clay interlocking tiles) overlap once at the head
- Batten gauge (double-lap tiles) — gauge = (tile length − minimum headlap) ÷ 2; double-lap tiles (traditional clay plain tiles, and natural slate) always have two thicknesses of tile at any point on the roof, so the usable gauge is roughly half the tile's own length
- Tiles per m² formula — tiles per m² = (1000 ÷ gauge in mm) × (1000 ÷ cover width in mm); cover width is the tile's effective covering width once any side lap or interlock is accounted for, which is narrower than the tile's overall width for most products
- Minimum headlap varies by pitch and exposure — steeper pitches and more sheltered wind zones generally allow a shorter headlap (and therefore a wider gauge, meaning fewer tiles per m²); shallower pitches and more exposed wind zones need a longer headlap under BS 5534:2014+A2:2018 — always check the specific tile's fixing data sheet for the site's actual pitch and wind zone rather than assuming a single figure applies everywhere
- Ridge tiles are calculated by running length, not area — ridge tile requirement = ridge run length ÷ typical ridge tile length (commonly around 450mm, though this varies by product), rounded up, plus one or two spares per job
- Hip tiles follow the same running-length method as ridge tiles, measured along the hip rafter line rather than the horizontal ridge
- Verge treatment — two systems — a traditional bedded/mortar verge or tile-and-a-half verge tiles (one per course, matching the field tile gauge) versus a proprietary dry-verge system (clip-fixed units, also typically one per course); either way, verge quantity is calculated per course, not per m²
- Number of courses on a roof plane — courses = slope length ÷ gauge, rounded up; this figure is also what's needed to calculate verge tile/unit quantities, since one is typically required per course
- Waste allowance — add 5% for a straightforward roof with few cuts and hips, up to 10% for a complex roofline with multiple hips, valleys, dormers or a high proportion of cut tiles
- Underlay and battens are separate calculations — this article covers tile quantities only; breathable underlay is typically ordered by roll to cover the full sloped area plus laps, and battens are ordered by running metre per course (number of courses × roof plane width) — see roof tile types for underlay specification
- Tile weight affects delivery and handling planning, not the quantity calculation itself — but it's worth cross-checking the total tile weight (tiles/m² × roof area × unit tile weight) against the manufacturer's data when ordering a heavier tile type, to plan crane/lift access and confirm the roof structure was assessed for the load — see roof tile types for typical weights by tile type
Quick Reference Table
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Try squote free →| Tile type | Typical cover width | Typical gauge | Typical tiles/m² (verify against manufacturer data sheet) |
|---|---|---|---|
| Concrete interlocking, large format (e.g. modern flat/low-profile tile) | ~300mm | ~330–345mm | ~9.5–10 |
| Concrete interlocking, standard profile | ~290–300mm | ~280–300mm | ~11–12 |
| Clay interlocking (pantile-style, single lap) | ~270–300mm | ~250–280mm | ~13–15 |
| Clay plain tile (double lap, traditional) | ~100–130mm | ~90–100mm | ~55–65 |
| Natural slate, standard size (e.g. 500×250mm class) | Varies with headlap | Varies with headlap/pitch | ~18–22 |
| Fibre cement slate | Varies with headlap | Varies with headlap/pitch | ~16–20 |
These figures are typical ranges for sense-checking a quantity takeoff only — always calculate the actual figure from the specific product's cover width and gauge as published on the manufacturer's technical data sheet, since these vary meaningfully between products within each category.
Detailed Guidance
Step 1: Measure or calculate the sloped roof area
Walk the roof (or work from an accurate roof plan) and establish, for each roof plane separately, the horizontal length along the eaves and the slope length from eaves to ridge. If the pitch is known and the horizontal rafter run (from the wall/eaves line to the ridge, on plan) is known, calculate the slope length as:
Slope length = horizontal rafter run ÷ cos(pitch angle)
Alternatively — and often more reliable on an existing roof — measure the actual slope length directly with a tape from eaves to ridge along the roof face (or along a ladder/roof access point at a safe working position), which avoids any error from an inaccurately assumed pitch angle.
Step 2: Work out the batten gauge for the specific tile
Get the tile length and minimum headlap from the manufacturer's data sheet for the specific product, pitch, and wind zone. For a single-lap tile (most concrete interlocking and clay interlocking tiles):
Gauge = tile length − minimum headlap
For a double-lap tile (clay plain tiles, natural slate):
Gauge = (tile length − minimum headlap) ÷ 2
The minimum headlap is not a fixed universal figure — it increases on shallower pitches and in more exposed wind zones under BS 5534:2014+A2:2018, so the same tile can have a different gauge (and therefore a different tiles-per-m² figure) on two different roofs. Always take the headlap from the specific manufacturer's fixing table for the actual pitch and wind zone of the job, not from a generic reference figure.
Step 3: Calculate tiles per m² and total field tiles
Tiles per m² = (1000 ÷ gauge in mm) × (1000 ÷ cover width in mm)
Multiply this by the total sloped roof area (all planes added together) to get the total field tile requirement before waste.
Step 4: Worked example — simple gable roof, large-format concrete interlocking tile
Scenario: A rectangular single-storey extension with a simple duo-pitch (gable) roof. Building length (ridge run) 6m, pitch 30°, horizontal rafter run from eaves line to ridge (including eaves overhang) 3.5m per side. Tile: large-format concrete interlocking, cover width 300mm, tile length 420mm, minimum headlap 75mm (per the manufacturer's data sheet for this pitch and wind zone).
Roof plane dimensions:
- Slope length = 3.5m ÷ cos(30°) = 3.5 ÷ 0.866 = 4.04m per roof plane
- Roof plane width (= building length) = 6m
- Area per plane = 6m × 4.04m = 24.24m²
- Two planes (front and back) = 48.48m² total sloped area
Batten gauge:
- Gauge = 420mm − 75mm = 345mm (0.345m)
Courses per plane:
- Courses = 4.04m ÷ 0.345m = 11.7 → round up to 12 courses
Tiles per course:
- Tiles per course = 6m ÷ 0.3m cover width = 20 tiles per course
Field tiles per plane (before waste):
- 12 courses × 20 tiles = 240 tiles per plane
- Two planes = 480 field tiles
Waste allowance (7% — straightforward gable, few cuts):
- 480 × 1.07 = 513.6 → round up to 514 tiles, then round up further to the supplier's pack quantity if tiles are supplied in fixed pack sizes (check with the merchant before finalising the order)
Ridge tiles:
- Ridge run = 6m (the full building length)
- Typical ridge tile length for this product = 450mm
- 6m ÷ 0.45m = 13.3 → round up to 14, plus 1 spare = order 15 ridge tiles
Verge tiles (tile-and-a-half, one per course, per verge):
- This roof has 4 verge runs (left and right end of each of the two roof planes), each verge run needing one tile-and-a-half unit per course
- 12 courses × 4 verge runs = 48 tile-and-a-half tiles (or 48 dry-verge units if using a proprietary dry-verge system instead — the quantity logic is the same either way)
Final order summary for this roof:
| Item | Quantity to order |
|---|---|
| Field tiles (concrete interlocking, incl. 7% waste) | 514 |
| Ridge tiles | 15 |
| Verge tiles (tile-and-a-half) or dry-verge units | 48 |
| Battens (12 courses × 6m per plane × 2 planes, +10% waste) | ~158 linear m |
| Breathable underlay (48.48m² + laps, rounded to roll quantities) | ~52–55m² allowing for laps |
This is the complete materials list for the roof covering itself. Fixings (nails/clips per BS 5534 wind uplift requirement), lead flashing at any abutments, and any hip or valley detailing on a more complex roofline are calculated separately and are not part of this straightforward gable example.
Step 5: Adjusting the method for a hipped roof
A hipped roof (sloping on all sides rather than having gable ends) is calculated the same way for field tiles — total sloped area across all planes, divided by tiles per m² — but needs hip tiles in addition to ridge tiles, and generally needs more cut tiles at each hip line, pushing the waste allowance toward the higher end of the 5–10% range (or higher on a complex multi-hip roof). Hip tile quantity is calculated the same way as ridge tiles: hip run length ÷ typical hip tile length, rounded up, per hip.
Step 6: Adjusting the method for a roof with dormers, valleys or chimneys
Any roof feature that breaks up the simple rectangular plane — a dormer window, a valley where two roof planes meet, or a chimney stack penetrating the roof — increases both the cutting waste (push toward 10% or slightly above on a genuinely complex roof) and adds specific detailing (valley tiles or lead valley gutters, soakers around the chimney) that isn't captured by the simple area-and-gauge calculation. For anything beyond a straightforward gable or hip roof, break the roof down into its constituent simple planes, calculate each one separately using this method, and add the specific junction details (valleys, abutment flashings) as their own line items rather than trying to fold them into a single whole-roof waste percentage.
Common takeoff mistakes
- Using plan (footprint) area instead of sloped area — this under-orders tiles on every pitched roof; the steeper the pitch, the bigger the shortfall
- Assuming one tiles-per-m² figure applies to every product in a tile category — cover width and gauge vary between manufacturers and even between ranges from the same manufacturer; always check the specific product's data sheet
- Forgetting ridge, verge and hip quantities are separate calculations — these are run-length based, not area based, and are commonly left off a first-pass area-only materials list
- Using a single headlap figure regardless of pitch or wind zone — the minimum headlap (and therefore the gauge) changes with pitch and exposure under BS 5534:2014+A2:2018; a headlap figure taken from a different pitch or a sheltered site can under-specify the fixing on an exposed one
- Not accounting for supplier pack sizes when finalising an order — many tile ranges are supplied in fixed pack quantities; round the final order up to the nearest full pack rather than ordering the exact calculated figure
Frequently Asked Questions
How many concrete roof tiles do I need per m²?
For a large-format concrete interlocking tile, typically around 9.5–12 tiles per m² depending on the specific product's cover width and gauge. For a smaller standard-profile concrete interlocking tile, the figure is usually somewhat higher, in the 11–15 range. Always calculate the exact figure using the formula in this article with the specific product's published cover width and gauge, since "concrete tile" covers a wide range of actual dimensions across different manufacturers and ranges.
Do I calculate roof area from the plan drawing or by measuring the actual roof?
Always use the sloped (actual) roof area, not the horizontal plan/footprint area. If working from a plan drawing, convert the horizontal rafter run to a slope length using the pitch (slope length = horizontal run ÷ cos(pitch)) before calculating area. If measuring an existing roof directly, measure the slope length along the actual roof face rather than relying on a plan figure that may not reflect the as-built pitch.
How many spare tiles should I keep after the job?
Beyond the 5–10% waste allowance built into the initial order (most of which will be used during installation for cuts), keeping a small number of genuine spares — commonly 1–2% of the total field tile count, or a minimum of a handful of tiles — is good practice so the customer has matching replacement tiles available for any future accidental breakage, particularly important for tiles that may be discontinued or hard to match years later.
Why does my headlap figure differ from a colleague's for what looks like the same tile?
Minimum headlap under BS 5534:2014+A2:2018 depends on the roof's pitch and wind zone (which itself depends on location, altitude and site exposure), not just the tile product. Two jobs with the same tile but a different pitch or a different wind exposure can legitimately require different minimum headlaps, and therefore have different gauges and different tiles-per-m² figures. Always take the headlap from the manufacturer's fixing table for the actual site conditions, not from memory or a previous job.
Regulations & Standards
BS 5534:2014+A2:2018 — code of practice for slating and tiling of pitched roofs and vertical cladding; governs minimum pitch, headlap, and fixing requirements that determine the batten gauge used in this calculation
BS EN 490:2011 — concrete roofing tiles and fittings; product standard referenced for concrete tile dimensions and specification
BS EN 1304:2013 — clay roofing tiles and fittings; product standard referenced for clay tile dimensions and specification
Approved Document A — structure; relevant where a tile weight/quantity calculation feeds into a roof structural load assessment
Approved Document C — site preparation and resistance to contaminants and moisture; weather resistance requirements for the completed roof covering
Marley Technical — Roofing Product Selector — tile specifications, gauge and cover width data by product range
Redland Roofing — Technical Library — product data sheets and BS 5534 fixing/gauge tables
NFRC (National Federation of Roofing Contractors) — roofing trade body technical guidance
Sandtoft Roof Tiles — Technical Data — gauge and coverage tables for clay and concrete tile ranges
roof tile types — full tile type comparison covering weight, minimum pitch, lifespan and BS 5534 fixing requirements referenced throughout this calculator
slate installation — slate-specific gauge calculation (gauge = (slate length − headlap) ÷ 2) and fixing detail for natural slate roofs
pitched roof structure — rafter sizing and roof structure background relevant to assessing load before specifying a heavier tile type
guttering quantity calculator — companion materials-takeoff calculator using the same measure-then-apply-waste-allowance method for guttering rather than roof coverings