Summary
Roof pitch gets specified in three different units depending on who you're talking to and what document you're reading — a structural engineer's drawing might give rise:run in millimetres, a tile manufacturer's technical datasheet gives minimum pitch in degrees, and an older survey report might describe a roof as "a 1 in 3 pitch" or "35% fall." A tradesperson who can convert fluently between all three, on site, without a calculator app, saves time and avoids the single most common site error on re-roofing jobs: specifying or ordering a tile or slate rated for a steeper minimum pitch than the actual roof.
Pitch matters commercially as much as technically. A roof measured at 17° that's being re-covered in a tile rated to a 17.5° minimum needs either a different tile, an increased headlap (which increases tile consumption and cost — see roof tile count), or in some cases a full underlay upgrade to compensate. Getting the pitch measurement and unit conversion right at quoting stage avoids a costly mid-job material swap.
This calculator covers the three pitch notations, gives conversion formulas and a ready-reference table for common pitches, and lists typical minimum pitch requirements by roofing material. For calculating the actual sloping length of a rafter once you know the pitch, see rafter length calculator.
Key Facts
- Pitch in degrees — the angle between the roof slope and the horizontal, measured with a protractor, digital angle finder, or calculated from rise and run
- Pitch as a ratio (rise:run) — traditional carpentry notation, e.g. "1 in 2" or "1:2" meaning 1 unit of rise for every 2 units of horizontal run
- Pitch as a percentage — rise ÷ run × 100; common in civil engineering and some manufacturer datasheets, especially for flat and low-pitch membrane roofing
- Conversion formula (degrees from ratio) — angle = arctan(rise ÷ run)
- Conversion formula (percentage from degrees) — percentage = tan(angle in degrees) × 100
- Conversion formula (degrees from percentage) — angle = arctan(percentage ÷ 100)
- 45° pitch — equals a 1:1 ratio and a 100% grade; the point where rise equals run
- Low pitch (under 10°/17.5%) — treated as "flat roofing" for material selection purposes even though it usually has a small fall; single-ply membrane, felt, or GRP fibreglass
- Standard pitched roof range — 30-45° is the most common range for UK domestic pitched roofs, balancing water shedding, loft usable space, and material efficiency
- Steep/traditional pitch — 45-55°+ common on Victorian terraces and steep slate roofs, sheds water fast but increases material quantity and scaffold/access cost
- Minimum pitch is material-specific, not universal — always check the specific tile, slate or membrane manufacturer's BBA certificate or datasheet, since minimum pitch varies with headlap, exposure zone, and underlay type
- Exposure zone affects minimum pitch — BS 5534 sets more conservative minimum pitches and headlaps for roofs in high wind/exposure zones (coastal, elevated sites) than for sheltered inland sites
- Rafter length increases with pitch at a fixed span — a steeper roof needs longer rafters and more covering material per m² of plan area, even though the footprint hasn't changed
- Digital angle finder / inclinometer — the fastest accurate on-site pitch measurement tool; laid against a rafter or roof slope, reads directly in degrees
- Roof pitch calculator apps and physical pitch gauges — widely used on site but always cross-check against a measured rise and run if the reading looks unusual, since a gauge resting on a bowed or sagging rafter will misread
Quick Reference Table
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Try squote free →| Ratio (rise:run) | Degrees | Percentage | Typical use |
|---|---|---|---|
| 1:12 | 4.8° | 8.3% | Flat/low-pitch membrane roofing |
| 1:6 | 9.5° | 16.7% | Low-pitch flat roof upper limit |
| 1:4.7 | 12° | 21.3% | Minimum for some low-pitch interlocking tiles |
| 1:3.7 | 15° | 26.8% | Common minimum for large interlocking concrete tiles |
| 1:3.2 | 17.5° | 31.5% | Typical minimum for standard interlocking tiles |
| 1:2.75 | 20° | 36.4% | Common minimum for plain tiles and some slates |
| 1:2 | 26.6° | 50% | Mid-range pitch, common on extensions |
| 1:1.7 | 30° | 57.7% | Very common UK domestic roof pitch |
| 1:1.4 | 35° | 70% | Common on traditional slate roofs |
| 1:1.2 | 40° | 83.9% | Steeper traditional roof |
| 1:1 | 45° | 100% | Rise equals run |
| 1.2:1 | 50° | 119.2% | Steep traditional/Victorian roof |
| 1.4:1 | 55° | 142.8% | Very steep, mansard-style upper slopes |
Detailed Guidance
Converting from a measured rise and run
On site, the simplest method is to measure the rise (vertical height gained) over a fixed horizontal run, then calculate.
Worked example: a roof rises 1.85m over a horizontal run of 3.2m.
- Ratio: 1.85 : 3.2, simplify by dividing both by 1.85 → 1 : 1.73
- Degrees: arctan(1.85 ÷ 3.2) = arctan(0.578) = 30.0°
- Percentage: 0.578 × 100 = 57.8%
This is a very common UK pitch — 30° covers the vast majority of standard interlocking concrete and clay tile ranges with normal headlap.
Converting from a stated ratio (traditional carpentry notation)
Worked example: a roof is specified as "1 in 2.4" pitch.
- Rise ÷ run = 1 ÷ 2.4 = 0.4167
- Degrees: arctan(0.4167) = 22.6°
- Percentage: 0.4167 × 100 = 41.7%
Converting from a percentage grade
Worked example: a flat-roof extension datasheet specifies a minimum fall of 1:80 (1.25%), and the existing roof measures 3% fall.
- Degrees: arctan(0.03) = 1.7°
This confirms the roof is a true flat roof (well under 10°) and falls under single-ply/felt/GRP material selection, with fall design governed by drainage requirements rather than a tiled-roof minimum pitch.
Minimum pitch by roofing material (indicative — always check the specific product BBA certificate)
| Material | Typical minimum pitch | Notes |
|---|---|---|
| Single-ply membrane / felt / GRP | Under 10° (design fall, not "pitch") | Governed by fall-to-drainage design, not headlap |
| Standing seam metal | 1-3° minimum with sealed seams | Product-specific, check manufacturer data |
| Large interlocking concrete tile | 15-17.5° | Increase headlap below 22.5° per BS 5534 |
| Standard interlocking concrete/clay tile | 17.5-22.5° | Product and headlap dependent |
| Plain tile (double-lap) | 35° typical minimum, some rated to 30° | Requires greater headlap at lower pitch |
| Natural slate | 20-25° minimum, exposure-dependent | Lower pitch needs increased headlap and exposure-rated underlay |
| Fibre cement slate | 17.5-20° minimum | Product-specific |
Why the same pitch can need different tile quantities
A roof at a fixed pitch still needs more tiles per m² of plan area if the headlap is increased — and headlap is often increased specifically because the pitch is nearer the material's minimum. This means a roof pitched close to a tile's minimum rating both restricts material choice and increases the quantity/cost of covering it. See roof tile count for gauge and headlap tables.
Frequently Asked Questions
What's the difference between "pitch" and "fall" on a flat roof?
"Pitch" is generally used for roofs steep enough to be measured in degrees against tile/slate minimum ratings. "Fall" is the term used for the very shallow gradient designed into a "flat" roof purely to drain water to outlets — usually expressed as a ratio like 1:40 or 1:80, not in degrees, because true flat roofs are never perfectly flat; they need a minimum fall for drainage even though visually they look horizontal.
Can I use a smartphone app to measure roof pitch accurately?
Smartphone inclinometer apps are accurate enough for a quick estimate but can be thrown off by the phone not being held flush against a true reference surface, and by any curvature or sag in the actual roof timbers. For quoting and material ordering, cross-check an app reading against a physical rise-and-run measurement taken with a spirit level and tape measure, especially where the pitch is close to a material's minimum threshold.
Why do older buildings sometimes have unusually steep roof pitches?
Traditional slate and plain clay tile roofing (pre-20th century) generally needed steeper pitches — often 45-55° — because the small individual units and shorter available lap depths meant water shedding relied more on gravity and less on interlocking or increased headlap. Modern interlocking concrete and clay tiles were specifically engineered to work at shallower pitches, which is one reason later 20th-century housing stock tends to have visibly lower-pitched roofs than Victorian terraces.
Regulations & Standards
BS 5534 — Code of practice for slating and tiling for pitched roofs and vertical cladding; sets minimum pitch, headlap, and fixing requirements by exposure zone
BS EN 12326 — Slate and stone products for discontinuous roofing and cladding; product performance and classification for natural slate
Building Regulations Part A — structure; roof pitch affects rafter loading calculations and structural timber sizing
BBA (British Board of Agrément) certification — individual tile, slate and membrane products carry BBA certificates stating validated minimum pitch for the specific product
BS 5534 — BSI — code of practice for slating and tiling
NFRC (National Federation of Roofing Contractors) — Technical Guidance — pitch and headlap guidance for roofing contractors
BBA — British Board of Agrément — product certificates including validated minimum pitch ratings
rafter length calculator — calculating true rafter length once pitch and run are known
roof tile count — tile density per m² and headlap/gauge tables by pitch
part a structure — structural requirements affecting roof pitch and rafter design