Roof Pitch Comparison
Compare two roof pitches drawn to the same scale, with ridge heights, rafter lengths and minimum pitch requirements for your covering.
Calculator Inputs
Both Pitches Drawn To Scale
Same span, same baseline, no lens distortion. This is what the two pitches actually look like against each other.
How These Numbers Are Worked Out
| Step | Pitch A | Pitch B |
|---|
Standard UK Pitches At Your Span
The comparison chart the photo results never give you: every common pitch, worked through your own building span.
| Pitch | Rise in 12 | Ridge Height | Rafter | Area Factor |
|---|
Choosing And Checking A Pitch
Measure what you haveInside the loft, measure the vertical rise over a known horizontal run. A 1.2 m rise over a 2 m run is 31°.
Convert to one notationDrawings use degrees, roofers often use x in 12. Compare like with like before deciding anything.
Check the covering minimumNatural slate needs 20°, plain tiles usually 35°, interlocking concrete tiles 15°. Below the minimum the warranty goes.
Check the ridge heightTake the ridge figure to your local plan. Height limits and Article 4 directions bite on height, not angle.
Price the slope areaMultiply plan area by the area factor before ordering covering, underlay and battens, then add for hips and valleys.
What Roof Pitch Actually Means
Pitch is the steepness of a roof slope measured from horizontal, and the UK uses three notations for it. Degrees is the common one on drawings: 30°, 35°, 45°. The "x in 12" ratio says how far the roof rises for every 12 units it runs, so 30° is 6.9 in 12 and 45° is exactly 12 in 12. Percentage grade is the same ratio against 100, making 45° equal to 100%. All three describe one angle: multiply the tangent of the angle by 12 for the x-in-12 ratio, or by 100 for the percentage.
What The Difference Looks Like On A Real House
On an 8 m terrace the gap between a 30° and a 45° roof is 1.69 m of ridge height, roughly the difference between a loft you can stand up in and one you cannot. Most Victorian and Edwardian UK housing sits between 40° and 45°. Post-war estate housing is commonly 30° to 35°. Interlocking concrete tiles made the shallow 17.5° to 22.5° roofs of the 1960s onwards possible. The drawing above puts your two figures on one span, which is the comparison a photo search cannot give you.
Minimum Pitch By Covering
The covering decides how shallow you can go, because a shallower roof drains more slowly and drives rain further up under the lap. BS 5534:2014+A2:2018 recommends a 20° minimum for natural slate, as Marley summarises. Plain clay and concrete tiles usually need 35°, with some single-camber clay tiles approved to 30°, while interlocking concrete tiles go down to 15° and slate-effect interlocking tiles to 17.5°, per Marley's low-pitch guidance. Fibre cement slate sits at 20° for moderate exposure with a 110 mm headlap and 22.5° for severe exposure in the Rivendale fixing guide. Standing seam zinc is the outlier at 3° as built, with 5° the figure SIG Zinc & Copper recommend designing to. Below that you are building a flat roof, where BS 6229 asks for a 1:80 finished fall designed at 1:40, as IKO explains. Laying a product under its minimum invalidates the warranty and building control can refuse to sign the work off.
What A Steeper Pitch Costs You
Slope area grows as 1 ÷ cos(pitch). A 15° roof covers 1.04 times its plan area, 30° is 1.15, 45° is 1.41 and 60° is 2.00. Going from 30° to 45° therefore adds 22.5% to the covering, underlay, battens and labour for the same footprint, before the extra ridge height is counted. That is the trade against a shallow roof's own costs: less loft volume, and a much narrower choice of covering.
Pitch, Height And Permission
Ridge height is what planning tends to care about. Raising an 8 m span from 30° to 45° adds 1.69 m to the ridge, enough to cross a height limit in a conservation area or under an Article 4 direction, and enough to matter for a rights-of-light objection. Check the ridge figure this tool reports against your local plan before committing to a steeper roof on a replacement or an extension, and against the permitted development limits on GOV.UK's planning permission guidance, which caps a roof alteration at the height of the existing highest part of the roof.
Method And Limits
The geometry is trigonometry on a symmetrical dual-pitched roof: ridge rise is half the span times the tangent of the pitch, and the common rafter is half the span divided by its cosine. Everything else on this page follows from those two lines.
- Assumes a symmetrical roof — span is halved evenly. A mono-pitch or asymmetric roof needs each slope's run entered separately, which this tool does not accept.
- Does not model overhang — rafter length runs wall plate to ridge. Add your eaves projection divided by cos(pitch), plus a cutting allowance and the ridge board thickness, to get an ordering length.
- Does not model hips, valleys or dormers — each adds slope area and cut waste that a single span figure cannot capture.
- Does not set out courses — headlap, batten gauge and the real tile count come from the manufacturer's fixing guide, not from the pitch.
- Does not check loading — BS 5534 changes fixing specification with wind and snow exposure. That affects how the covering is held down, not the geometry reported here.
Minimum-pitch figures checked .
Frequently Asked Questions
What is roof pitch?
The angle of a roof slope from horizontal, written in degrees, as an "x in 12" ratio, or as a percentage grade. 30° is 6.9 in 12 and 57.7%.
What is the most common roof pitch in the UK?
There is no single answer. Victorian and Edwardian housing is typically 40° to 45°, post-war estate housing commonly 30° to 35°, and shallow interlocking-tile roofs from the 1960s onwards sit at 17.5° to 22.5°.
How do I convert degrees to an x-in-12 ratio?
Multiply the tangent of the angle by 12. 30° gives 12 × 0.5774 = 6.9 in 12; 45° gives exactly 12 in 12.
What is the minimum pitch for natural slate?
BS 5534:2014+A2:2018 recommends 20°. Smaller slates and exposed sites often need more, so check the supplier's headlap table.
Can I use plain clay tiles on a 30° roof?
Usually not. Most single and double camber plain tiles need 35°, though some single-camber clay tiles are approved down to 30°. The tile's own data sheet decides it.
What is the lowest pitch I can tile?
Interlocking concrete tiles go down to 15°, and slate-effect interlocking tiles to 17.5°. Below that you are into metal or membrane systems.
How low can a zinc standing seam roof go?
3° as built is the absolute minimum; 5° is the recommended design figure, because site tolerance tends to reduce the pitch actually achieved.
What counts as a flat roof?
Anything below roughly 3°. BS 6229 asks for a 1:80 finished fall, which is 0.72°, and recommends designing at 1:40 so the finished roof still achieves it.
How much more material does a steeper roof need?
Slope area is 1 ÷ cos(pitch) times the plan area. 30° is 1.15×, 45° is 1.41×, so 30° to 45° is a 22.5% increase.
How do I measure my existing roof pitch?
Measure the vertical rise over a known horizontal run inside the loft, divide rise by run, then take the inverse tangent. A 1.2 m rise over a 2 m run is 31°. Enter it here as degrees, or as 7.2 in 12.
Does a steeper roof give more loft space?
Yes, and quickly. On an 8 m span, 30° gives 2.31 m of ridge height above the wall plate and 45° gives 4.00 m. Usable head height needs roughly 2.2 m under the purlin.
Does roof pitch affect planning permission?
Indirectly, through ridge height. Height limits, conservation areas and Article 4 directions all bite on the ridge figure rather than the angle, so check the height this tool reports against your local plan.
Is rafter length the same as the timber I order?
No. The figure here is wall plate to ridge on the centreline. Add the eaves overhang divided by cos(pitch), plus a cutting allowance and the ridge board thickness.
Why do two photos of the same pitch look different?
Lens focal length, camera distance and the width of the building all change the apparent angle. A 45° roof on a 4 m span looks steeper in a photo than the same 45° on a 12 m span. Drawing both on one span is the only fair comparison.
Can I compare a pitch in degrees against one in x-in-12?
Yes. Each side of the comparison has its own unit selector, so you can enter 35° against 6 in 12 and the tool converts before comparing.
Sources
- Marley – BS 5534 code of practice for roof slating and tiling
- Marley – What do I need to know about low pitch tiles?
- Marley Eternit – Fibre cement slates fixing guide
- SIG Zinc & Copper – Minimum pitch for a zinc roof
- IKO – What is BS 6229:2018?
- GOV.UK – Planning permission in England and Wales
Last updated: 2026-09-04. Minimum pitches are published guidance figures; the manufacturer's data sheet for the product you buy is what governs. This page is an estimate only and is not structural, planning or professional advice.
About this tool
Created and maintained by CoryVu. Read how we check our tools and consult the sources and assumptions on this page.
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