When designing an extension, a new build house, or even a garden shed, the angle or pitch of the roof is an important consideration. Here in the UK, the roof primarily deals with rainwater (rather than massive snow loads, desert sun, forest fires or ridiculous wind speeds). They will have to cope with sunshine, fluctuating temperatures, wind loading, salty marine air and occasional snow loads, but these are not usually the forefront of design choices.
In its most simplistic form, the water needs to run off the roof, sounds like an obvious point, but it can be forgotten when dealing with ‘flat’ roofs!!
The angle or pitch of a roof will determine what materials are suitable. Conversely, sometimes a preferred material will limit the range of angles possible. For example a straightforward lean-to extension, such as this one, is limited by the first floor windows, and new eaves (the gutter position) height. The resulting triangle between these fixed points gives a maximum roof pitch. In the example on the link, the roof pitch was constructed at 18˚. This is generally considered too shallow for natural slate but is ok for man-made ‘slates’ as they fit together more tightly.
The risk is that wind-driven rain can be forced between the slates, and the effect of capillary action draws the moisture up and in. At 18˚ more basic (and economically priced) rooflights are available, and warrantied. A shallower roof pitch would have allowed a taller eaves, or lower upper floor window sills (if the building had them), or allowed the extension to project further into the garden. A shallower pitch however would have reduced the roof materials possible, and limited the available opening rooflights that wouldn’t leak.
Every material has different minimum (the shallowest possible) roof pitches, and most have maximums, but the maximum is rarely exceeded in normal situations. I have listed out below a series of materials and their respective typical minimum pitches. Each manufacturer has different ratings, and there are subtle variations within each, but this forms a decent guide to work from in a ball-park manner.
A flat roof is defined by BS6229:1982 as being 10˚ (1in6) or shallower. In reality most people refer to flat roofs as being much flatter than this, and pitches up to 10˚ usually being referred to as shallow pitch roofs. The angle or pitch of a flat roof is usually referred to as the ‘fall’.
Flat roofs are rarely perfectly flat, and should have a fall of 1in80 or approx 0.7˚ to shed rainwater. However a design fall pitch of 1in50 (about 1.1˚) is recommended to account for site tolerances and structural deflection to avoid the risk of ponding.
GRP-type membrane = Min 1in80 or approx 0.7, although ponding wont affect the material and could be 0˚
Single ply membrane = Min 1in80 or approx 0.7, although ponding wont affect the material and could be 0˚
Felt = Min 1in80 or approx 0.7˚
Lead = Min 1in80 or approx 0.7˚, Lead roofs will need steps to accommodate weathered movement joints to cope with the expansion and contraction
Green Living Roof = Generally less than 8˚. Specialist suppliers vary slightly, but generally the living roof layers are laid over single ply.
SHALLOW or LOW PITCHED ROOFS
Profiled metal sheeting = typically 1.5˚ minimum fall
Zinc standing seam = absolute min 3˚ but usually 7˚ recommended especially for longer runs
Copper standing seam = usually 6˚
Glass (patent glazing type) = as low as 5˚ but 15˚ recommended.
Cedar Shingles = 14˚
Man-made slate = as low as 15˚-17˚
Concrete interlocking tiles = 17.5˚
Natural slate = as low as 20˚ (Special fixings are required) but usually 22.5˚ is the standard accepted minimum.
Interlocking clay tiles = 25˚
Plain clay tiles = 35˚
Thatch = 45-50˚
This blog is focusing on minimum pitches as this is usually the tricky part of getting roofs to work. Its not to say that a nice steep roof wont look good or perform any worse/better in these materials, just that the limitations are less critical. When consulting minimum pitch information, usually you are tight on space. It can be a balance to find the right material, and a suitable pairing with rooflights, which are often only suitable for steeper gradients to avoid leaking, or require a kerb upstand so water flows around them rather than over them. As an example, most Velux-type rooflights have a min pitch of 15˚.