Designing roof gardens and podiums with a small fall, or roof gradient, is usually necessary to ensure that rainwater can quickly and easily drain off. If there are no suitable drainage solutions, the water will form ponds on the roof surface. This is called standing water and is defined as water which remains on the roof surface for more than 48 hours.
While standing water will not directly cause problems, it will decrease the expected lifespan of the roof surface. Other problems caused by standing water are increased water ingress if there are any waterproofing defects and damage if the standing water freezes in winter.
By following the BS6229 guidance, there should be no standing water on rooftops and consequently, fewer problems in the long term.
BS6229 is a code of practise which describes current best practise in the design, construction, care and maintenance of roofs with a flat or curved surface at a pitch less than 10 degrees to the horizontal. Although BS6229 looks at gradients for all roof types, this article will focus on gradients for roof gardens, terraces and podiums.
BS6229 was first published in 2003. Since then, the design, scope and application of many roof surfaces has changed. Therefore, an updated BS6229 was published in 2018, known as the 2018 Standard. Unlike many revisions of similar standards, the 2018 BS6229 Standard involves significant changes from the 2003 Standard.
The gradient fall recommended as part of BS6229 is a minimum fall of 1:80 achieved on the completed roof. To clarify, a 1:80 fall refers to an 80-inch run in the horizontal plane and a 1-inch rise in the vertical plane. The resulting angle is approximately 0.7° from the horizontal. A good online tool for calculating the degree rise from a gradient ratio can be found here.
1:80 or even 1:60 falls mean that the roof build up can be designed as if it were flat. This is due to the fall being so gradual.
However, because of the word ‘minimum’ within the BS6229 and the desire the avoid standing water at all costs, the trend has been to design roof spaces with a gradient of 1:40 and 1:30.
The significance of this is that as soon as the gradient increases beyond 1:60, the roof space has noticeable gradient. This necessitates all associated roof build up to cater to the gradient in terms of loadings, fixings and structural. This adds unnecessary complexity and cost to a roof garden or podium.
In conclusion, a 1:60 fall is best for a balance between standing water risk and ease of design/lower cost. Any gradients steeper than a 1:60 fall are going to add increased and unnecessary costs and design complexity.
Remember, the guidance in the BS6229 is for 1:80 falls. Therefore, even a 1:60 fall can be viewed as ‘too safe’ and unnecessary.
On a normal roof (ie not a roof garden or podium), steep falls are not so problematic and even recommended. This is because the build-up is far more simple. However, on roof gardens and podium spaces where a complex build-up is necessary it is worth carefully considering the pros and cons of steep falls.