Advanced Metal Knowledge
Designing roof gardens and podiums with a small fall or roof gradient is necessary to ensure that rainwater can quickly and easily drain off. This article looks into what legal requirements must be adhered to when designing a roof and other important considerations.
Designing roof gardens and podiums with a small fall or roof gradient is 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 defects and damage. Freezing water in the winter can damage the waterproofing surface.
By following the BS6229 guidance, there should be no standing water on rooftops and consequently, fewer problems in the long term.
BS 6229 is a code of practice which describes the design, construction, care and maintenance of roofs with a flat or curved surface. 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 have changed. Therefore, an updated BS6229 was published in 2018, known as the 2018 Standard. The 2018 BS6229 Standard involves significant changes from the 2003 Standard.
The gradient fall recommended is a minimum fall of 1:80 achieved on the completed roof. This means a 1:80 fall refers to an 80-inch rise 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 is so gradual it can be designed as if it were flat.
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. This trend avoids the risk of standing water however, on a roof garden, this can introduce challanges.
As soon as the gradient increases beyond 1:60, the roof space has a noticeable gradient. This means that all associated roof build-up must cater to the gradient. With this gradient, challenges around loadings, fixings and structural design integrity arise. 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 will unnecessarily increase 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.
Efficient Drainage: A steep gradient roof, typically with a fall of 1:60 or steeper, ensures efficient rainwater drainage. Steep gradients help prevent the formation of standing water on the roof surface, in accordance with BS6229 guidelines. This, in turn, reduces the risk of water ingress, damage, and freezing-related problems, particularly in winter.
Increased Complexity and Cost: Steeper gradients, such as 1:40 or 1:30 falls, introduce design challenges related to loadings, fixings, and structural integrity. This complexity can lead to higher construction costs for roof gardens and podiums.
Design Challenges: Steeper gradients require more complex roof build-ups, which may demand specialized designs and materials. This can complicate the construction process so be sure to consult with specialist manufacturers.
Limited Versatility: Steeper gradients limit the range of design options for roof gardens and podiums. The design must accommodate the pronounced slope, potentially reducing the flexibility to create various functional spaces on the roof.
In summary, a steep gradient roof with a fall of 1:60 or steeper offers efficient drainage and helps avoid standing water, but it can lead to increased design complexity and costs.
On a normal roof (ie not a roof garden or podium), steep falls are not so problematic. This is because the build-up is far more simple. However, on roof gardens and podium spaces, there is a more complex build-up. It is worth carefully considering the pros and cons of steep falls.
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