Rainfall levels and siphonic roof drains

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BLÜCHER® was founded in 1965 and has since grown into one of Europe’s leading stainless steel drainage specialists. In this guest post, Frank Netherwood (Technical Manager from BLUCHER UK Ltd) explains the siphonic drainage principle and how it’s used in BLÜCHER products.

Frank Netherwood: Whilst a gravity system is simple to understand, the siphonic system is a little more complex …

The siphoning principle

Siphonic roof drainage makes use of the siphoning principle to carry water horizontally from multiple roof drains to a single downpipe and to increase flow velocity. Air baffles at the roof drain inlets restrict air from entering the top of the system, which causes the system to run full and exclude air.

The action of water dropping down the downpipe causes a negative pressure to form at the top. This negative pressure can be harnessed to suck water along a collector pipe installed horizontally and connecting the outlets at high level.

There are a number of advantages associated with a siphonic system, including a reduction in the number of downpipes and where they are placed, which in turn affects the amount of internal drainage and underground drainage.

Drains that work in siphonic and gravity applications

Traditionally, manufacturers of roof drains would produce either a range of products for siphonic applications or a range for gravity applications. But BLÜCHER EuroPipe push fit stainless steel drainage pipework can be installed in both gravity and siphonic applications. This means that BLÜCHER had a requirement to produce a range of drains that would cover both gravity and siphonic applications as well.

BLÜCHER roof drains have outlets ranging from 40–110mm. Typically, siphonic applications run in small diameter, while gravity applications usually run in larger diameter. Rather than produce two types of drain, BLÜCHER has developed a single range that uses a simple siphon plate to turn it from a gravity roof drain into a siphonic roof drain.

Designing drainage systems for heavy rainfall

Generally, all roof drainage systems are designed around rainfall intensity. Rainfall levels are determined from BS EN 12056-3:2000, using the projected building life together with a suitable factor of safety. However, as recent downpours have shown, sometimes even the most safety conscious design can be tested to its limit.

BLÜCHER also sought to innovate elsewhere to reduce the potential of a roof drainage system to be overloaded. Looking at the design of gravity systems, it was evident that in periods of high rainfall intensity there was often a build-up in the head of water. Usual practice is to install an overflow drain that will attempt to clear the rainfall should the system become overloaded and this practice should still stand. However, BLÜCHER’s Boost leaf guard increases the flow rate of a gravity roof drain.

In effect, the Boost guard eliminates some of the air passing into the drain after the head of water has reached 30mm, which in turn increases the flow rate of the gravity drain by virtue of a reduction in turbulence and therefore clears the water more quickly from the roof. It is a simple but effective means of building in a secondary barrier of defence against significant rainfall intensity.

So, there are various factors that have to be considered when designing a roof drainage system. With that in mind, BLÜCHER offers a full design and specification service for customers. Contact information and details of the roof drainage range are available on ESI.info.

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