Drainage Questions? Ask Wavin!

We asked you recently what you most wanted to know about drainage – we know it can be a bit of a minefield. The experts at Wavin have picked out some of the best ones and put together this handy Q&A for you!

Is your question below? If not, there’s still bags of useful info here to help you swat up on all things external drainage – enjoy!

What is the calculation for working out how many stormwater crates you need per plot?

This calculation is not exactly straightforward, so you wouldn’t want to do it manually! Luckily, the Wavin MyPortal has a clever online tool, the Aquacell Configurator, that can work this out for you.

Can there be any adverse effects from oversizing wastes (foul and ground)?

Yes. If a foul drainage pipe is oversized, there is a risk that solids could get left behind, which could potentially cause a blockage. With oversized ground drainage, the contact surface area is increased, which results in a shallower water depth, creating more friction.

Is a back fall better than no fall?

Neither is good! There must always be a fall when installing any gravity drainage pipe. There should not be a back fall or no fall.

Why is a French drain French?

Interesting question! French drains may have been invented in France, but they’re actually named after Henry Flagg French (1813-1885), who defined and popularised them in the 19th century.

Does a boiler condensate pipe have to go in to a storm drain or a small soakaway?

A boiler condensate pipe can either discharge into a foul drainage system or a small soakaway, but it should never discharge directly into a surface drainage system.

What is the maximum depth that Wavin plastic external drainage can be laid?

Wavin’s plastic drainage systems can be laid up to 10 metres in depth, depending on ground conditions and water table height.

I’m working on a site of 70 flats where only Durgo air admittance valves are installed on all stacks with no underground ventilation. What is best practice with ventilation?

There must always be atmospheric vent pipes installed on all developments. Air admittance valves only let air into the system, which helps to alleviate negative pressure. However, without a traditional atmospheric vent too, you could have problems with positive pressure, resulting in either foul gases bubbling through water traps or slow drainage, which can cause blockages.

How can you avoid drainage crates and soakaways filling with silt and debris?

When installing any geo-cellular crate system, silt traps should always be used to prevent any silt or sediment entering the tanks.

How much fall is too steep?

There is nothing in the regulations to say what gradient or fall is too steep. In fact, in BS EN 752:2017 it states that research has demonstrated that high velocities of flow in drains and sewers laid to steep gradients do not cause increased erosion of pipes or deposition of solids for commonly used sewer materials.

We hope that answers some of the questions you’ve had on below ground drainage. However, if there’s anything else you’d like to know, you’ll find lots more information on Wavin’s website. You can read more from Wavin here…..

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