Soakaway Size And Proximity To Road

[EViS]

When calculating the size of soakaway that would be required for a set roof area, should you calculate the actual roof area (i.e. the steeper the roof, the greater the surface area), or do you use the building's gross floor area (i.e. irrespective of the roof pitch)?

Furthermore, our site slopes down towards the road, so installing a soakaway in the back garden would involve some almighty deep trenches to be dug to provide the correct fall for pipes to carry rainwater from the front downpipes. We can quite easily site the soakaway in the front garden, it would be just over 5 metres from the house, but only a metre or so from the road. Can we position a soakaway this close to a highway?

 

[RNReindeer]

This is an excellent guide to soakaways:
http://www.pavingexpert.com/drain08.html
but I don't think it will answer your question about proximity to the highway.
You might need to consult your local authority.

 

[freddiemercurystwin]

BC seem to take a view on proximity to a highway, got a photo maybe?

 

[noseall]

In my experience BC would rather you built a soakaway closer to the road than a building.

We have come across this scenario often when dealing with rainwater on front elevations.

They also take into consideration as to whether the ground is sloping towards the building or away from the building when deciding the 5m proximity rule.

 

[EViS]

I can't quite figure out what size soakaway we require for our 100m2 roof area.

Following Paving Experts formula, Vol = roof area × (rainfall rate/3000), so Vol = 100 × (50/3000), I come up with a soakaway size of 1.7 m3.

Another popular formula is roof area/40, so 100/40, which results in a soakaway size of 2.5 m3.

Which is the correct formula to follow?

 

[jeds]

I wouldn't risk it that close to a road, even if a BCO allows it. And if they insist on it ask them for a copy of their valid PI insurance in case anything goes wrong. My advice is go into the storm water drain.

 

[EViS]

There is no storm drain, soakaway it is. Any advice on which formula is correct?

 

[freddiemercurystwin]

Normally the BCO will tell what he wants, being familiar with the local ground conditions, have you actually asked him? Otherwise 100÷40 is 2.5! That formula is touted on many a Building Control guidance sheets so I would expect that to be acceptable, once you go over100m² then you need to be a mathematician and percolation tests etc are required, technically. Sometimes BC will let you drain surface water into a foul sewer if there is no other option.

 

[jeds]

If the site isn't suitable for a soak away - which yours isn't from your description - and there is no storm water drain, you are entitled to go into the foul drain. If there is one of course.