Which roof area to account for when calculating soakaway

[garyo]

Hi Chaps,

I'm building a rear extension on full-plans and currently drawing up my soakaway. It's a semi detached house and the existing roof shares two downpipes with the neighbour: the front one discharges directly on to the ground, and the rear one goes in to the sewerage system. The plot is on a gradient which means this works well.

I assume I'll have to add a new downpipe at the rear of the house and connect this to a new soakaway in the rear lawn. When calculating my soakaway size, should I just use the roof area that I'm adding, or do I need to take in to account my whole roof? What about the neighbours roof?

The pink area in the diagram below is the new extension.

Another idea: I had to dig a pretty large soakaway in the front lawn last year for the new porch roof - The maths say that this is of sufficient size for the new extension too, so I'm tempted to propose that I link the front house downpipe in to this existing drainage system. Would this be allowed given that the rear would still be draining in to the sewers?

Thanks

Gary

Untitled

• garyo
• 21 Jan 2012
• 1

 

[RedHerring2]

When I did mine I only calculated for the new extended part of the roof, even though some of the old roof now feeds into the soakaway.

You could decide on your guttering layout and take it from there.
e.g, the new hip end to your new extension is defintely to be included. But the side return to a valley, you might decide youre going to feed it into the sewer, but it really should be fed, including up to the valley, into your new soakaway. Anything else might upset the BCO.

Similarly,what I assume was the original hip end, that is now the side/end pitch, you could drain all of it to soakaway 'cos you can't do some into the original downpipe and the rest into the soakaway.

 

[Static]

Anthing that feeds into the new soakaway should be part of the calc.. its there to stop it backing up every few years in a heavy storm..

 

[garyo]

Cheers guys - I've specc'd a new 1.2 m^3 soakaway in the rear lawn, and I'll adjust the guttering to suit.

Gary