Drainaways - Calculating size

[davespaves]

I am just looking into having the front of the house flagged.
To conform with suds I believe I will need to install a soakaway.
The driveway is about a third of the width of the house and the garden two thirds.
Rainfall runs off the existing drive to the street, and on the garden side drains straight into the lawn.
I am hoping to put new flags down to cover the existing front drive and existing front garden except for a 1.2m border by the pavement .
This means the driveway will need a gulley to drain into a soakaway and the newly flagged garden will drain into the 1.2m soil border.
When working out the size of the soakaway can I just calculate for the driveway area which is the part that needs to be stopped from draining into the street and which is just about one third of the total new flagging.
Thank you for any help on this.

[lutonlagerlout]

I had the building inspector out today and he says they currently work on 1 M3 of crates for every 15M2 of drive or roof
hope this helps
LLL

[davespaves]

Thanks for that but what I am trying to establish is, I plan to have about 90 square metres of new flagging but only about 30 square metres would drain onto the pavement, the other 60 will drain into a wide soil border.
Do I need to calculate soakaway capacity for 90 square metres or just the 30 square metres that would end up draining onto the pavement and road.
Thanks.

[davespaves]

Thanks for that but what I am trying to establish is, I plan to have about 90 square metres of new flagging but only about 30 square metres would drain onto the pavement, the other 60 will drain into a wide soil border.
Do I need to calculate soakaway capacity for 90 square metres or just the 30 square metres that would end up draining onto the pavement and road.
Thanks.

[Tony McC]

they currently work on 1 M3 of crates for every 15M2 of drive or roof

That seems excessive, but then it may be that there is really poor ground permeability where you are, Tone.

It all gets very complicated, trying to determine how big a soakaway is required, but the key is that ground permeability. I keep trying to write a simple explanation of how to do a basic 'back of a fag packet' calculation for crate requirements, but it always ends up with me getting dizzy and having to lie down in a dark room.

As a very, very, very rough guide, I've been working on the principle that, if a 300x300x300mm trial hole formed at invert level of the intended soakaway (so that means this 300x300x300mm hole would lie underneath the base of the lowest crate)...if such a hole is filled to the top with water and that water drops by 150mm or more overnight or in 24 hrs, then use 1 crate per 15m² which is equivalent to 1m³ of crates per 60m² of roof or paving.

Obviously, if the local BCO have published guidance, that would supercede anything I suggest, and it's often worth giving them a call to check if they do have any guidance, and if so, what they would suggest, as they, better than almost anyone else, would know what's what locally when it comes to groundwater drainage.

[lutonlagerlout]

yep pretty much all clay round these parts,with just a few areas of chalk and sand
the main BCO for Luton is a decent sort and said to me that his hands are tied now 1M3 per 15m2 of crates too not "clean rubble"
and BTW I used the PAVE1 discount code at plastics express last week and saved myself 30 quid
Happy days
LLL

[Tony McC]

It's about time you got something back from this site, Tone - you put in a lot of unpaid work so very much deserve a little reward now and again! Drink it wisely!