Soakaway - How big does it need to be ?

Discussion in 'Builders' Talk' started by Barn Builder, Apr 16, 2010.

  1. Barn Builder

    Barn Builder New Member


    I need to build a soakaway for our extension/rebuild.

    I am unsure how big it needs to be.

    Our roof area at the front is approximately 41.5 sq mtr.

    I can't seem to quite understand the calculation. Do I just divide by 40 which would mean the soakaway needed is just over 1 cubic metre below the inlet pipe?

    Can the hole be filled with rubble? This would reduce the volume etc...

    I will buy some of those plastic crates (aquacell) and fill the hole with those to maintain the volume if that is what is necessary. But they are quite expensive :(

    Can anyone set me straight on this.

    It seems to me that the days of a little hole filled with rubble are gone, if you need to meet the building specs.

    Please advise.



    P.S. - (I will need to do something similar for the back of the house, in fact a bit bigger as the rear roof area is about 55 sq mtr).
  2. clueless jon

    clueless jon New Member

    You can use a dumpy bag with a few holes in - it's about the right size and works well. fill with rubble and cover with an offcut of DPM or something. That's what i did and the council had no problems with it. i beleive the size is fine for 40msq
  3. doing a bit

    doing a bit New Member

    the soakaway needs to be well away from the foundations of the building to stop washing out of fines and potential subsidence, i would have said 1m square and 2m deep, back fill with 2 inch stone and use landscape fabric to stop soil washing back into the stone, this will eventually block up as french drain does the same
  4. Barn Builder

    Barn Builder New Member

    What surprises me about these rubble filled soakaways, is that they don't actually meet the volume requirements from what I can see.

    It seems that the true volume of a rubble filled hole is only about 30% of the size of the hole.

    I can't understand how this meets the regs as most average soakaway holes seem to be just over 1 metre cubed.

  5. Measure2cut1

    Measure2cut1 New Member

  6. the old un

    the old un New Member

    I note from your previous post that you are doing an extension both to the rear of your property and to the front. As both of them exceed the sizes of permitted development you should have applied and received planning permission.
    This being the case you have now got another problem on your hands. If you were building on permitted development, it is a 50/50 chance you would have no problems with the building inspector with regard to soakaways as they are not a statuary inspection, providing they are at least 5m away from your property, BC do not normally bother you.
    However, after the introduction of SUDS and the relevant code of practice, it would appear that central government issued a directive to all local planning controls that soakaway design is now part of the planning permission and another box for some pen pusher to tick.
    They may not have caught up with you yet but there is a very strong possibility that they will do so before signing you of. We got caught out on this with on our last house we have finished.
    Once you understand the formula, soakaway design for 100m2 and under is very easy, over 100m2 very complicated.
    The secret is to design your soakaways for a soakaway for every 25m2 area of roof, thereby you only have to design for 10mm rain fall in 5 minutes assumed to be the worst possible case, all as Building Regs Part H.
    We enclose a copy of our soakaway design. We have deleted the address of this property as we do not want any one to know where we are based, as we will tell you ways that we have heard but never practised, of how people do possibly cheat on this.

    Soakaway Design at (For guidance only)

    Total area to be drained. 99.33 m2, therefore form four number soakaways to drain 24.83m2 per soakaway.

    A design rainfall of 10mm over 5 minutes has been assumed to give the worst case all as Building Regulation Approved Document H.

    Percolation Test.
    Average time to seep away from 75% to 25% = 45 minutes.
    45 minutes by 60 seconds =2700 seconds divided by 150 = 18.
    Therefore (Vp = 18).

    Filtration Rate. f=10-3


    Therefore f= 0.0003

    Outflow Volume. O=as50xfxD

    therefore O=2x0.0003x5 = 0.003

    Area to be drained. 24.83m2 x 10mm rainfall = .2483m3 storage capacity less outflow 0.003 = 0.2453m3 total clear storage volume

    Soakaway size. 1.100x1.100x1.100 depth below invert = 1.331m3 with 20% void = 0.2662m3

    Therefore the above size soakaway will suffice.

    If you do not understand the formula, the percolation test, or the best way to form a soakaway, then come back and we will point you in right direction.
    Not for one moment, are we saying that you should, but we have heard how people do cheat.
    By measuring the roof on plan, and then plead ignorant that they did not realise the roof area would increase by approx 33% when measured on elevation.
    Percolation test. By digging the test hole in the top 300mm of the ground instead of at the bottom of the invert.
    By accidentally making a typing error in the (Vp).
    By knowing that it is one chance out of a hundred that any of this will be checked, All that is required is the actual design, so that the box can be ticked.

    old un. For guidance only.

    Noticed your post ref. flue to gable wall. Will drop you a post about it that may be of interest.
  7. inkpad

    inkpad New Member

    your ground conditions will dictate the size of your soakaway (as the percolation test described above will determine)

    Incidently Old Un, I have always considered the surface water drainge to be part and parcel of the 'Drainage' inspection, there is no difference between foul and rain in the notification/inspections that I am aware of, as such the soakaway gets inspected along with the pipework
  8. inkpad

    inkpad New Member

    great post btw Old Un ;)

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