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Sign-off on a soakaway design


catrionag

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Just as I thought we were close to getting our Building Warrant approved, Building Control have said they want signed-off calculations for the soakaway that the architect has designed. Who would do this? Would it be the company we got to do the site investigation? 

 

The soakaway we have currently is the maximum size that will fit in the plot while abiding by all the rules about distance from boundaries/roads/buildings (19m2) so I'm really hoping that the findings from the SI support the current size...

 

Alternatively, if anyone can recommend a person/company who would be able to do that sort of thing I would be very appreciative!

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What have you submitted?

 

They will as a minimum want percolation test results and then calculations using the approved method that is detailed in the BR technical standards book, showing the soakaway area required, and then a site layout drawing showing that will fit with all necessary clearance distances.

 

 

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We submitted percolation test results, and the architect has submitted his own calculations, but in his words he was 'winging it' a bit having not designed a soakway in a while, so I'm assuming that Building Control want the sign-off of someone suitably competent.

I'm just wondering who that is - whether we can do it ourselves since we have all the information, but then who signs it off?

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I am not sure if this does actually need a "qualified" person to sign it off, but the calculation method is in the technical handbook, so why don't you run the calculations using the percolation tests you have and see what you come up with.

 

If your soakaway only just fits in the available space then no doubt BC are concerned you or your architect have massaged the figures to make it fit and are seeking confirmation that is not the case.

 

Post your percolation results here and one of us will have a go and see what area you need.

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28 minutes ago, catrionag said:

whether we can do it ourselves

That will be up to the Officer.

 

If the Architect has 'winged it' with calculations,  you should ask him to resolve it and within his fee.

It is not just a matter of ticking it off, as it does have to work.

 

It does need to be right as nobody wants overflow, especially from the treatment tank however clean it is supposed to be.

 

On the matter of being an appropriate person, who knows?. I have the qualifications but am not registered in Scotland, and our Warrant application was responded to that we needed a registered SE for the structure. However my drainage proposal appears to have been accepted.....as they have no further questions.

 

So yes post it here. Perhaps privately tell one or more correspondents of any concerns you have , as this page is public,

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Our architect did the permeability tests and designed the soakaway to suit.  However these calculations had to be signed off by an "expert" that had the right computer program.  Ultimately the whole treatment system had to be approved by SEPA, including the soakaway and they insisted on a pumped system for the outflow to soakaway.  Beware SEPA approval took an age.

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14 hours ago, JohnMo said:

the right computer program.

The 'right' computer programme is expensive. However it is simply a spreadsheet that iterates the rainfall possibilities and then the volume required is the worst case.

A cheaper alternative is available online, or you can even make your own, but they may not be accepted unless you have the credentials yourself.

 

Using the recognised programme shows the Building inspector that the information is likely to be accurate.

However, garbage in/garbage out apples, and I have seen proposals that are simply wrong.

 

By all means, do your own assessment for comfort, and to pre-consider the most practical solution., but I think you need expert presentation.

 

BTW in some areas of mass house building but where the drainage is inadequate, building inspectors are forced to agree lower standards for rainwater.

 

 

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Thanks so much everyone! 

 

I did take a look at the spreadsheets that were available but felt quite out of my depth to be honest - my Higher B in Maths is not serving me well here. 

 

The percolation rate from the SI was given as 1.5 x 10-5 (depth of test hole was 600mm)

The Impermeable Area I'm taking to be the roof area? Roof area is 128m2. Footprint of the house is approx 80m2.

 

As mentioned above the available space for the soakaway is 19m(underneath the driveway, so it would be filled with hardcore/gravel)

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6 minutes ago, dangti6 said:

Might be misinterpreting your final statement, but why not use crates? The older method of filling a soakaway with material reduces the volume and therefore requires a larger 'hole'.

Apparently we can't have crates since it's under the driveway - supposedly we need the gravel/hardcore in order to support the weight of vehicles. Unfortunately there is no space for it anywhere else on the plot as it would be too near buildings or boundaries.

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1 minute ago, catrionag said:

Apparently we can't have crates since it's under the driveway - supposedly we need the gravel/hardcore in order to support the weight of vehicles. Unfortunately there is no space for it anywhere else on the plot as it would be too near buildings or boundaries.

You could use crates but would then need a load bearing “bridge” to support vehicles.

some crates do have a load bearing rating but no idea what it is.

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So I'd basically been advised by the bloke that's doing our groundworks that we'd need to use a hardcore infill for the soakaway since the driveway is going to be the only access onto site and will need to take the weight of heavy plant/lorries etc. I hadn't realised they would be available rated for such heavy loads, so I'll check back in with him!

 

Otherwise I guess it would mean we'd just need to dig up the driveway again after the build to put the crates in?

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51 minutes ago, catrionag said:

Thanks so much everyone! 

 

I did take a look at the spreadsheets that were available but felt quite out of my depth to be honest - my Higher B in Maths is not serving me well here. 

 

The percolation rate from the SI was given as 1.5 x 10-5 (depth of test hole was 600mm)

The Impermeable Area I'm taking to be the roof area? Roof area is 128m2. Footprint of the house is approx 80m2.

 

As mentioned above the available space for the soakaway is 19m(underneath the driveway, so it would be filled with hardcore/gravel)

Hold on, are you talking about just the soakaway for waste water from a treatment plant?  Or SUDS drainage to dispose of rainwater?  or BOTH?

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Just now, ProDave said:

Hold on, are you talking about just the soakaway for waste water from a treatment plant?  Or SUDS drainage to dispose of rainwater?  or BOTH?

 

No treatment plant at all! Just SUDS drainage to dispose of rainwater. Sorry if I've been using the wrong terminology, it's all new to me :)

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7 hours ago, catrionag said:

filled with hardcore/gravel)

 

If you buy single sized stone, such as gravel, at least 2/3 is stone and  only 1/3 is gaps for the water.

So you need a lot of it, gravel isn't good under a drive, other crushed stone is expensive, and you also need a porous top surface,

Over time even this 1/3 tends to fill with silt.

I don't know your builder but it is unlikely  he knows all this.

 

Crates can take artics as long as you get the right ones. The shallow ones allow the water to disperse into a greater area of ground, but are more expensive, and depend on porous surface.

The boxy ones are fed from drains.

Whichever method, you have to allow for the amount of rain and the percolation.

The builders merchants (especially the drainage ones) can get the sums done for you, if you know the input numbers.

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