Soakaway size

[Russdl]

My drainage plan shows that I need a whopping 6.8m³ of soakaway which seems pretty enormous.

 

According to the calculations from the drainagepipe.co.uk website I need 2.0m³ other on line sources lead me to a similar figure but I can't find a definitive calculator as opposed to these 'ball park' calculations. Does any one know what the proper calculations are? I have the results from my DIY percolation test.

[Temp]

The BRE have a guide. They normally charge for it but I found a copy online here..

 

https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.ecomerchant.co.uk/productattachments/index/download%3Fid%3D29&ved=2ahUKEwig2frsr5DpAhWUtXEKHYSmBFMQFjABegQIARAB&usg=AOvVaw3u7CprWtvZacdYCQlxh2OX&cshid=1588257907298

Edited April 30, 2020 by Temp
Problem with link

[Carrerahill]

1 hour ago, Russdl said:

My drainage plan shows that I need a whopping 6.8m³ of soakaway which seems pretty enormous.

 

According to the calculations from the drainagepipe.co.uk website I need 2.0m³ other on line sources lead me to a similar figure but I can't find a definitive calculator as opposed to these 'ball park' calculations. Does any one know what the proper calculations are? I have the results from my DIY percolation test.

Who did your soakaway calcs/plan? Does it take into consideration more factors than the others such as ground make up, rain fall, flood risk, size of roof, existing situation etc.

 

Might be that the 6.8 takes into account more than other do. 

 

2m isn't really that big, that is 2 IBC's - a big catchment area could soon fill that, or a small catchment area in a poor draining area could soon overwhelm that over a rainy period such as we had at the beginning of the year, soakaways need just that, somewhere for it to go, waterlogged ground equates to no soaking away!

 

 

Edited April 30, 2020 by Carrerahill

[Russdl]

@Temp thanks for that, I'll get reading.

 

@Carrerahill it was the Architect. It's 2 soakaways 1 to the north of the house, 1 to the south. We have no flood risk, we're on chalk. According to the on line stuff I've found so far a roof our size requires a 2m³ soakaway. I'll see what the BRE stuff says.

[Carrerahill]

12 minutes ago, Russdl said:

@Temp thanks for that, I'll get reading.

 

@Carrerahill it was the Architect. It's 2 soakaways 1 to the north of the house, 1 to the south. We have no flood risk, we're on chalk. According to the on line stuff I've found so far a roof our size requires a 2m³ soakaway. I'll see what the BRE stuff says.

OK then I'd ask myself has the spec been created by an architect who is clearly therefore not a civil engineer (or even better, a water engineer) doing a design which mitigates any liability by totally going overboard?

 

 

Edited April 30, 2020 by Carrerahill

[Russdl]

@Carrerahill No idea. It seems excessive to me, you could just be correct.

 

I'm currently struggling through that BRE guide that @Temp linked to. I don't mind maths, but I don't like maths that includes lots of letters!

[Mr Punter]

As a rule of thumb, a 1m3 soakaway does about 60m2 area, so take you area, divide by 60 and that should give a suitable soakaway size in m3.

[Russdl]

@Mr Punter Exactly. Using the following calculation:

 

Length of Roof x (Width of Gable/2) x 1.419 (for our 40° roof pitch) 

 

I get 100m² so I recon I need 2m³ of soakaway, not 6.8M³, but before I have a word with Building Control about this and explain that I think the Architect has got it wildly wrong I want to make sure I'm not missing anything.

[joe90]

@Russdl could you not ask your architect where he got his figure from?, then tell him what your figure is!

[LA3222]

Do you not need to conduct ground infiltration test in order to correctly size a soak away? It's all well and good knowing how much water is coming off the roof but a key component of the problem is how quickly that water drains away?  

[Russdl]

@joe90 That is the obvious answer but I want to know how to do it myself because as it stands I think he's wrong and I think I have all the data I need to produce an answer myself. I don't want to be fobbed off.

 

32 minutes ago, LA3222 said:

ground infiltration test

 

@LA3222 As I said, I've got the results for the percolation test, I take it they are one and the same thing? And from @Temp's link I've got a BRE document with lots of formulas, pictures wiggly lines and graphs so I think I'm on my way to an answer.

[Temp]

There must be a spreadsheet out there somewhere to bash the numbers for you? I'm mean one based on the BRE method.

Edited April 30, 2020 by Temp

[Temp]

Found one here..

https://www.yourspreadsheets.co.uk/soakaway-design.html

 

Scroll down for download link.

 

PS can't view it using the tablet I'm on. Needs macros enabled.

Edited April 30, 2020 by Temp

[Russdl]

@Temp Brilliant, thanks again. 

[LA3222]

9 hours ago, Russdl said:

 

@LA3222 As I said, I've got the results for the percolation test

My bad, I missed that part on your initial comment. Sizing the soak away up is easy then, I created my own spreadsheet to do the calculations using the formulae given in the BRE Digest, but it looks like you've been given a link to one. I'll have to check that out to confirm my own calculations are correct.

[Russdl]

@Temp That link wouldn't open for me last night. I gave my computer overnight to have a long hard look at itself and it's performance but it clearly doesn't give a damn because I still can't open that link (probably a Mac thing?)

 

@LA3222 Were you able to open the link? If so, was your spreadsheet correct?

[Jeremy Harris]

I can't seem to find the calculations we did for this, but remember that I used the chart in building regs to get the requirement, then worked from there with the roof area.  I installed 20 heavy duty Aquacell drainage crates, wrapped in terram, under the drive, with a volume of about 196 litres each, so a total volume of 3,920 litres/3.92m².  This was overkill, but the percolation rate here isn't that fast, as these crates drain via a narrow strip of permeable soil above the clay.

[LA3222]

15 minutes ago, Russdl said:

@Temp That link wouldn't open for me last night. I gave my computer overnight to have a long hard look at itself and it's performance but it clearly doesn't give a damn because I still can't open that link (probably a Mac thing?)

 

@LA3222 Were you able to open the link? If so, was your spreadsheet correct?

I had a quick look on my phone but the free version has loads of locked cells - not sure from glancing whether they are the ones you need to be able to change.  I'll open it on my laptop tonight and compare the two then to see how it is.

[joe90]

@Russdl just did a quick search and found this https://www.eden.gov.uk/media/1072/bre_digest_soakawaysdesign.pdf. Any good? (Not read it yet got household chores to do ?

Edited May 1, 2020 by joe90

[Russdl]

@joe90 That very similar to what @Temp sent me originally. With the two of them open side by side it should be a lot easier to get the data from Fig 1 and Table 1 without endless scrolling up and down, ta.

[Russdl]

@Jeremy Harris I don't remember seeing that chart. I'm off back to the regs!

[Russdl]

The first stumbling block in the Regs is the phrase 'Effective Design Area' I can't find a definition of it anywhere

 

 

What is the 'effective design area'?

 

Drainagepipe.co.uk clearly define it (aimed squarely at the innumerate) and it's what I'm using to convince myself that I only need 2m³ of soakaway. Does anyone know any different?

 

 

[Jeremy Harris]

I'm pretty sure I just used the calculation method given in Part H 3.23 to 3.30.  Wish I could find the spreadsheet I put together, as I remember pondering over how to calculate this.  It's probably at least 3 PCs ago, though.

[Temp]

17 minutes ago, Russdl said:

The first stumbling block in the Regs is the phrase 'Effective Design Area' I can't find a definition of it anywhere

 

 

What is the 'effective design area'?

 

Drainagepipe.co.uk clearly define it (aimed squarely at the innumerate) and it's what I'm using to convince myself that I only need 2m³ of soakaway. Does anyone know any different?

 

 

 

I think that's telling you how to calculate the effective collecting area. 

 

Lets say your house had a footprint/plan area of 1000sqm and a weird roof that was half flat and half had a 45 degree pitch the effective area would be 500 + (500*1.5) = 500+750 = 1250sqm.

 

 

 

[Russdl]

@Temp Surely that can't be right though can it, to have an effective roof area that is 25% bigger than the actual roof area. 

 

That extract from the drainage pipe website that I posted above would give an effective roof area of 750m² for a conventional 45° roof of 1000m² plan, so only 75% of the footprint.