Jump to content

Recommended Posts

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 2.0mother 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.

Link to comment
Share on other sites

Link to comment
Share on other sites

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 2.0mother 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 by Carrerahill
Link to comment
Share on other sites

@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.

Link to comment
Share on other sites

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 by Carrerahill
Link to comment
Share on other sites

@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.

Link to comment
Share on other sites

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?  

  • Like 2
Link to comment
Share on other sites

@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.

  • Like 1
Link to comment
Share on other sites

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.

Link to comment
Share on other sites

@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?

Link to comment
Share on other sites

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.

  • Thanks 1
Link to comment
Share on other sites

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.

  • Thanks 1
Link to comment
Share on other sites

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

 

424005229_Screenshot2020-05-01at09_34_33.png.815a76bd30e2447a6018baf363ab69dc.png

 

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?

 

2034525722_Screenshot2020-05-01at09_37_21.png.7b92b12340513077d78b0db068df0c5e.png

 

Link to comment
Share on other sites

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.

  • Haha 1
Link to comment
Share on other sites

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

 

424005229_Screenshot2020-05-01at09_34_33.png.815a76bd30e2447a6018baf363ab69dc.png

 

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?

 

2034525722_Screenshot2020-05-01at09_37_21.png.7b92b12340513077d78b0db068df0c5e.png

 

 

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.

 

 

 

Link to comment
Share on other sites

@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.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

×
×
  • Create New...