joe90 Posted May 1, 2020 Share Posted May 1, 2020 I think they calculate that way with multipliers because most people can’t measure the gable slope distance (actual distance from ridge to guttering). Link to comment Share on other sites More sharing options...
Mr Punter Posted May 1, 2020 Share Posted May 1, 2020 If this is still about soakaway sizing, just use the calc I gave - 1m3 for each 60m2 of plan area. Does not matter what sort of roof goes on, just length x width. The stuff about the roof slope is just for gutters and downpipes, not soakaways. Link to comment Share on other sites More sharing options...
Russdl Posted May 1, 2020 Author Share Posted May 1, 2020 @Mr Punter It is, and your calculation gives me 2m3 not the 6.8m3 that are on the plans. Is that calculation going to be sufficient for me to convince the BCO that I only need 2m3? I guess there is only one way to find out! Link to comment Share on other sites More sharing options...
Russdl Posted May 1, 2020 Author Share Posted May 1, 2020 @Temp @LA3222 My computer has sorted itself out and I've now been able to open that spreadsheet. Sadly it hasn't helped a huge amount because I'm just too thick to work out how to input my percolation test results into the spread sheet so I've bitten the bullet and told my BCO inspector that I only need 2m3 not 6.8m3 ? I'll let you know what he says. Link to comment Share on other sites More sharing options...
Jeremy Harris Posted May 1, 2020 Share Posted May 1, 2020 2 hours ago, Russdl said: @Mr Punter It is, and your calculation gives me 2m3 not the 6.8m3 that are on the plans. Is that calculation going to be sufficient for me to convince the BCO that I only need 2m3? I guess there is only one way to find out! FWIW, I think that the calculation I did came up to be under 2m³, I only increased the size because I got a good deal on a lot of 20 Aquacells, we had a 15 tonne digger on site, and were already getting rid of hundreds of tonnes of soil, so another few tonnes was neither here nor there. Turned out to be handy, as much of the drive surface drains down into it as well. Our BCO (LABC Salisbury) wasn't the slightest bit interested in seeing any calculations, or even inspecting the soakaway. Link to comment Share on other sites More sharing options...
Russdl Posted May 1, 2020 Author Share Posted May 1, 2020 1 hour ago, Jeremy Harris said: Our BCO (LABC Salisbury) wasn't the slightest bit interested in seeing any calculations, or even inspecting the soakaway. Bugger. Perhaps I shouldn’t have asked the question! We’re using ‘Spire’ a private BCO. Link to comment Share on other sites More sharing options...
Big Jimbo Posted May 1, 2020 Share Posted May 1, 2020 I have always found bco's to be pretty clued up about soakaway. My new house needs 8cubic Mt. If its round the back, I hope you can get a digger round there after the house is built.... If you get sussed. 2cubic Mt is tiny for a whole house. Good luck. Link to comment Share on other sites More sharing options...
Russdl Posted May 1, 2020 Author Share Posted May 1, 2020 @Big Jimbo I’m not going to commit until I have the BCO agreement (especially as I’ve now asked the question). What calculations did you use to get to the 8m3 figure? Link to comment Share on other sites More sharing options...
Big Jimbo Posted May 1, 2020 Share Posted May 1, 2020 I can't remember buddy, but I used 2 different one's and they both came out all but the same. 2cubic meters I'd defo bloody tiny for a whole house though mate. Link to comment Share on other sites More sharing options...
Russdl Posted May 1, 2020 Author Share Posted May 1, 2020 @Big Jimbo the several versions of the calculations I’ve done have all come up with 2m3. To me it’s seems 6.8m3 is massive, but if it transpires that is what I’m supposed to have then that’s what I’ll do. I’m not trying to cheat the system here, it’ll be my back garden and I don’t want it to turn in to a swamp, but if I only have to dig 2 and a bit m3 instead of circa 7m3 then I’ll go with the former. 1 Link to comment Share on other sites More sharing options...
Pete Posted May 2, 2020 Share Posted May 2, 2020 13 hours ago, Big Jimbo said: I have always found bco's to be pretty clued up about soakaway. My new house needs 8cubic Mt. If its round the back, I hope you can get a digger round there after the house is built.... If you get sussed. 2cubic Mt is tiny for a whole house. Good luck. +1 to this. My BCO told me what he wanted and I did it. If I remember rightly, I have a 1.3 c/m soakaway for garage roof which is 12.5m x 6m and 3 .6m soakaway for house roof and roof terrace which is 10m x 10m and 5m x 9m. I have another soakaway for a lower roof but it follows the same ratio (ish) Link to comment Share on other sites More sharing options...
Vijay Posted May 2, 2020 Share Posted May 2, 2020 is it the same calculation if you have water harvesting? Link to comment Share on other sites More sharing options...
Russdl Posted May 2, 2020 Author Share Posted May 2, 2020 (edited) @Pete Thanks for those numbers. 1.3m3 for 75m2 and 3.6m3 for 145m2 seem pretty reasonable and does make the 6.8m3 for my 108m2 look a bit on the large size. Hence the original question. I've asked BCO now anyway so I'll wait and see what he says. Edited May 2, 2020 by Russdl Link to comment Share on other sites More sharing options...
Jeremy Harris Posted May 2, 2020 Share Posted May 2, 2020 Those numbers fit with what I remember for our roof size, as well, as I was pretty sure we ended up putting in about double the volume that was actually needed. Our roof area is ~90m² and I seem to remember that the soakaway requirement was around 2m³, but we made it larger just because we could, and because the percolation rate was a bit variable. Link to comment Share on other sites More sharing options...
Pete Posted May 2, 2020 Share Posted May 2, 2020 16 minutes ago, Jeremy Harris said: Those numbers fit with what I remember for our roof size, as well, as I was pretty sure we ended up putting in about double the volume that was actually needed. Our roof area is ~90m² and I seem to remember that the soakaway requirement was around 2m³, but we made it larger just because we could, and because the percolation rate was a bit variable. We have the fastest draining land in the West so no problem Link to comment Share on other sites More sharing options...
redtop Posted May 4, 2020 Share Posted May 4, 2020 we have 200m sq of flat roof and the soakaway, approved by BC, was sized at..... 1m cubed. And all done with no perc test. Anyhow I am planning on putting in more than that, probably 3 to 4 m cubed once I get round to running the calcs Link to comment Share on other sites More sharing options...
Russdl Posted May 4, 2020 Author Share Posted May 4, 2020 @redtop Thanks for those figures, they seem as wrong as mine. Or maybe they are both correct and I just haven't fathomed out how the figures are determined. From what I've gleaned so far, a roof pitch below 10° is not factored in anyway so your Effective Roof Area would be 200m2. With no percolation test then I guess you would use the standard calculation to determine the required soakaway size: Area x (Rainfall Rate/3000). Rainfall rate for the UK would appear to be 50 (thingamabobs) and the 3000 is, er, I've no idea. Anyway 200 x (50/3000) = 3.333. It looks like you should have 3.3m3 - so pretty much bang on what you're planning on. Link to comment Share on other sites More sharing options...
CC45 Posted May 4, 2020 Share Posted May 4, 2020 just checked - our soakaway was 12.6m3. 150m2 house footprint + garage @ 70m2 footprint. Roof area bit more than these so prob ~ 230m2. The soil infiltration rate was OK to good from memory. Link to comment Share on other sites More sharing options...
Russdl Posted May 4, 2020 Author Share Posted May 4, 2020 10 minutes ago, CC45 said: our soakaway was 12.6m3. 150m2 house footprint + garage @ 70m2 footprint Wow! Link to comment Share on other sites More sharing options...
Adam2 Posted May 4, 2020 Share Posted May 4, 2020 In mine there was a fair bit of hard standing that will likely drain to acco that increased the area a lot. Ended up with about 12m^3 of crates. And this was on sandy soil so perc test showed good drainage. Private building control just wanted to see some calcs and i used the supplier rather than my SE as his initial thoughts were much higher vol and he wanted a few K extra ffs. Link to comment Share on other sites More sharing options...
Russdl Posted May 5, 2020 Author Share Posted May 5, 2020 @Adam2 thanks for the info. Interestingly it’s starting to appear to be pretty random from project to project. 1 Link to comment Share on other sites More sharing options...
Russdl Posted May 9, 2020 Author Share Posted May 9, 2020 To update this. I contacted our BCO and explained that I thought our architect had made an error in calculating our soakaway requirements. He had drawn two, one north of the building for a roof area of 110m2 (4m3 of soakaway crates) and one south of the building for a roof area of 80m2 (2.8m3 of soakaway crates). The initial response from the BCO was mild confusion as to why there were two soakaways to which I had no answer. The second response was "...Can’t see why the design for the 100m2 soak away is not ok..." in other words, he was pretty vague about the whole thing. So I responded by telling him I'll put 3m3 of soakaway south of the building and he's happy with that. What I've learned is that calculating soakaway size is really straightforward if you use the 'rules of thumb' and a real dark art if you try and include percolation results in the calculation. It would also seem to be a dark art for my BCO (either that or he couldn't be arsed to enter all the data he had into his spread sheet?). I've also learnt that watching water drain away for the percolation test is therapeutic but otherwise a complete waste of time. 4 Link to comment Share on other sites More sharing options...
Russdl Posted May 14, 2020 Author Share Posted May 14, 2020 (edited) So I dug a big hole (well, my grave digger mate did) and dropped in 14 crates which equated to 2.8m3. The invert level into these crates is about 150mm below the top of the crates, maybe a bit more, so that removes .5 - .75m3 of storage so I've effectively got around 2.2m3 of soakaway. The BCO just wanted to see photos but at the last minute changed his mind and wanted to see the crates in situ. I was mildly concerned that he was going to change his mind and say it needed to be bigger so awaited his visit with a bit of apprehension. He turned up yesterday. He liked the cladding on our house and took some photos. He liked the (maybe) temporary name I'd given to our slightly contentious house and took a photo. He had quick look in the hole, took a photo of the membrane wrapped soakaway crates and drove off. Perfect. Edited May 15, 2020 by Russdl Pictures added 2 Link to comment Share on other sites More sharing options...
Russdl Posted May 23, 2020 Author Share Posted May 23, 2020 It seems that I have a ‘little mind’ because this ‘little thing’ pleased me no end! I’ve been itching for it to rain since I put the silt trap in, surely there are better things for me to be getting on with! C1C99E6F-B98A-45DF-9887-6046077BD851.MOV 2 Link to comment Share on other sites More sharing options...
Ferdinand Posted May 23, 2020 Share Posted May 23, 2020 (edited) I can tell you that percolation test holes half full of water are not things to go and inspect just to see how it is doing if you are dressed up to go out on the posh. Edited May 23, 2020 by Ferdinand 1 Link to comment Share on other sites More sharing options...
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