Soakaway for an outbuilding 37sq metres

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Hello All,
We have had a flat roofed outbuilding(7.5m x 5m) built by a builder on building notices. I trusted that he and his men knew what they were doing. He called in a BC inspector for final sign off certificate last week. It failed on 2 items, one of them was the soakaway that I nor the inspector saw until it was all covered over. I dug down today briefly and found the 100mm plastic pipe about 125mm deep below the grass with 1cm dia stones scattered around and it was sloping upwards rather than with a fall downwards........ Not a good start! I didn't dig at the 5m end away from the building yet to find out what was there.
The soil is clay, so should it go to a soakaway firstly? If it can go to a soakaway, how deep should it be and is 5m away from the building and 2m from my boundary OK? Also should it go to a special receptacle at the end of 5m?
I think the water table is quite high as we are on flat ground 1km from the sea.
If a soakaway isn't allowed in clay where does the surface water have to go? Into the main drain?
Please advise anyone that can help.........
Thanks
Penny
PS the other failure by BC was the strength of the roof which is yet another thing to sort out! Should it be my problem or the builders to do?! We are still on good terms at the moment.....I think.
 
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Yes, the soak-away hole and all drains leading to it should have been inspected and passed.

However, the fact that it is in clay means that water is unlikely to soak away in any case. The 'soak-away' becomes a catchment pit. There should be either a soak-away run-off pipe or the storm drains need re-directing elsewhere.
 
Rain water does not need to go to a soakaway. The requirement is that it disperses into the ground in such a way that it does not affect any structures by making the ground soggy. So this can mean around the building, or to a concentrated point away from a building such as a soakaway.

There are different types of clay, and it depends on how porous it is as to how big a soakaway needs to be.

If the ground is not suitable or large enough for natural drainage, then connecting to existing drains can be an option.

It's implied that the builders have responsibility to build in accordance with building regulations, unless your contact says otherwise. But the council will always come to you as the landowner/applicant for being in breach of the regulations.
 
Woody, when you say an option is connecting to existing drains, do you mean any of the existing drains not just surface water drains? The bungalow rainwater appears to go into the main drain which is also the foul drain. Can it be connected into that? And if we do, would we have to get/do an infiltration test to prove the clay is unsuitable? If the test proves that it is unsuitable clay due to lack of porosity do we have to dig one of those deep drains into those crate things or can we simply connect to the existing drain? The garden is quite big but the other bungalows are quite close to our boundary so to take the soakaway clear by say 10m it would be closer to them than us.
I'm unsure of the action plan to arrive at where we should direct the water.
Thanks
 
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There is an order of preference for rain water disposal - harvesting, ground, watercourses and lastly existing drains.

If the first is not possible or feasible, then you go to the next on the list, and so forth.
 
Thanks Woody but how do we establish what is not feasible? The ground is saturated at the moment and the water table is probably as high as can be without actually flooding and the soil is clay right down to the 1m deep footings. If we carry out a infiltration test now won't it show that we can't use a soakaway as it is full up with water. As there isn't a water course nearby do we have to put in a rain water 100mm pipe to join with the combined SW and FW drain? Apologies if I'm not getting it.
Penny
 
Oops. The plans for my flat roofed (well less than 10 degree roof pitch) 70 sq m outbuilding also called for guttering to drain via soakaway. Seemed feasible to me (after all, the rain falling on the ground had been soaking away for the last no idea how long....) but BCO disagreed- when he came out to check the foundation trenches he told us 'footings are fine, that's good solid clay, but you can't have a soakaway in that, you'll have to run drainage to the main drains'. Cursed him at the time but actually it has been a bonus- there's a toilet in there now, and a washbasin, and a washing machine. Cost me a certain amount of time and effort digging a 33 metre trench and setting it to a fall but that's done now (and signed off) and having running water and toilet in the garage is bliss.

You're not allowed to drain rainwater via the mains drainage system on new builds (as far as I know) but for older properties, if the ground isn't suitable for a soakaway then mains drainage is the Plan B under instruction from your friendly BCO. If you don't have mains drainage (or want to keep the 'no surface water drained here' discount) then you could try some sort of water harvesting/slow irrigation scheme (1000 litre IBC tanks to catch the rainwater, some sort of pressure regulated/siphon/pumped nonsense to water the garden and provide grey water for flushing toilets) but payback time is in the decades with current sewerage and water charges even if you're on a water meter. Bottom line- if Building Control tell you to drain gutters to the mains drainage then that's what you do. Mind, given you are in the land of hosepipe bans you might want to have a think about storing some of your rainwater for a sunny day (imagine how annoyed your neighbours will be when your lawn is lush and green and theirs is parched and nasty. Double the irritant factor when they dob you in and the Water Police find you not guilty on account of your IBC storage tanks filled with rainwater from the winter.....)

For that sized building, you probably needed planning permission- did the plans specify soakaway or was drainage not mentioned? I'm not familiar with construction details for soakaways but browsing on here tells me that a typical soakaway would be a cubic metre of largish (50mm) stones buried in the garden 5m from the nearest building in any direction. If you don't have one of those then your builder has possibly been trying to pull a bit of a flanker.

Fag packet calculations for a 1 cubic metre soakaway (assuming no access difficulties)- minidigger and driver for half a day £100, small skip (to get rid of the cubic metre of soil/clay) £100, 3 tonnes large ballast £60, 5m x 110mm waste pipe £15. Total £265

Fag packet calculations for trenching to mains drainage (assuming no access difficulties and adequate fall). Minidigger and driver for half a day (£100), small skip (assume 500mm average depth trench x 300mm wide. Each metre of trench will produce 0.15 cubic metres of spoil. Some will go back in the hole, some will need to get gone) £100. Pea shingle £25/ton, allow 3 tons/cubic metre so .18 tons/metre (£4.50 per metre) of trench. 110mm pipe £2.58/metre single socketed, couple of inspection chambers £30 each. Total fixed £260 plus £7-ish per metre of trench

NB These prices are for the North East of England. You should probably double the labour and hire charges in Surrey. Material prices- the stone might be more expensive, the pipe and inspection covers are from T'Interweb so shouldn't be region specific.

So your builder owes you the fixed costs of the trench, you'll need to chip in a cost per metre of the trench (assuming the original plans called for a soakaway). You may be able to haggle for a better deal- a prudent builder would have called BCO to inspect the hole for the soakaway before filling it with whatever. By the way,those are cost figures- no profit included.

As for the roof strength- oh dear oh dear oh dear. Again on my little project (7m span), at an early stage BCO told us he'd accept manufactured roof trusses if we showed him an invoice from a truss making company with BBA /ISO certification, if we went for rafters he wanted to see calculations. I wanted rafters (didn't build to 4500 just to look at a load of timber struts) so got calculations from James Jones (their JJI beams are pricey but very useful- I'm no Superman but managed quite easily to lift and set 7 metre rafters on my own), fixed and strutted them as per their spec, gave BCO the calcs and the spec sheet and a look at the job and off he toddled, happy as a happy thing. That fix is down to the builder- if he can prove that his roof structure complies with the relevant legislation (doesn't have to slavishly follow the Approved Documents, they are one route but not necessarily the only route to compliance) then you're both laughing. If he can't prove compliance then he needs to modify it so it does comply. That's down to him to sort out unless the original plans specified magic fairy zero mass film for the roof covering and you've changed your mind and put 4 inch concrete slabs on instead.
 
There is an order of preference for rain water disposal - harvesting, ground, watercourses and lastly existing drains.

If the first is not possible or feasible, then you go to the next on the list, and so forth.
I have found that nine times out of ten, Mr BC will be familiar with local ground conditions and will know if a soakaway will be acceptable or not and I’ve not heard of having to do surface water into anything other than a soakaway or the mains for a domestic extension (so far) and have never had to do a percolation test for a domestic extension, normally a 1m cube will do it. So I would have expected the priority list to be soakaway, surface water drain, foul or combined drain.

In clay I would not expect it to be acceptable.
 
Thanks Woody but how do we establish what is not feasible? The ground is saturated at the moment and the water table is probably as high as can be without actually flooding and the soil is clay right down to the 1m deep footings. If we carry out a infiltration test now won't it show that we can't use a soakaway as it is full up with water. As there isn't a water course nearby do we have to put in a rain water 100mm pipe to join with the combined SW and FW drain? Apologies if I'm not getting it.
Penny
Were the builders following design instructions or was it a design and build arrangement? The basic process with drainage is that you are obliged to pursue some form of natural drainage, which usually means a soakaway. Only if a soakaway is unsuitable can you move to the next option which is surface water drainage and if that's not available you can then go to a foul drain. All as explained above. For a soakaway to be suitable it must be 5m away from any building and 2m away from any boundary and the soil must physically drain at a minimum rate, determined by a percolation test. The soakaway is really an attenuation pit designed to hold enough water to take care of a downpour. When it pours with rain the pit fills, when it stops pouring, or slows down, the pit slowly releases the water into the ground. The pit must be sized to match the percolation rate. Lower percolation = bigger pit. Who did the design?
 
Oh crikey, thanks for all this info. It's brilliant everyone.
Well the bungalow is in West Sussex 1km from the sea and is not our main home in Surrey. It's meant to be our retirement home for later if anyone's interested! It won't be long now!
We asked about planning and it was said that it is not needed. It has a flat roof. The new part of the building is an extension of a flat roofed semi detached garage so is 2.5m away(a garage width) from the boundary so planning not necessary we were told.
A structural engineer is coming today with the builder to see if extra work on roof is needed. The building inspector reckoned that the span should be a max of 1.84m but it is 2.4m with 100mmx50mm joists. The builder put in a 152mmx152mm column type beam down the middle.

When they come I will then show the builder the soakaway pipe going uphill. Only a small pit appears to have been dug at the 5m end of the soakaway about 0.5m x 0.5m. I haven't dug down to see how far it goes. The pipe going uphill was enough for me! The builder left the ground workers to their own devices unfortunately. We have had a soil pipe put in the corner of the garage before the floor slab went down)for a WC at a later date but it is at the wrong end of the garage for rainwater disposal as the roof slopes to the other end. It may be possible to join it on another side of the building though.
I will ask the BCO if he thinks the soil for a normal soakaway is OK anyway.
I'll keep you updated............
Penny
 
Bear in mind that people oftern quote pre-calculatd span tables when stating how long roof joists should be, but it may be possible to do a dedicated calcualtion to prove that the joists can be longer than the tables state.

Instead of having an ugly beam below the joists, it may be possible to insert exta joists instead. The engineer can advise.

Don't accept an inferior job just because your builder messed up. You will be living with it.
 
Yes, an inferior job it may turn out to be. I don't think inserting extra timber joists is an option now. As the roof was also being insulated with celotex between the joists, plaster board was used to hold it in place. For some reason the builder decided to plaster it as well and all over the big steel beam. Not sure if it was a deliberate case of covering things up that might not be quite right as far as building regs go. I know it's not going to look too good with more steel beams over the nice plaster. Anyway the engineer will be doing some calcs and has hinted that 2 x 100mm deep steel beams may be needed which is a bit of a bind as the nice lighting will have to be moved thro' 90degrees as well to avoid them.
The engineer did say he would look at the best option regarding the roof. Let's hope he does.

The soakaway is being redone tomorrow to the BC inspector's requirements anyway thankfully. No photos of the first attempt that were promised have come to light.
Penny
 

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