The Brew Cabin

Posted: **Sun Apr 28, 2013 8:36 am**

i am currently building an extension. i do have issues with finished ground levels which i will try and explain. basically i live on a slight hill where all the ground from the left and the rear slopes towards my house. in order to extend my semi detached house and maintain a level internal floor, i have had to dig into the ground to my left which now means that the finished DPC on that external wall will be 550mm above the original DPC level of the house. i have dealt with this with building control by doing a semi tanking up to the external DPC. Basically internal DPC from under the concrete floor being raised up the inside edge of the inner skin to external ~DPC level, and external protection damp proof boarding placed against the external wall before backfill. There is no option to step down the outside ground level below existing DPC since the neighbour to my left owns that land.
currently i have a great deal of water settling on the trench filled footings which currently drains into a sump which i have cut into the ground. since the back corner is the lowest point in the garden it is no surprise, and to add to that the soil in my area is basically brick grade clay so all the ground water just runs off the top.
I have wondered about the best way to get rid of this water as follows.
1. Fill the gap between the external wall and the ground outside with concrete, from the top of the trench fill to the existing level of the concrete until it is level with the ground level of my patio on the opposite side of the extension, then fit some sort of gully there to try and move the water away. this will be 225mm above internal DPC level, so 375mm to external DPC. however this external DPC will still have to be stepped up to accommodate the highest ground level which will be 550mm above internal DPC.

what i am worrying about is, "where will all that water go if i do that.? will it just rest against the concrete i have used to backfill against the external wall or will it find its natural way to anther route.

the sump i have dug into the ground to collect this water is right by the side of the trench filled concrete, and is about the size of a builders bucket. This filled with water to the top in about an hour.

part of me is thinking that this is bound to happen since it is the lowest point in the garden at present until i back fill, but mu concern is that both the rear gardens and the side neighbours gardens to slope to this point.

is it best to try what i am doing or should i also go to installing an external sump and pump.

Posted: **Sun Apr 28, 2013 9:38 am**

you have totally baffled me with your explanation, a picture says a thousand words.

Posted: **Sun Apr 28, 2013 1:30 pm**

you need to install a separation drain against the boarding then have a 4" perforated drain at the bottom taking the water away into the top water system, you dont want that build up of hydraulic pressure against your tanking

we used a system from fosroc and basically it creates a 1" void that the water runs down in to the drain, it was in 8x4 sheets and its a layer of permeable geotextile that goes towards the ground then a spacers to give the void then a non permeable layer that goes against the tanking protection(very dense insulation foam)

Edited By GB_Groundworks on 1367152254

Posted: **Sun Apr 28, 2013 2:33 pm**

could you clarify what you mean by the top water system, and how would l get it there since it is all in lower ground a the moment.

regards

Posted: **Sun Apr 28, 2013 5:19 pm**

top water is where your gutters and grids go as opposed to the foul system where your shit goes but on older properties they are combined

Posted: **Sun Apr 28, 2013 6:30 pm**

ah , thanks. but that seems to be the issue here. the water is settling lower than any drainage system for rain water or surface water drainage. the back ground of the house slopes towards the house, and so does the left hand side. the only way to make the water run off from the corner would be to back fill the outside of the footings by three inches using some cement or concrete, then put down the plastic agricultural drainage pipe and cover with gravel. My only concern in doing this is that i am worried that this will allow the water to get under the concrete or cement that i would put down to raise the ground to slope the water away, since it seems that the water is coming up from the trench fill footings which are only 225mm lower than the concrete slab floor level anyway. or am i worrying too much? as it is the water would then follow its natural path as it did before, and drain into next doors garden. something i don't want but it has always been like that. the ground is brick making quality clay, so soak aways are useless. as it is most people have resorted to putting rain water into hoppers, and into the soil drainage. a system i know they don,t like but there is no other option`.

Posted: **Sun Apr 28, 2013 7:17 pm**

but your drains must fall away from your property and get deeper run them into the deeper bit if you can, or make a chamber and put an auto sump pump in?

Posted: **Sun Apr 28, 2013 7:37 pm**

You have hit the nail on the head. There are no surface water drains at all. However I have run French drains to the right of of the garden following the slope, so if I were to rais the level in the problem left hand side corner I could pick that up with some drainage as previously described. I did think about putting a sump in at the problem end but if I can do as per my previous reply that could work . It is hard to believe that her are no surface water drains her, but there are not. 1950,s estate, built on a shoe string. The front of the hous is lower but again itvisbimpossible to get to that point form the problem corner. The extension in 14 mtrs long from back to front with no room to get drainage. As I said I did think of a sump pump placed externally, but thought it was a bit overkill?. Then there was the issue of wher to pump it to? Would you suggest a sump under here circumstances or would you give he French drain idea as previously mentioned a go? .

The Brew Cabin

Land drain or sump pump.