Building over sewer problem

problematic

Hi,

I'm looking for some advice on building over a sewer.

I own a house built in the 1890s which has a sewer running through the garden (around 150mm diameter, and around 700mm deep). We have been building an extension and have applied to Thames Water for permission to build over the sewer. Unfortunately I'd not really researched this properly and took some word of mouth advice that this being granted shouldn't be an issue. We decided to go ahead with building the extension without waiting for the approval (a decision I feel I might be about to regret..). The rear wall of our extension is directly over the sewer; we followed Building Regs advice about how to lay the foundations and have ended up with four foundations, two outside the building and two inside, over which there are two steel joists perpendicular to the rear wall, and a steel directly under the rear wall. Thames Water are happy with all of this, except they say the rear wall should be offset by 600mm from the sewer (which would effectively reduce the size of our extension by 600mm). By now we have the walls and roof in place so I would ideally like to avoid having to move the rear wall!

In my naivete it seems to me that there is sufficient space to access the sewer if necessary as there is a gap of 3 metres between the foundations at either side of the house. Does anyone have any experience of how flexible Thames Water are likely to be about this? What are our realistic options at this point?

Thanks in advance for any advice,

Andrew

anotherbaldrick

Not quite clear on the layout you have, is their space to divert ie: move the sewer over at a future date ? If so I would have a static water test done on the sewer and if it proves sound approach Thames water with the suggestion that if in the future their is a problem you can and would carry out a diversion to the sewer. At 700mm deep it could be hand dug and 3m gives adequate room. you would only need a 300mm wide trench providing the ground is stable

problematic

Thanks for the quick response - I'll try to be clearer about the layout.

Our house faces north, with a garden to the south. It is a mid-terraced house approximately 5 metres wide. The sewer in our garden runs east to west and I believe continues into both our neighbours properties. We are building an extension on the back of the house which is the full width of the house (i.e. ~5m) and to a depth of 4 metres. The rear wall of the extension we have built is directly over the sewer. The four concrete foundation footings we have laid are all 1 metre cubed. Two of the footings are 'inside' the extension; they are at the east and west edges of our property so are approx 3 metres apart. The other two footings are in the garden and are in a similar arrangement. Effectively the four footings are in a rectangular layout, with the sewer running through the middle. We then have two steel joists that run north-south, one sitting on the two 'western' footings and one on the 'eastern' footings. Directly about the sewer we have a joist which runs east-west which sits on the two north-south joists. The rear wall of the extension is built on top of that joist.

I think that moving the sewer might be quite difficult - given that our rear wall spans the entire width of the property I'd imagine we'd have to begin the movement of the sewer in the neighbour's gardens so that it is 600mm clear of our extension at all points. They might be willing to let us do that but it would be nice to avoid it if possible..

anotherbaldrick

Do you not have a manhole or inspection chamber on this run of sewer taking discharges from your property ? If you do how do you access it if it is below the extension wall. This person who advised there was no problem building over, was he a professional engineer or someone who thought he knew it ?

presumably you have read this http://www.thameswater.co.uk/cps/rde/xchg/prod/hs.xsl/693.htm

keystone

problematic wrote: »

Does anyone have any experience of how flexible Thames Water are likely to be about this?

You wont be able to second guess TW and neither can I. You'll have to wait for them to communicate back to you.

Cheers