Builders? Underground garage design.

[Driller]

We want to have an underground garage built on basement level. The adjoining walls (ie house and garage) must be touching as we want to build a terrace on top of the garage.

We are in the middle of having quotes done but I would like to understand exactly what we need to ask for especially in terms of waterproofing and drainage underground.

When we renovated the house I dug a trench around the foundations and placed a drainage composite and drain up against the foundations.

If the underground wall of the garage is in contact more or less with the wall of the house, how is the waterproofing and drainage going to be assured interms of materials used and how they are placed? What will stop water pushing up between the two constructions?

As I said there is a basement to the house but at this time no connection is planned between basement and garage although we may want to knock through in the future.

(the drain pipes and water containers on the left are temporary until an underground version is done).

[Tuna]

Why not build it a little bit away from the house (say a foot or two) and cantilever the roof beams over to the edge of the house? That way you don't have to worry about how the two sets of walls interact and your drainage can do whatever it needs to do.

[Driller]

Tuna said:

Why not build it a little bit away from the house (say a foot or two) and cantilever the roof beams over to the edge of the house? That way you don't have to worry about how the two sets of walls interact and your drainage can do whatever it needs to do.

Thanks for the reply. The trouble with doing that is that it's going to be a huge squeeze to get into that space to do the necessary work on treating the wall and putting the drainage in. IMO it would look a little strange too.

There are definately techniques available when two foundations walls are built in close proximity but I can't find any info on them.

[eps]

I'm not quite sure why you're building another wall next to the wall...

I would get some drawings made up, to see how things hold together.

The main issue, for me, here would seem to be the foundations and how they interact with each other and the drainage, I guess foul water, but then also rainwater..

Is the property in France? Not sure about how the system works there, in the UK it would be Planning Permission and then Building Control drawings, these would cover all the technical aspects of the project and would also be used to price from, otherwise there are too many unkowns and items possibly specified but maybe not..

[Driller]

eps said:

I'm not quite sure why you're building another wall next to the wall...

I would get some drawings made up, to see how things hold together.

The main issue, for me, here would seem to be the foundations and how they interact with each other and the drainage, I guess foul water, but then also rainwater..

Is the property in France? Not sure about how the system works there, in the UK it would be Planning Permission and then Building Control drawings, these would cover all the technical aspects of the project and would also be used to price from, otherwise there are too many unkowns and items possibly specified but maybe not..

Thanks for the reply,

The building of a new wall next to the existing wall is to support the roof section of the garage.

Would you key directly into the masonry of the existing wall then?

No foul water problem (no sinks etc planned) only rainwater an issue.

Yes, the property is in France but my questions are only of a technical nature not administrative.

[mk1fan]

The wall is already supporting not only the ground floor but also the horizontal loadings from the ground so I expect it'll cope just fine if the horizontal loads are replaced with another floor. Alternatively, run the roof to load the new garage walls.

Tieing the new structure to the existing will be very simple - just copy what's already there. And land drainage around the existing foundations can be easily altered to run round the garage.

[Driller]

mk1fan said:

The wall is already supporting not only the ground floor but also the horizontal loadings from the ground so I expect it'll cope just fine if the horizontal loads are replaced with another floor. Alternatively, run the roof to load the new garage walls.

Tieing the new structure to the existing will be very simple - just copy what's already there. And land drainage around the existing foundations can be easily altered to run round the garage.

Oh it'll support the load all right, the walls are 50cm thick stone I just don't fancy someone hacking chunks out of our existing walls from 1893.

Could explain what you mean by "run the roof to load the new garage walls."?

I suppose my main question was how could impermeability be assured if a (solid obviously) breeze block wall was built adjacent to the existing one to support the garage roof/terrace floor...

[mk1fan]

I mean you can run the roof joists parallel to the existing house, resting on the new walls.

'Impermiability' can only be assured with proper detailing. There's plenty of tanking options that could be used.

Sounds like you need to invite a professional to visit the property and discuss matters with you.

[aeropilot]

Driller said:

We want to have an underground garage built on basement level. The adjoining walls (ie house and garage) must be touching as we want to build a terrace on top of the garage.

Why do you need to build a wall up against the existing basement wall..?

Assuming the new garage floor level isn't going below the existing basement floor, which would have implications on you existing wall foundations, I can't see the need to seperate the structures.