Basements....

[JDLDIY]

Hi there. I'm trying to decide what to do with our house. One thing I keep coming back to is that I've always wanted a basement with whatever we do with it.

What I'd like to know is, how is a basement retro-fitted to an existing property - what is the process? I might have to underpin the existing house anyway.....

Any thoughts or pointers gratefully received!

 

[noseall]

What I'd like to know is, how is a basement retro-fitted to an existing property - what is the process?

Demolish existing property - dig a huge hole - build expensive tanked structure - back-fill.

Oh, and hope you don't have a high water table.

 

[RonnyRaygun]

I agree with Noseall - if you are going to go to the expense of fitting a basement then it may well be more cost effective to rebuild, and then you can also redesign the whole house.

However, if you did want to retrofit a basement there are a few ways of doing it.

One way is to pile inside the perimeter of the original building to create new walls - usually only possible if there is already a large void or cellar underneath the house.
Similarly a reinforced concrete box can be formed - the existing ground floor walls can sit directly onto the new reinforced concrete walls of the box. This has to be done in stages much like underpinning.

If there is no large substructure void it becomes much more difficult. I did one once on a detached house and the basement extended beyond the footprint of the house on one elevation. The procedure was something like as follows:
1: A series of piles were bored through the floor of the house and beyond where necessary.
2: At ground floor level pockets were cut into the existing perimeter walls.
3: The piles were trimmed to length and an RC slab was cast on top of the piles and into the pockets in the masonry.
4: Now the house was supported on the RC slab the area below the house was excavated.
5: New basement walls with adequate foundations were formed in RC around the perimeter of the basement.
6: The piles were trimmed down to basement floor level.
7: The basement floor slab was then installed.

As you can see, not a simple process, and one that was probably only cost effective because the house was in Hampstead!

Also, as Noseall said, water tables can play havoc with houses with basements - they really can float if not anchored down. If you didn't see last week's Grand Designs, watch it on 4od to see what we're talking about. Anchoring down to piles designed to resist uplift will overcome this problem but again, there is additional cost involved.

 

[angelboy]

I've understand why basements are installed in high property priced areas, such as London, because the figures work out. It can be cheaper to build one than pay increased property prices, especially stamp duty costs of million pound houses. I'd question the financial viability in other parts of the country.

I was going to put one in my new build but got cold feet as to the potential costs. I know someone who built one on his new build for £20k (I think he has a full thread of the build on another forum - let me know if you want the link) but if you get a contractor in then it's more than likely going to be 4 or 5 times that. I'll have a go at one someday though as I think it's a very useful way to gain valuable space - plus I want a cinema room!

Correct me if I'm wrong but I think there's only really three ways to build one, tanked, waterproof concrete or drain with pump. Preferably a combination of a few would work best.

 

[RonnyRaygun]

Correct me if I'm wrong but I think there's only really three ways to build one, tanked, waterproof concrete or drain with pump. Preferably a combination of a few would work best.

That's just how you stop them leaking...

 

[bernardgreen]

Sinking a dry basement into ground with a high water table requires a lot of thought. There can be a lot of up-thrust when the cellar tries to float up out of the water , like a bucket pushed down into water the cellar will exert a force upwards.

Year ago a flooded cellar under a house was drained and tanked. It had to be re-flooded when creaks, groans and eventually cracks were noticed in the house.

 

[angelboy]

Correct me if I'm wrong but I think there's only really three ways to build one, tanked, waterproof concrete or drain with pump. Preferably a combination of a few would work best.

That's just how you stop them leaking...

Sorry, I meant in addition to other standard building methods.

 

[JDLDIY]

Many thanks to you all, all very interesting! I think it's something I'm going to investigate further as whatever we do I think we will have to go deep due to a deep clay layer. I'd just love the space a basement would give us - games room, home business, granny flat, etc, etc.

As far as the water ingress issues - yes, from what I've read so far this has to be carefully considered. We would have to assume that here the water table is high and so tanking wouldn't be the best option, possibly using ICF with waterproof concrete would be the way to go. Then how that would be anchored down I don't know.

angelboy - yes I'd love the link to the basement build please, many thanks.

If I was to retro fit a basement to my place it would have to be bigger to then match what would be the extended footprint, when the ground floor has been extended, if that makes sense. I would hope that therefore that might make it easier/more cost effective as you'd be digging down next to the existing walls to basement floor level, then under. I dunno. I think I might get some specialists round for some advice. I certainly don't want drains and pumps involved, that just sounds like stuff that'll go wrong eventually.

Any more thoughts/advice gratefully received.

 

[angelboy]

Have a read through this build. I went to see the guy and the cinema basement blew me away. Really nice bloke as well.

https://www.avforums.com/threads/not-another-new-cinema-room.538125/

 

[Blagard]

If your water table is high I would forget it. I have on occasion in my career worked with guys dealing with defects. One thing I recall clearly is the hassle with swimming pools and basements (new build). If built perfectly then great, but just one mistake or less than perfect installation can really give you some grief! Stopping leaks is very expensive and rather hit and miss. Plug one leak and the water simply finds the next weak spot and it can go on like that forever!

 

[JDLDIY]

The one thing which keeps coming up then is the height of the water table. How do I figure out how high it is? Doing a google at the moment on it.......

 

[bernardgreen]

the water table. How do I figure out how high it is?

Dig a small hole until you find water coming in the hole from the sides of the hole. Wait until the water settles at a stable level in the hole. That is the water table.

 

[JDLDIY]

But what would be good and what would be bad?

 

[bernardgreen]

The lower the better, you have to accept what it is and design accordingly. Unless you are prepared to install expensive drainage the water table will determine it's own level depending on the local geology and rainfall.

For a dry basement a high water table adds difficulty as the basement is then like a boat in water and will try to float on the water. To prevent it lifting up the basement has to be heavy enough to not float up.

 

[Blagard]

The water table can vary considerably during a year, with it normally being lowest at the end of Summer. Quite simply plenty of rain will usually raise the level and drought will lower it. So the pit method of finding the level is only any good at the time the pit is done. Any water level above the basement floor means your construction needs to be perfect or it will find a way in. As I mentioned before if the work done is not perfect you may never stop water getting in.