Underground shed...?

WillC1423 · 4 Sep 2012

Hi All,

We've got a fairly small garden, but really need a shed for bike/tool storage etc. I was going to build a shed using our valuable garden space, but then had a cunning plan! Why not build underground. This is more sensible in our garden then in many places, as the garden has a significant slope. We're currently building an extension, and about 1 metre from what will be the new back on the house there is a 5' retaining wall.

We can therefore obviously just dig straight into this, stick in a floor, walls, roof and some doors, and suddenly we have a shed.

I was wondering if anyone else has tried a similar project, my main concern, for obvious reasons, is damp, followed by how much structural strength I'll need to stop the earth surrounding it from causing it to collapse.

Any feedback would be much appreciated.

Cheers.

nanuuk · 4 Sep 2012

About the damp, I like this approach:
http://www.youtube.com/watch?v=A3EAJex1RVo
He just assumes it'll fill with water, lays a drainage aggregate as foundation, then adds a sump pump.

Whether or not it works is another matter, but seems sensible enough.

For whether or not it'll collapse, it's presumably the same calculation as for a retaining wall. The thickness of the wall needs to be about half the height retained, but you can make it thinner as you go up to the top. You can make it thinner still, due to the fact that it has supports at either end (the adjoining walls), I'm nowhere near capable of calculating exactly what difference that'll make though!

Norcon · 4 Sep 2012

There seemed to be enough fall for an excavated drain. Much more sensible than a pump.

nanuuk · 4 Sep 2012

Norcon said:
There seemed to be enough fall for an excavated drain. Much more sensible than a pump.

Obviously if there was fall then you don't need the pump, the principle is the same. Impossible to say which is more sensible without seeing the site.

WillC1423 · 5 Sep 2012

As we can dig straight in, I was planning on having a gentle floor slope so that any water that enters flows out under the door, this seems like the most sensible option to me.

Sounds like my retaining walls might need to be thicker than I had imagined however!

freddiemercurystwin · 5 Sep 2012

You'll need a structural engineer to design the walls, to not do so is asking for trouble. Also to just let the whole thing leak is just stupid. Everything inside will just get damp. The cheapest and most effective way and easiest for diy way of keeping the walls/floor dry is bobbly membrane system from someone like Newton.

WillC1423 · 5 Sep 2012

I wasn't planning on letting it leak, it was a just in case scenario. I was planning on some kind of membrane, I'll take a look at what you've suggested.

JohnD · 5 Sep 2012

it's usual to make the cell out of reinforced concrete. It can be done with blocks but the walls have to be absurdly thick.

you will either have to get the design calculated, or you can copy a similar design in similar soil of similar depth if you can find one.

Try to make it watertight but assume that seepage will occur so allow for drainage (you can get special flooring blocks with a drainage gap under) and good ventilation to prevent damp and condensation.

If the finished floor area is less than 30m² you can call it a fallout shelter and it is exempt from planning and building regs, if it is away from other buildings (you can look up the exemption)

shelters do not have to be notified to the council, so they are not on record, so in the event of being used, people will not know you have got one and try to take it off you.

freddiemercurystwin · 5 Sep 2012

JohnD said:
shelters do not have to be notified to the council, so they are not on record, so in the event of being used, people will not know you have got one and try to take it off you.

Interesting, is that right John? What if it was bigger than 30m²

This issue comes up every now and then on here, I'm yet to see anyone link to any legislation though despite posters requests.

Norcon · 5 Sep 2012

nanuuk said:
Norcon said:

There seemed to be enough fall for an excavated drain. Much more sensible than a pump.

Click to expand...

Obviously if there was fall then you don't need the pump, the principle is the same. Impossible to say which is more sensible without seeing the site.

What you posted in the link we could complete in four days including the excavation. All shuttered , re-barred, lidded and sealed.

The stairway access would be much better entering from the top. So flooding is almost impossible.

If you launched it off a ship ramp into the ocean I'd be 95% confident it would not leak.

SimonH2 · 6 Sep 2012

If you are building an extension, can you take that down a level and add a cellar to it ? Obviously it's going to be difficult at one side since I assume your house foundations won't be that deep, but perhaps there's scope for having a cellar under part of it.

This site might give you some ideas on techniques available to keep water out.

JohnD · 26 Oct 2012

for anyone who cares, I found the Building Regulations definition for shelters at last. I knew I'd seen it somewhere.

"CLASS VI
Small detached buildings
1. A detached single storey building, having a floor area which does not exceed 30m2, which contains no sleeping accommodation and is a building—

(a)no point of which is less than one metre from the boundary of its curtilage; or .
(b)which is constructed substantially of non-combustible material. .
2. A detached building designed and intended to shelter people from the effects of nuclear, chemical or conventional weapons, and not used for any other purpose, if—

(a)its floor area does not exceed 30m2; and .
(b)the excavation for the building is no closer to any exposed part of another building or structure than a distance equal to the depth of the excavation plus one metre. .
3. A detached building, having a floor area which does not exceed 15m2, which contains no sleeping accommodation. "

http://www.legislation.gov.uk/uksi/2000/2531/schedule/2/made

chapeau · 27 Oct 2012

That shelter clause lets you put beds in it.

So you can have a shed with bikes and no beds, or a shelter with no bikes but beds. And I really cannot be arsed to argue with a clever dick who says you will need a bike to get around after the bomb drops. You get the idea.

Build the thing, make sure it's not going to collapse on your head or leak so your bikes go rusty. I'd assume it would reduce the value of your house.

gissle · 27 Oct 2012

SimonH2 said:
If you are building an extension, can you take that down a level and add a cellar to it ? Obviously it's going to be difficult at one side since I assume your house foundations won't be that deep, but perhaps there's scope for having a cellar under part of it.

This site might give you some ideas on techniques available to keep water out.

This sounds like a better idea. It sounds like a hell of a job building an underground shed, and I could see it leading to problems.

Are you going to reinforce the roof? Obviously people might be walking on top.

gissle · 27 Oct 2012

Also, how would you get in and out?

Also if you are digging away near to your house, you could affect the foundations.

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Underground shed...?

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