Can I build my big garage on a raft?

[Standy5050]

Hi,

I'm trying to get a garage 8 x 5 metres built. I got someone to draw up plans and they have said I need to build the foundations 1200mm deep and they have to be filled in bays rather than as a trench - all of which seem a bit OTT to me. It's basically to protect the garages on either side from damage from making the ground unstable. Basically, this pushes me WAY over budget and is therefore prohibitively expensive.

A friend suggested it might be possible to build it on a concrete raft. Does anyone agree/disagree?

BTW - the garage is 8 x 5 in concrete block and render with concrete tiles.

Interested to hear what y'all think!

 

[noseall]

Raft foundations are often misinterpreted as slab foundations, but this is not the case.

A true 'raft' foundation, although principally resembles a concrete slab, is in fact an engineered and reinforced structure that takes some thought and expense to build.

This is why trench footings are preferred, are simple and are less expensive.

Why the (1200mm) depth? Is it made up ground. I guess from your o.p. that the neighbouring garages are very close?

 

[Standy5050]

Noseall. Thanks for the reply. I have no idea why the foundations have been drawn as 1200mm. Everyone I've told this to seems to think that it's madness. The neighbours' garages will be right next to mine. Pardon my ignorance, but what do you mean by made up ground?

So a raft is expensive, is it? Nooooo. The whole point of this is to try and get the price down. Do you reckon it'd be more expensive to do that than dig the trench in bays? Is there any way of doing this cheaper?

 

[inspector01]

8 x 5m means the building would be controlled by Building Regulation (i.e fee payable to the council). From experience strip foundations will be cheaper than a regulations compliant raft. Where are you based? Are you on clay? This may be the reason 1200mm foundation depth has been suggested. If you are digging close to neighbours buildings you will need to be careful not to undermine their foundations. The person who drew your plans should be able to advise you.

 

[Standy5050]

Thanks - Yeah, I've obtained permission and everything, just trying to work out how to do it within budget really.

My builder said that the problem is the strip foundations are drawn as being dug in 1 metre bays and then tied. This means that he will not be able to get a concrete lorry to deliver because each bay will be a much smaller volume so the concrete will need to be made by hand. He also said that the building regs inspector will have to come out after each bay is complete - or more times. The cost of labour has rocketed and taken me well over budget. I'm just trying to figure out a way of getting the costs of this down.

 

[Chukka63]

Are there any drains nearby as they could factor into the depth.

 

[Standy5050]

I'm pretty confident there aren't drains nearby.

 

[inspector01]

I had a job like this a while back. In the end the best solution was to dig 1m2 pads every 2.4m and span the gap using a 150x100 concrete lintel below ground. The wall is then built directly off the lintel.This solution will reduce the volume/cost of concrete and the labour.

I can help out with a design for Building Control if you want. I have some photos of the job I can send to you if you pm me your email address.

 

[Chukka63]

hand mixed concrete would be cheaper,the labour will be more expensive but not that much.and a few metal rods placed in the concrete would be a few quid.i cant see the problem myself.im assuming the "strip" foundations only go along 1 wall????

 

[Standy5050]

The strip foundations will be across two walls (i I understand your question).

 

[Chukka63]

the other option is like inspector mentioned but called a pad and beam foundation.instead of using lintels you use rebar and then you may be able to space the pad further apart maybe???.

 

[inspector01]

A 2.7m PCC lintel costs around £25 making it very cost effective. Spans over this distance may have to consider buckling and UB encased in a 50mm concrete cover can be used, although the steel cost would be approximatley £16/metre.

 

[Standy5050]

By the way - something that I haven't mentioned is that the garage will be detached. Does that make any difference? Could a pad and beam system still be used? It certainlyy sounds like the most cost effective in this case.

Is there any reason why the pads can't be dug in one go and then filled the next day? I'd imagine this to be much cheaper in terms of labour.

 

[Chukka63]

you could dig and pour the pads in 1 day,then sort the beams the next day or so.

 

[lostinfens]

Is the problem then just the proximity of 1 of your neighbours walls, it's difficult to envisage how it could be more. If that is the case then surely your problem is only down 1 side. Whichever, I can see that pouring in sections is expensive from a labour point of view- so why not do it yourself.