Footings...

[Headsintheshed]

I'm sure this has been discussed many times, so apologies in advance, but I'm struggling to get firm advice on depth and width of footings required for a single storey, single skin, block built detached garage. The ground is soil then clay. It is compact and hasn't been "disturbed" for over 40 years if that makes any difference. I'm planning to build a large garage with a flat roof and double width garage door opening. Building Control won't advise on footings and have said that my builder or Architect will advise me, BUT they could over-rule and tell me to go deeper / wider if they think it needs it when they inspect. An architect visited me and said to just work under a Building Notice (since it's just a simple garage and plans aren't really necessary). If I start work under a Building Notice and my builder has underestimated the depth etc then I will incur additional costs in further digging. So, what are people's thoughts on footing depths and widths please? I will be laying 4" type 1 and 4" concrete with rebar as the base. Thanks

 

[oldbutnotdead]

The Approved Documents are your friend here- have a look at Part A pages 36 onwards. Precis- for single skin block you're looking at 700mm deep, 450mm wide, 200mm or so thick. Dig your trenches, get BCO to come and have a look. If the bottom is firm undisturbed clay then you'll be fine, if it is made ground (ie there's bits of brick and infill at the bottom) he'll make you go deeper- which will get expensive in terms of shoring, spoil removal, concrete so make sure there's no evidence of made ground in the trench bottom. Surprises me that Building Control are being that way, ours up here are very helpful on stuff like that

 

[tomfe]

What size is your planned floor area?

 

[^woody^]

If it's shrinkable clay, then you may be better off forming a raft, rather than strip foundations.

 

[Headsintheshed]

If it's shrinkable clay, then you may be better off forming a raft, rather than strip foundations.

I'm not familiar with a raft - can you expand on the difference please. Thanks

 

[^woody^]

A raft moves when clay moves. Strip foundations might need to be really deep, otherwise they move unevenly and crack the building.

It depends on what clay you have there, and nearby trees.

 

[Headsintheshed]

Thanks. I've had 7 conifers removed (about 15ft tall) and their stumps ground out which were along the boundary and there is a very tall conifer in the neighbour's garden about 15m away (it's probably 15-20m tall).

 

[Headsintheshed]

What size is your planned floor area?

60m2 in an L shape. Clockwise each wall: 8m, 10m, 3m (with single garage door), 4m, 5m (with double garage door), 6m.

 

[stuart45]

You need to know whether the clay is the shrinkable type, and if so if it's high, medium or low.
There is an NHBC calculator that BC are also guided by.
https://www.labcwarranty.co.uk/resources/foundation-depth-calculator/

 

[tony1851]

Thanks. I've had 7 conifers removed (about 15ft tall) and their stumps ground out which were along the boundary and there is a very tall conifer in the neighbour's garden about 15m away (it's probably 15-20m tall).

Oh dear; google 'soil heave'.

 

[freddiemercurystwin]

No wonder he wants to avoid BC.

 

[Dhjones]

If all of the foundation runs are at the same depth and it's clay all the the way round, there's no issue. Get down to around 750-900mm, ring inspector, when he says, which if clay he probably will,,,,, "are you going to fill it?" (With concrete that is) your instant reply is, awww yeah, filling it right up, it's easier for us, you'll be fine and be granted with a, let us know when ure at dpc.
Have fun