Garage footings - am i going over the top?

[Ctxrob20]

Hi All,

A few years back I self-built our house and now i'm adding a single skin garage with a hipped roof, that's attached to one of the walls (a garage was not approved originally by planning but i've got it through at a fresh attempt).

As the garage will only be single skin block work with a central pillar (6x3m), do i need to go to the extent of putting in full trench fill footings? It will never be built onto of due to the location, so that's not an issue.

Building regs suggest that if the garage is attached, the foundations must be the same standard as the original house. That would mean 1m deep and 600mm wide, which seems massive overkill. Am i interpreting the regs correctly?

Does anyone have any insight they can throw my way?

Thanks,

Rob

 

[Notch7]

The depth of foundations is determined far more by the ground bearing capacity of the ground and its stability than the weight of the superstructure

It all depends on what depth you reach stable sub soil, whether there are any trees in the zone of influence and whether you have shrinkable clay soil.

If the house was built on 100 deep footings, Im guessing its good soil, in which case maybe you could get away with a bit less depth, it all so depends on where you reach hard ground

 

[frutbunn]

Assuming shrinkable clay type sub soils the minimum depth is 1m, subject to ground conditions, proximity of trees etc., for sand/gravel 600mm is generally accepted. 450mm width is generally accepted as the minimum for single leaf buildings, subject to ground conditions.

 

[Ctxrob20]

Thanks both, I dug the footings last time and reached hard ground almost immediately so it's pretty ideal. I'm always one to overdo things, so will probably go for a 450x1000 trench fill and suffer the cost, but it just feels like overkill for such a small load. I'll ask the BI and see what they come b ack with.

Cheers,

Rob

 

[^woody^]

I's not the loading and its not so much the bearing capacity, but its to do with how much and how evenly the ground will move over the seasons.

If like you say you have hard ground, not clay, and no large tress or shrubs nearby, then the ground is likely to be stable and you could just go down to 600mm or so for a strip, or about 300mm for a raft. But if this is being inspected by building control then they will want it deeper for strips, but should accept a raft.

 

[JBR]

About 20 years ago I built an attached garage on to our previous 1930s semi-detached house whose foundations are the usual couple of feet deep.
The building control gentleman insisted, however, that I should dig down to a layer of peat which was known to be about six feet down. This I did (reluctantly) and removed the layer of peat which I found to be no more than a few inches thick!
Naturally, the garage is still there standing perfectly soundly - but so is the house with its much smaller foundations!