Garage raft questions?

[angelboy]

I'm rebuilding a garage and have searched the forum (and paving expert) for info on concrete rafts but still have a few questions regarding the design.

The garage is 30sqm so doesn't require building regs but I'm confused as to what constitutes 'adequate construction' against the 'minimum guidelines' of building regs?

It's 3.4m x 8.8m, single skinned brick construction with brick piers at 3m centers, hipped roof construction with clay rosemary tiles.

I was planning to built a steel re-enforced raft, 150mm on 75mm sand blinding over a DPM then 150mm MOT 1 and 300mm wide x 400mm deep foundations (not re-enforced) extending for the brick piers.

If it were going through building regs then the spec for the foundation depths would be 2.2m due to being 2m from a Hawthorn hedge and medium density clay (NHBC guidelines). This would then bust the water table which is about 1.5m below ground.

What can I get away with without going overboard. The garage I'm rebuilding has been perfectly fine for 60 years although I don't know what the walls are sitting on until I pull it down.

 

[noseall]

Why are you aiming towards a raft foundation as opposed to a simpler (better) strip foundation?

 

[Deluks]

Assuming only one wall will be 2m from the hedge, the other side will be even further away so will be well clear of any roots. So only 1 or 2 sides will be on potential dodgy ground. You could do normal 1 metre strip on the furthest walls and pad and beam on the hawthorn side, even this may be overkill but for peace of mind...
Plus will probs be cheaper than reinforced slab. By all means chuck some A142 mesh into 4" slab for the floor in any case.

 

[angelboy]

noseall: It was just advise from the brickie, which I think was along the lines of getting the concrete poured in one go and having a base to store the materials from the old garage.

Deluks: I already have some mesh from an old job, not sure if I have enough though, it's stored an hour away from where I am so I'll have to check. That was the thing really, I know what the regs are but don't know what I can get away with for the least cost (same old story).

The current garage, which is double skin, was built over 60 years ago on strip footings. Until I've dug down I'm unsure what depth the footings are but if it's managed 60 years on 60 year old regs then I'm sure that todays regs will be a massive overkill.

The point (my wife pointed out!!!) was that the council and warranty companies are ****ting themselves over liabilities so they're over egging the custard (haven't used that one in a while!). We don't fall into building regs so where do they draw the line? To me, they're basically saying that if the building doesn't fall into the 'current' regs then they're not bothered how you build what you have to build. Let it fall down after 5 years, they don't give a big rats arse coz you can't go after them.