Raft foundation - do I need to underpin?

[CortinaV8]

I've a garage which was originally a single but has been doubled in size in the past (early 1980's)

The new section was built on a raft foundation which seems to be roughly 20cm deep at the edges, the slab has cracked and one corner has moved slightly, I suspect due to a rainwater downpipe discharging directly onto the ground in that area. I fitted proper drainage about a year ago and have not noticed any further movement.

I want to replace the current flat roof with a pitched tiled roof on pre-built trusses in the spring next year.

Given the thickness of the slab is this going to be thick enough to support a tiled roof or would it require underpinning, if so it would probably be just as easy to pull the raft section of the garage down and rebuild it as it's only single brick?

 

[Dhjones]

The 200mm (20cm) the perimeter of the garage sounds like the 'toe'. It should then step up which the internal skin is built off and is also floor level. So would be around 400 thick. Although this still isn't really thick/deep enough. Concrete under the walls in a raft should be as deep as your average foundation depth, with a lot of hardcore underneath wackered down so it's as solid as the concrete itself. The pad to the floor area is then around 200mm thick.
Something doesn't sound as it should be.

Underpinning a raft won't see any benefits, unless there piles, but too late for that. If underpin, and movement occurs, the new concrete underneath will just shift away from the raft, then it becomes an expensive slab of useles concrete.
Structural engineer is your favourable option here

 

[Blagard]

If it is a raft it should have been reinforced and to find a corner cracking is not what I would expect with a properly constructed raft. I would not be inclined to load it up any more with a heavier structure unless a SE is prepared to back that!

I have never heard of a raft having the thickening as deep as your average foundation depth! - Not sure what Dhjones is saying. The thickness of the edge thickening would normally be a fair bit more than a traditional strip foundation needs to be and would be reinforced as well. 200mm thick at the edge does not sound thick enough to me to indicate a properly formed raft.

 

[Dhjones]

If it is a raft it should have been reinforced and to find a corner cracking is not what I would expect with a properly constructed raft. I would not be inclined to load it up any more with a heavier structure unless a SE is prepared to back that!

I have never heard of a raft having the thickening as deep as your average foundation depth! - Not sure what Dhjones is saying. The thickness of the edge thickening would normally be a fair bit more than a traditional strip foundation needs to be and would be reinforced as well. 200mm thick at the edge does not sound thick enough to me to indicate a properly formed raft.

Was merely saying the Concrete strip in content sounds like it's the toe of the raft.
I've been lucky enough to have completed a few rafts now along the way, and every design I've been supplied by SE has Involved the perimeter of the build being a deeper footing, steel links and bars with mesh over all tied. So it looks like an 'L' but laid on its front. (Turned clockwise)

 

[chappers]

they come in many shapes and sizes the last one we did was similar to what Jones describes it had a 200mm toe thickening to 450 for 600mm and then tapering to a 150 slab with a 250 cube thickening at an abutment with another building. It was also cantilevered on a 350 tapered thickening somewhere down the middle

 

[CortinaV8]

From what you guys are saying and to be honest what I thought myself is it's a bit of a lash up.

I think the easiest solution for me would be to take it down and rebuild on proper strip footings like the other side of the garage, it's only single brick construction and I should be able to clean up and reuse a large amount of the bricks (if I can be arsed with the work)

 

[Dhjones]

So why has the raft foundation been done? Who would do a raft if a standard dig would suffice?
If the raft is there to serve the purpose, a normal dig won't.

 

[Blagard]

Who would do a raft if a standard dig would suffice?
If the raft is there to serve the purpose, a normal dig won't.

A cowboy or uninformed DIYer
Well if you think about what appears to be a raft but is not properly reinforced it will cost considerably less than a strip foundation. As a minimum I would want to open up the crack to see if there is any at all.

 

[JohnD]

for DIY sheds and garages, a "thickened edge slab" used to be a recommended method of construction. No reinforcement.

The Portland Cement trade association used to include it in their promotional leaflets.

I didn't know it had fallen out of fashion.

 

[CortinaV8]

Yeah, Raft is probably the wrong terminology? What I have is what JohnD has suggested, a slab with a thickened edge.

I'm guessing it was built that way to save the costs, the original portion of the garage is on standard strip foundations roughly 1.5m deep (exposed some of this when digging down to install the drainage) can't see why laying standard foundations would be an issue?

 

[Grantx]

Two points: re-using the brick will make the garage look cheap and s**t; you will need a double skin (or at least, piers) to support the roof. Do a job on the cheap and it will look like a cheap job.

 

[CortinaV8]

Cheers for your input Grant/Cockwomble

 

[^woody^]

A slab with or without a thickened edge is still a raft.

The thing is with rafts, they will move as one over the seasons which is normal. If you underpin one part of a raft then you stop that one part moving, so the cracking you tried to stop ends up happening anyway, but for a different reason.

 

[Dhjones]

No, the slab is a slab, a raft is the raft as a whole.

 

[^woody^]

No, the slab is a slab, a raft is the raft as a whole.

So if I build walls on the edge of a slab, it magically becomes a raft? Thats cool.