Hi all, I'm in the process of digging my foundation for a new front porch, and to my pleasant surprise there is already a footing in place. The existing foundation runs away from the house 1 side and the front where I intend to build, leaving just 1 run to dig and tie. The house is a prefab timber build (scotswood 50's house) which is constructed on a what looks like an 8" slab. The existing porch foundation looks like it was added at a later date but again is only around 200mm deep. Usually I'd just break out and dig to a required depth but seriously this concrete is ridiculous in strength. I plan on using 140mm concrete hollows left over from a bar build I did in the summer, so not the lightest of blocks! Am I mad even considering leaving the existing in place? Or could anyone suggest anything else apart from the obvious? TIA Mike.
New porch on existing foundation
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It may or may not be OK. It's the ground below that would be the issue, not the slab.
There were/are lots of 60s houses with porches built on slabs like this, and most of those are fine, some are not.
Even if you got someone to look at the ground, I doubt that they would be able to give you anything other than a best guess.
There were/are lots of 60s houses with porches built on slabs like this, and most of those are fine, some are not.
Even if you got someone to look at the ground, I doubt that they would be able to give you anything other than a best guess.
Ground is very loamy here. I suspect 1 of the reasons why they chose to build a mass amount of this style prefab in the 50's here. I know I'm asking a crystal ball question with out being able to see it for your own eyes. But, should I even be considering building on it?
Dig to the same depth alongside it, find out what it's sitting on.
If you wanted a wood porch then anything is probably OK. As you want block then you don't want topsoil under it.
Porches are fundamentally unstable, as they have 3 sides so want to tip over outwards as the weight of the outer wall pushes downwards and there's nothing to balance it. Have seen old porches tipping away from the building at crazy angles.
We've got very deep topsoil, but it's never infinite. There will be clay, rock or sand at some depth - about 1m in our case.
If you wanted a wood porch then anything is probably OK. As you want block then you don't want topsoil under it.
Porches are fundamentally unstable, as they have 3 sides so want to tip over outwards as the weight of the outer wall pushes downwards and there's nothing to balance it. Have seen old porches tipping away from the building at crazy angles.
We've got very deep topsoil, but it's never infinite. There will be clay, rock or sand at some depth - about 1m in our case.
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Yes, there is nothing inherently wrong with a slab like that. The main house is on one for a reason, so potentially the porch would be equally fine on a slab - in fact any addition should be on a similar slab to the main house so that they move in unison.
Presumably this slab had a previous building on it, so if the slab is still level in both directions, I'd expect it to be fine to build on. Slabs (rafts) are very stable, and have excellent load bearing capacity over poor ground
Are there any similar properties with an original porch for you to assess? If there are and there are no problems, then I'd suspect that the chances of success are greater than of failure.
As an added precaution, would it be possible to build a lighter timber framed structure?
Thanks for the advice. This afternoon I dug around the existing as advised, turns out this a more a slab rather than a strip foundation. Tied well into the house slab too (seems the row of terrace houses all on 1 big slab) was this the norm back in the 50's?
Anyway... There is a sub base below but only seems around another 150mm deep. If this concrete was a fairly standard mix, I would have just bit the bullet and had it up, but honestly think I'd need a pecker on mini digger to have half a chance
.The fact that the house is prefab timber suggests it's probably not going to be adequate for concrete block.
Either build lightweight or get one of these...
www.screwfix.com
Mine hasn't been defeated by any of the 1950s hard as rock concrete I've encountered here.
Either build lightweight or get one of these...
Titan 16.4kg Hex Shank Electric Breaker 230V - Screwfix
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Mine hasn't been defeated by any of the 1950s hard as rock concrete I've encountered here.
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