Porch foundations

3 Jul 2012
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West Midlands
United Kingdom
Hi guys,

I'm extending an outhouse (single skin brick work) to install a upvc porch. I'm wondering how deep the foundations need to be for this construction? As the area of extension is less than 3m2 its not subject to regs. The soil conditions are hard clay.

I was going to go 8 inches wide and 8 inches deep, but was unsure of where to finish the foootings relative to where the bricks would start!

Also, I'll be raising the floor level to the house (the door on the left of the picture). What sort of depths do I need when I fill in the middle with a concrete slab e.g. concrete, insulation, dpc, sand, hardcore etc?

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Eight inches might be a little shallow for clay, which can be prone to shrinkage in dry weather and expansion in wet weather.

Why not take your footing to the depth of the footing of the original outhouse, whatever that is? If that's not had any problems, you should be OK going to the same depth.

Eight inches wide might be difficult to work - perhaps go a little wider (12"?).

For your floor, usually 3" - 4" concrete on dpm on 6" hardcore finished with sand. Why bother with insulation? - waste of time in a porch.
dig footing to match existing outhouse and to make it an easier dig and allow a good toe on the concrete I usually go around 400mm wide for single skin light construction such as upvc porches and usually a minimum of 600mm deep might sound a bit belt and braces but at least it stays where you want it. matching the existing foundation sounds ok in principal however I have underpinned houses built on 4 inches of concrete
also for the little extra work and cost I'd add a minimum of 50mm sdn polystyrene insulation you will feel the benefit on your feet.
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dependent on job type that how we work thats all with rebar ect how we was showed by an old school builder.
Thans for the advice guys. Footings went down a treat! Bit hot the day I had to mix it up sweated buckets :)

Got the engineerings in now I need to fill in the middle. I was thinking 75mm concrete DPM/25mm sand blinding and some hardcore underneath that nicely compacted. Is this OK?

I'll be tiling on this afterwards.

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