Conservatory foundatios.

[Tristar]

Yes it's me again.

Conservatory footings have now been dug to support a dwarf wall 450mm high. Footings have been dug to 580mm deep and 400mm wide all round. There is 450 mm difference in height from top to bottom of slope, over the 6.2 m length of structure.

Is this depth and width OK? (my plan spec reads 1000mm x 450mm) The difference in height will clearly affect the footings depth, as, by my calculation, given level foundations, the depth of concrete on the high end of the building will only be 130mm, unless the footings are stepped up, which I hope is the plan.

Any comments gratefully received.

 

[Tristar]

Nice word that, foundatios, think I'll patent it!

 

[Shytalkz]

Footings have been dug to 580mm deep and 400mm wide all round

Straight out of the GGF "How to dig conservatory foundations without any regard to soil conditions" brochure so beloved of subbies working for conservatory companies, no doubt...

I would guess that they are intending to step them (though you can never be 100% sure of intentions as any fule no). Where is floor level intended to be relative to these excavations, is the floor going to end up lower than the top of the footings at the high end?

 

[Tristar]

Thanks ST

The floor level will be 210mm above the top of the trench at the high side. The floor spec is 100mm hardcore, DPM, 100mm steel reinforced slab, 75mm Celotex insulation, and 75mm (give or take) screed. say 350mm.

Cheers

 

[Tristar]

I guess if we put steel reinforcement in the footings, that may be OK. Any comments?

 

[Shytalkz]

I would assume that they are indeed going to step the foundations, with the top at the higher end being set either at formation level (underside of hardcore), or at slab formation level. As long as the foundations are a min of 225 thick, then that's ok.

There's no need for rebar in stepped footings, as long as the ground is ok along the length of the foundations* and the steps are formed in accordance with accepted practice.


* My reference to the GGF detail was a flippant one in that I've dealt with too many conservatory problems where the foundation complies with that detail, but with no consideration for the condition of the ground type that was being excavated. Quelle suprise, the conservatory separates from the main building. One day all this kind of work will come under BRegs...

 

[Tristar]

Thanks very much Shytalkz, ( apologies for reposting, but as you can see, time is short) I will ensure that the footings are stepped, and also he will hace to take away some more topsoil to allw the full 350mm depth below floor level. Call me an old cynic, but I suspect he may try to grade the hardcore to avoid taking any more soil away.

Thanks again, you have set my mind at rest.

 

[Shytalkz]

...to take away some more topsoil...

All top soil over the area, I hope...!

 

[Tristar]

Well he hasn't taken any away from the slab area yet. How much is he supposed to take? As it is a slope some of the area will have to be built up.

 

[Tristar]

Shytalkz, by overlap I assume you mean double depth for 600mm run? So I will ask for two steps with the minimum depth of 255mm of each 'single' run. Is he supposed to take away all the topsoil? He hasn't taken any away from the slab area yet.

 

[Shytalkz]

Yes, 450 deep for 600. He can always strip the topsoil after casting the footings, he's probably left it in to avoid having to shutter up the sides of the excavation if the top of the footing would be above the strip level.

 

[Tristar]

Got you, yes I think that makes sense. How much SHOULD he remove? Obviously enough on the high side to get the undersite in, but the rest will need levelling up somehow.

 

[Shytalkz]

Just read the other thread - now locked. If the footings to the building that it's butting up to are deeper, due to fill (made ground), then there is absolutely no point in casting the foundations at a higher level, if this is also fill material.

Although this work lies outside BRegs, you can invoke that (for a fee) and get a BCO over to check, before he pours. Although if he's doing that on Tues, you're going to be pushed to get one out on that timescale.

 

[Shytalkz]

Got you, yes I think that makes sense. How much SHOULD he remove? Obviously enough on the high side to get the undersite in, but the rest will need levelling up somehow.

All topsoil needs to come out: if the floor slab is cast on organic material, that will degrade over time and down the slab will go.

 

[Tristar]

I'm now getting a bad feeling again. Are you saying that, even though the ground has consolidated over the past 30 years, and the trench bottoms all appear to be solid, he should still dig down to the original footings level?