The Brew Cabin

Hi,
I've just taken up my old patio which was bedded down on 50mm sand/cement screed with 50mm of fairly loose sandy gravel by the looks of it. This was all on top of Chiltern orange clay with large flint nodules in. A bugger to dig but it seems pretty stable.

I need to remove about 6" of this clay so that the new patio has a good clearance from the dpc but can I get away with just 50mm of compacted type 1 followed by 50mm of 10:1 sand/cement screed then the slabs? The old patio had been down for about 5 years an seemed fine, no cracks or bulges.

One last thing, and I know it's not really to do with slabs but, on the same clay, do you think the same 50mm type 1 sub base followed by 125mm of concrete, 50mm of insulation and 65mm screed is OK for a conservatory floor? Or should I give the floor a thicker sub base?

Thanks very much for any comments or suggestions you might have.

Regards

For the patio, you could lay the flags on just a 50mm bed of 10:1 sand/cement and omit the sub-base altogether, given that the sub-grade is so hard, but, if you must use a sub-base, then 50mm would be adequate for this type of construction, as it is basically a fill layer, rather than a structural layer (which would need to be at least 75mm thick).

However, for the conservatory floor, I'd go for a 100mm sub-base and then 100-125mm of concrete over a damp proof membrane, as any movement beneath the conservatory floor is much more of a problem than movement/settlement beneath a patio.

Have you thought of asking your local Building Control Office what they normally recommend for a conservatory floor? That's what they're there for! :)

Thanks for that. I will ask the Building Control Officer, I never thought of that. With regards the sub base for the patio, I don't actually need to add the 50mm as a filler as I am going to have to dig the clay and flint away in the first place. I just wasn't sure if it was OK to screed straight over the clay & flint. Should I go over the clay with something to tamp it down flat first ? It wont be very level otherwise what with the tops of flint nodules sticking up etc.

Eliminate any high spots (flint nodules and the like) from the sub-grade, and then use perhaps 25mm of sub-base material to 'regulate' the sub-grade - that is, to get it all reasonably level so that the same depth of bedding will be used everywhere. You should compact the regulating sub-base material; a wacker is ideal, but, if you don't have one, a punnel or even just stamping with the boots should be ok.

What you need to avoid is the tops of any flint nodules poking through, as these may develop into 'rocking points' once the rest of the paving is laid.

Thanks for the advice and for putting together such a brilliant site. I owe you a Guiness!

My mouth is a-watering in anticipation. :)