Paving slabs on concrete foundation - Mixed sub layers/how to approach paving

[monotwin]

Hello. Firstly, just wanted to say this website is fantastic, it's so well run and laid out :) thanks in advance.

This is my first post, forgive me, I have a lot of DIY experience but I have never really had to do much groundworks and I want to get it right first time.

So I am looking at laying indian sandstone in my garden as a patio/pathway, roughly 20m2, it will not have any vehicles on it, just foot traffic, kids playing etc. I know a fair amount of this is covered on the website and I have gone through everything but I am still not clear on what I should do as I believe there are probably quite a few answers regarding different levels and compostioins of the sub base and sub layer.

Hence, I have a few questions!!

1) Roughly half of the area has a concrete foundation where an old garage was. It has been down since the house was built in the 1930's. It has a few small cracks; I would presume it has settled and there will be minimal movement going forward. Is it possible to lay directly onto this foundation? If so what sort of mortar mix should I be putting down?

2)The conrcete foundation 'steps down' about 30mm to another concrete foundation. I obviously need to make it level with the other concrete foundation. Is it possible to do this with appx 30mm of compacted Type 1 aggregate? Or do you think a concrete screed is required to bring it up to level? I will be edging the perimeter if this has any baring on whether I can use such a thin depth of aggregate.

3) Beyond this is an area of thin tarmac on grass, which I plan to excavate and bring up to the correct level of the concrete foundation.

I suppose my main question IS: Is it best to work to the highest level of concrete foundation (which represents the largest proportion of the area to be paved) and excavate the lower concrete foundation along with the grassed area. Or could I just build up the lower concrete foundation and then excavate all of the 'soft' material where the astro will eventually go?

Thank you all so much in advance.

[Tony McC]

1 - yes, but what impact will this have on surface levels? Is there any conflict with DPC? Use a 6:1 mix, moist slump.

2 - No: the largest particle size in Type 1 is 37.5mm so bigger than the 'step' you need to make up. Use more of the mortar bed mentioned above, but build-up levels first, allow that first overlay to harden, then lay the flags in the usual way.

3 - If DPC levels allow, pave over the concrete base and excavate/sub-base the grassed areas to suit.

Does that help?

[monotwin]

1 - yes, but what impact will this have on surface levels? Is there any conflict with DPC? Use a 6:1 mix, moist slump.

2 - No: the largest particle size in Type 1 is 37.5mm so bigger than the 'step' you need to make up. Use more of the mortar bed mentioned above, but build-up levels first, allow that first overlay to harden, then lay the flags in the usual way.

3 - If DPC levels allow, pave over the concrete base and excavate/sub-base the grassed areas to suit.

Does that help?

Thanks for the response, very helpful.

The DCM is 180mm above the current concrete surface. I know this could be an issue....do you have any advice on that? I could possibly get the new slab edge about 100mm from the wall and use gravel/decorative stones to fill the void - would this help at all?

Thanks in advance.

[Tony McC]

A whole page of advice - here

[lutonlagerlout]

ideally it is never a great idea long term to pave over partial foundations
at some point the ground around it may move slightly,either up or down and your lovely patio will crack
so its either best to break it out and install a new subbase
or you could just take your chances

180mm to dpc is tight, does it fall away or toward the house?
LLL