re-laying a patio

[barnsley7]

i'm re-laying a patio. It's 11m x 8m.
I've not lifted up the old patio yet so not sure what the base is like, but it looks like its had a solid foundation down for a few years because there is no sagging. its nice and flat. I will be taking up the old flag stones.

Just been on an aggregate calculator and for 11m x 8m and 5cm depth of sharp sand - it suggests that I need 7 tonnes of mortar! this seems a bit extreme... is this right? I don't want to use dot and dab on each corner but if the existing foundation is good, could I leave most of existing old mortar there and just partly fill in with new mortar and use about half new and half old mortar? using about 3 tonnes of mortar?

 

[JohnD]

OOI, will the new one be higher, and will it bridge your DPC?

 

[Munroast]

depends whats under the existing slabs, they may all be lying on sharp sand, you could just loosen that up a bit with a rake then relay. even if it has had some cement dust mixed it then it will still loosen off and can be reused.
of course they may have been layed on wet mortor, in which case you will have a big job.

 

[barnsley7]

thanks, yes i guess i really do need to lift a slab and have a look first. And I'll try to stay 15cm below the DPC

 

[barnsley7]

I've had a look underneath the slabs. There's about 2cm of mortar on top of sand. The mortar comes up easily so I can get rid of that and use existing sand with some new sand mixed in.
Dpc is 13cm from existing patio.