Laying on concrete base

[Spadger]

Hi, I have a fairly new house which is surrounded by a concrete based patio, I want to extend it and also lay either slate or sandstone slabs.
I have dug out the extended area, put in my subbase as per this site. What I would like to know is is it ok to lay the slabs direct onto the concrete and if so what mmix would be recommended to set the slabs on it. Cheers in anticipation.

[London Stone Paving]

I'm assuming that you have used MOT type 1 for your sub base?

Your bedding layer should go straight on top of this subbase. A lot of people have different ideas about bedding layer mix. I always swore by 4 parts sharp sand to 1 part cement in the consistency of bricklaying mortar. Anything from 4:1 to 6:1 is ok. The right amount of water is crucial here. Too much and you will make a right mess, not enough and the mix wont be pliable enough

All the best

Steve

[Spadger]

Cheers Steve, was just wondering if I'd be ok putting straight onto the concrete ,cheers for reply and will get to it, just replacing all the fencing posts b4 I go any further so not to have to do the job twice,
John

[London Stone Paving]

What do you mean ? "staright onto the concrete"

[Spadger]

There is already a concrete patio / path all around the house what I want to do is extend the patio area out further into the garden and lay slabs over the whole lot , my thought were to dig out the new area , put sub base down, then lay slabs on the original concreted area and continue over the new ly prepared area.

[Pablo]

Yes concrete will make a perfect base for your flags to be laid onto as long you don't breach the 150mm below your DPC rule. Bring the new subbase level to the concrete and compact the heck out of it. Best think about where the water goes now because concrete can sometimes have only a marginal fall in it and lawns and planting will struggle if the drained area is to large. Mortar wise 4:1 with a taste of plasticiser then add water until it's like a stiff building mortar and you'll be grand but make it too sloppy and you're screwed.

[Forestboy1978]

I heard that you shouldnt put plasticiser in a gritsand or concrete mix cos it causes air bubbles!?

I'd love some clarification on this as I avoid plasticiser for this reason though feel it would make my life a tad easier!

[rab1]

you shouldn't add plasticisers to concrete as it weakens the mix. concrete is the sructual part and motor is the bond if you get my drift.

[Pablo]

Standard batch mixed concretes and screeds usually contain a plasticiser but it's different to the kind we use in mortars. Adding it to bedding mortars for domestic work is standard practice and causes no negative effects to the finished strength it can also aid with adhesion.

[Forestboy1978]

i get ya