The Brew Cabin

I am planning to lay indian sandstone flags at the front of my property. I plan to lift 80% of the old path, but for obvious reasons I want to leave the original concrete steps and a 2m x 2m x 10 in concrete block. If I select the thinnest flags (25mm) to cover the block I will be about 9 in below the damp proof layer and 2 in below the bottom of the air-vent grates which should be OK. Due to the variation in thickness I plan to follow you instructions for 'Laying Riven Paving' for the whole area.
The major concern I have is dealing with covering the old concrete steps. Are there any major considerations with laying cut flags over previously layed concrete steps ( 3 x 1.2m x 15 cm x 15 cm ).

The only real problem is getting the flags securely bonded to the steps, and this is best done, in my experience, using a bond-enhancing additive with the laying mortar. Ronafix is an ideal product to use, if you can find it at your local BM.

You need to lay these flags on a full bed of mortar, and it needs to be a standard wet mix, ie, have the same consistency as bricklaying mortar, so that the flags are firmly bonded to the mortar and in turn, the mortar is bonded to the concrete steps.

The steps themselves may need some prep. You need to remove any loose material or crud, and blasting them with a power washer would do no harm. For ultimate adhesion of the beedding mortar, the old concrete can be 'etched' with acid before strating work. This effectively eats away the top fraction of a millimetre of old concrete and exposes a fresh, laitence-free surface that improves the bond. This might be seen as a bit OTT for a small residential job, and I'd normally only suggest it if the old steps were in a bit of a state, but it doesn't do any harm and can only give a better result.

Try to keep the bedding mortar to 8-25mm depth, and make sure the treads have a bit of fall so that they drain properly in/after rain.

That's about it - let us know how you get on.