Using slablayer over old mortar before laying slab - Laying larger patio over existing one.

[Tate1]

Hello everyone. I could do with some recommendations please. Immediately outside rear French doors to rear of home I have an small patio 2400x2400mm which I intend to replace and extend. The existing slabs are 30mm thick and 600x600mm. They have been laid on sub base and bed of mortar that’s not flat but 4 blobs of mortar under each one. I’ve dug out an additional 6 metres of ground around the existing patio, laid new sub base of MOT1 to height of old patron mortar. I’ve allowed for slope and compacted. I intend to use 2.5mm of slablayer (for ease) on top of entire area and lay 20mm thick porcelain tiles. My concern is that the old mortar needs lifted. It’s 4 months old and I just want to put slablayer on top. Should I consider anything??

[Tony McC]

Have I got this right: you plan to lay new porcelain paving over the top of spot-bedded sandstone flags?

That is a bad idea. A very bad idea. Very, very bad. It is almost certainly doomed to failure.

The old paving MUST be removed, along with the comedy bedding. The sub-base should be regulated to ensure you will have a consistent bed depth for the new paving, and then lay the porcelain in accordance ith the manufacturer's installation instruction.

Personally, I would not lay porcelain on anything other than a concrete base with outdoor grade tile adhesive, but I know some manufacturers are pushing the 'lay on mortar' method.

Read more: Laying Porcelain

[Tate1]

Thanks Tony McC. Not quite. I’ve lifted the 4 month old sandstone flags and can now see the spot mortar underneath. My intention was to use blue circle slab-layer to not only fill the empty parts of the spot mortar but build it up by another 25mm in accordance with manufacturers guidance to be optimal. My thinking was that the old sub base and spot mortar will therefore act as base. That’s the part I had reservations over hence raised the question. I then intend on adding a primer on back of tile before laying.
If I was to go ahead as described above but lay another sandstone slab instead of porcelain would this eliminate the concerns you raised or does the old mortar absolutely need to be lifted?

[doobin]

If you're really going to do that, smash the old slabs with a sledgehammer a few times (to make sure there are minimal voids below them) before filling the area with a very wet mix with some SBR (important) This is a total bodge but will give you a slab that might withstand foot traffic, depending upon underlying ground conditions. Then use an outdoor tile adhesive to bond the porcelain to the slab once it's cured.

Remember, it's only a bodge if it doesn't work.... Trouble is it might not. Your call.

Sorry for talking dirty on your forum Tony!

I love shitty new builds with 450 precast slabs- they keep my bills paid as its the first thing people want to change!:D

[Tony McC]

I'd rather get rid of the old mortar, make sure there's an adequate sub-base, and regulate it as necessary, then lay on all new mortar bed

I probably wouldn't run to the expense of buying a pre-mix basic mortar when I can get sharp and and cement for about half the price!