Slate DPC, and max mortar bed size

I'm doing a single story extension to a 1930's property with a slate DPC. The DPC mortar bed is approx 40mm thick. How should I go about matching this when tying in the extension to the original brickwork? What's the maximum depth mortar bed, and would I need to pack it out with something? A builder has suggested adding SBR to the mix. Would this be sufficient? Thanks in advance.

That builder is not one.

Pick the engineers up 20mm, to leave 20mm for the DPC. This assumes the new brickwork above will follow the guage of the existing.

In any case the DPC should be at floor level, so you work to this and not a 40mm joint

Thanks for the suggestion. Ideally I need to follow the gauge of the courses above and below DPC as one side of the extension is an extension of the gable wall end, plus the 2-3 courses below DPC are in blue brick but above ground level and rather visible. Are there any other solutions you can think of?

I use Sovereign waterproofer in my mix for rendering and tanking, it's also a brilliant plasticizer, it cost a few quid more than the likes of Cementone and other makes on the market but I've found it far superior so by using this in your mix instead of a plasticizer you have a win win waterproof job, the only problem I encountered was my spread complaining because I used it in both the scratch and finish coat of rendering, due to the scratch coat being waterproof the top coat took ages to dry and he was twiddling his fingers waiting to sponge it up..

Jamie Thompson said:

Are there any other solutions you can think of?

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A 40mm joint