Should you use a plastic DPC beneath coping stones on a Victorian terrace parapet wall

[Pol_Briz]

My roofer identified damage to the original coping stones on the parapet wall between my roof and next door's roof, and wants to put a plastic DPC and new cement coping stones. But I've been reading about how you shouldn't use modern materials on old houses that were built with stone, brick and lime. Does this also apply to the parapet wall or should this be OK? It looks like it's probably already been rendered with cement.
Thanks

 

[footprints]

It was my understanding that garden walls do not have any DPC.

 

[Pol_Briz]

Sorry, I should have stated that this is between my roof and my neighbour's roof

 

[footprints]

Ah! Silly me not reading correctly, you mean the small wall separating the houses on a Victorian terrace roof.
Traditional DPC material would be slate as a rule, but I cannot see any harm in using modern DPC for a repair, I must say I didn't think the capping stones had a DPC on those roofs but I can see it could help reduce water ingress I suppose.
The main problems are where the old render or internal plaster was breathable and replaced with non breathable materials or flexible lime mortar replaced with hard modern cement.
Modern DPC is water tight and so is slate.

 

[JohnD]

IIRC they used to have a lead DPC, but this is not something I've been interested in.

You mention rendering. This is usually applied to hide bricks damaged by spalling or something. Parapets are prone to water penetration and freezing. In some districts blue engineering bricks were used for the top course, which are impervious, if used with hard mortar. One thing I do remember is that if the mortar gets waterlogged and freezes, the coping gets a little longer, and a little looser, every winter.

 

[datarebal]

Dont use plastic .use lead wide enough to dress over the top of the render .

If your wall was lime mortar the lead tray would not cause any issues with breathability in the face brickwork.
It would help lessen water getting into the wall from above .
Well fitted copings will too but often they are not fitted well !