I am building an extension and have a query regarding the DPM and DPC layer, I am going to bring the DPM up and out over the block and then put a layer of 9" DPC on top. my question is what is the correct sequence regarding mortar layers is it ? Block, DPM, DPC mortar and then Block or should there be a layer of mortar between the DPM and the DPC ? I am just wondering if it is a bad idea to put essentially two plastic layers on top of ach other with nothing in between them ?