cavity tray

I have just been told by the building inspector that because the outside wall of my sons single storey extension will become an inside wall, we need to fit cavity trays above the tiles at lead flashing level. He suggests cutting out the mortar joints for 4 bricks at lead flashing level and putting a 9 inch DPC through to the inside blockwork, curling it up to make a tray. On the slope he suggests staggering this. I understand the theory of what he is saying but would appreciate any ideas on how to go about it. How would you join the short length of DPC to make it on continuous piece? We have not finished rafters yet so probably best to do the cavity trays first? Thanks for any input. Neil