My ancient house has a roof of clay roman canal tiles. These but up to the wall of next door's property, which is higher than mine. The joint is made with a mortar flashing, which has reached its sell by date and is cracking in places allowing water to pass behind it.
I need to remake this joint. Is there a better way than mortar? Is there a bitumen based product that can be trowelled on to form the flashing fillet.
A metal flashing is not possible because the wall is rendered and not mine to cut in to. A few years ago I used flash band to temporarily cover parts of the flashing. This has been successful but does not look good or last very long.
Advice would be welcome.