we have a stone built house with a small porch at the front with slate sloped roof. it predates our ownership and whoever built it obv decided flashing with mortar was easier than leadwork - unsurprising given the irregular stonework. it seems to have kept the water out succesfully for many years until last winter but is now leaking. I'm thinking just tor replace with new mortar. i realise its not the optimum solution but cutting in lead flashing is beyond my skills and confidence level and the old mortar seems to have done the trick for many years. what mix mortar should is use? any additive useful necessary? is there a premxed product that might be suitable?