Traditionally bricklayers left a raked out joint around a chimney or roof/wall abutment where lead flashing was to be installed. This left a gap of about 10mm high, some 20 - 25mm deep, and the lead flashing was tucked into these joints and lead wedges hammered in on top of the flashing to hold it in place. After all this there remained a gap of 7 - 10mm to be pointed up or filled. So a strong mix of cement mortar was used and the joints pointed up and finished to match elsewhere.
These days it is common to cut into a normal mortar joint with an angle grinder and form a slot about 3mm high and 25mm deep. The flashing is then tucked in and wedged in place as before, but now there is only a small gap over the flashing to be filled. This can be done with a cartridge gun and a sealant made for this particular applicatin. It is widely available in most merchants.
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