Having located the crack in a chimney, secure an extension ladder to the fascia board with a rope and use a ladder that hooks over the roof ridge to reach the stack.
If the crack is vertical, it is easier to pour the mortar mix down the crack rather than trowelling the mix into the cavity. First, clear all loose dust and brick and mortar chippings from the crack by scraping it out with a cold chisel and then by using a stiff brush. Wet the inside of the crack – taking care not to wet the face of the brickwork – and then press a length of wide, heavy-duty adhesive tape over the lower part of the crack. Cramp a piece of timber over the tape with a sash cramp to help hold it in place.
Mix one part of cement to three parts of sand and wet it until it has the consistency of a runny paste. Insert a funnel into the crack above the tape and pour the mix into the cavity. Allow it to set for about four hours, remove the board and tape, and repeat the process working progressively up the crack.