You may be better off consulting a brickie who specializes in heritage brick laying and repairs first. The reason I say this is because you may exacerbate the problem by rendering such brick and mortar work. I'm assuming by rendering, you are referring to a modern render that uses portland cement as part of the mix? Unibond will probably also cause problems with preventing the bricks and soft lime mortar from breathing. Remember, especially with a chimney, moisture can also enter the brickwork from behind, as well as any moisture introduced to the brickwork by the Unibond.
Cement works on modern bricks because of the controlled uniform temperatures under which they were fired. They have a much harder impermeable skin on them that repels moisture. Earlier bricks vary hugely, even from the same batch. Some are no better than the sun-dried bricks that are often found in some old buildings and walls. Heritage consultants always say that if you do paint such brickwork it should be with a limewash paint like you find in the Porters range, made from the original recipes. They allow the bricks and mortar to still breathe. If you go the Unibond/render way, you may find that it does last for many years- On the spalled pieces of brick that expand and come off with it :awful:
What you have there are Bricks and Mortar in name only, that may look the same, but are completely different to modern bricks, mortar, and techniques. They were laid using the method explained by Dihusky, and they are walls/chimneys etc that flex, breathe, and will stand for hundreds of years, but only if they are maintained correctly. Any short term satisfaction at a job that looks well done will almost certainly lead to long term misery for either yourself or any future buyers/owners of your property. It can also affect its value if someone who knows what they are looking at sees a heap of work that they know is going to give them problems in the future.
At one of my other properties in Eden Park in Vic, I have an old chimney that has brickwork around 110 years old. When I was getting quotes for a new roof, I asked each plumber how they would do around the chimney. The cheapest quote came from a nice chap who told me that they would simply cut or grind a groove down the brickwork at the same angle of the roof and run an apron flashing. Immediately that rang warning bells for me, as you never remove material from heritage bricks with power tools, only as a last resort sometimes when removing old mortar, and only then along the center line of the mortar, removing the rest with a hand tool. I eventually settled on a guy who explained to me how it needed to be step flashed and why, protecting the integrity of the bricks. Overall they were the most expensive quote, but they obviously knew what they were doing and the workmanship looks as beautiful as the day it was made.
erfect:
