I am planning to fix the chimney on our old shed as quite a lot of water leaks in there when it rains. The chimney appears to be made up of four large cast blocks or something similar, not a regular brick chimney.
The shed is from 1893, doesn't follow all building regulations and will continue to do so in the future.
What has been done is that a chimney sheet has been placed so that it sits perhaps 0.5cm apart from the chimney and then sealed with mortar sloping from the chimney out onto the sheet.
In cross-section, it looks something like this, the straight line is the chimney, the slanted ones are the mortar. The two lower lines are, on the left, the chimney, and on the right, the sheet. Very proud of my ASCII drawing (google "ascii art" to see people who are a bit more skilled).
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I know it would have been easier with a picture, but I don't have one available at the moment.
The mortar has deteriorated over time and is therefore largely missing, so I thought I would knock away the remaining mortar and apply new mortar to make it watertight while maintaining its appearance. The big question then is - does lime mortar/hydraulic lime mortar adhere to sheet metal? They hardly had any cement in 1893.
If someone happens to have the same question as me, the mortar adheres well to metal. You might need a bit more water and it helps to spray water on the metal for the mortar to stick. It will be loose when applied - so handle it carefully. But once it starts to harden, it actually holds reasonably well.
Such a joint should in no way bear weight but just act as a cap for the metal.
The joint is entirely freehand without molds, so it is not entirely even.
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