Some say it is because the North wind is stronger. That sounds ridiculous but in a strange way, it probably is to do with the North Wind. The best theory I have heard regarding why old chimneys lean is that the North wind is generally colder than the South, and that means that the North side of a chimney stays marginally colder for longer than the South side. As the smoke from the fire rises up the flue it will cool and condense on the inside surface of the chimney. Because the North side is colder than the South more will condense on the North face than the South. The smoke is a mixture of hot gasses given off from the burning coal or wood in the fireplace. Amongst the hydro carbons and other very toxic gases given off from your typical fire, are substances that will leech into the brickwork of your chimney and form a sticky tarry residue. This residue will cause the mortar in the brick joints to slightly expand due to a chemical reaction. Because slightly more occurs on the North side than the South, the North of the Chimney gets fractionally taller and hey presto, it leans to the South.
Another aspect of this residue that is a great nuisance is that it will discolour interior decorations. Internally, it does not matter how many times you paint over it, will come back. So the best way to irradicate it on the inside of your house is to fix some foil backed plasterboard over the affected area and decorate to taste. This residue will also leave a dark stain externally. The latter can often seen on the outside of a Victorian house and shows clearly the path of the chimney flue up the wall.
Chimneys built since the 1960s should have a special liner of clay which the residue cannot penetrate thus alleviating the problem. Incidentally these liners have to be installed the right way up to be effective and I have known brick layers get this wrong.
It is worth noting that now chimneys and fireplaces are back in vogue we are having to relearn the art. Actually it is more of a science and since the empirical knowledge built up over hundreds of years has been forgotten many mistakes in the construction of fireplaces has led to problems, some of which have cost lives.
Written by Tony Keller – Building Tectonics.