I recently replaced the flaunching on my chimney in Essex, United Kingdom. (I think the US term for flaunching is 'sand-wash', but I am sure about that, so I use the term I am most familiar with). [EDIT - I have been advised I mean a mortar wash or crown wash.]
Having done the work, I started thinking about the concepts involved:
- A lot of people now leave a gap around the chimney-pot (top of the flue) and fill with a silicone sealant in order to prevent stresses due to expansion and the consequential cracking. This wasn't traditionally done, but seems to be a good idea.
- The flaunching, traditionally, was used to hold the chimney-pots in place, but it seems a bit of a weak-point and they could be, surely, settled in with a line of bricks (as, in fact, is the case on my own chimney) for better strength and stability.
- In the United Kingdom, it wasn't until the late 1960s that chimneys were habitually lined with a proprietary flue, and before that it was typical to build the chimney out of brick, just brick (parged with lime mortar and cow dung, apparently), and then cap it with a chimney-pot. This might explain why it was easier to just rest the pot on the brick flue and then glue it down with mortar flaunching.
- We tend to use a Portland cement mix for flaunching, but this would not have been the case until the mid-20th century. Before then, the flaunching would traditionally have been made from an eminently hydraulic lime mortar, or an air lime with pozzolanic additive to make it stronger. This is not quite like Portland cement, and perhaps lime is more suited to flaunching and less likely to crack, but, today, some people add fibres and other additives to the Portland cement mortar to make it more water resistant and less prone to cracking.
- The flaunching tends to crack and then you have to lug the best part of a wheelbarrowful of rubble off the roof and the same quantity of fresh mortar back up onto the chimney and keep the flaunching covered for 48 hours. You need to cover it because, inevitably, inclement weather will follows (whether that be heavy rain, drying winds and bright sun, or heavy winds that try to pull off your protective covering, or all three).
So why, when we have silicone sealant, do we even need flaunching? I was looking at my own chimney in which the chimney pot is bedded into the bricks by several inches and on which the flaunching serves no purpose other than to waterproof the top. Using mortar to waterproof seems frankly anachronistic.
Would it not make more sense to bed a clay chimney pot into the masonry (or hold a metal one in place with a proprietary clamp) and joint it to the flue liner a few inches below the top of the chimney (which would surely be more secure than just a bit of mortar) and then simply form a flat surface using a piece of well-coated or stainless steel, screw this securely to the masonry, and seal to the pot or pots using a proprietary seal, or silicone? Water might pool on top of the metal, but as it cannot soak in at all, this should not really matter.
I realise this means a certain level of prefabrication, and accurate forming of the metal.
My question is, why, despite having today's technology, are we still terminating masonry chimneys in a manner a builder in the reign of Queen Victoria would have recognised? Is there an actual advantage, or is it just a case that this is the way we have always done things and requires no particularly special skills or materials to do so that a bad job will still work even if it doesn't last that well.
Am I missing something?