Why Soot and Mortar is Falling Down Your Chimney

Soot with parging

Spring and summer is a time of year when I start receiving more calls about soot and mortar falling down chimneys.

It can be quite worrying for some customers, whose minds start racing and wonder whether their chimney is about to fall down. Thankfully, that is rarely the case. It’s usually more to do with the fact that the chimney is simply responding to its environment and also its age/condition.

If you think about it, chimneys take a fair old battering over the years. Some are well over 100 years old and, before the mid-1960s, most chimneys in the UK were simply made of brick and had a thin layer of mortar. This is callled parging, and, over the years, the parging weakens and falls off, exposing the brickwork underneath.

Soot deposits then attack the brickwork, so bits can fall off. The mortar joints between the bricks are particularly susceptible, and in some the joints revert mostly to sand. Think of a house which needs repointing – something you can’t do on the internal wall of a chimney as it is largely inaccessible. This is why sometimes whole courses of bricks can collapse and block a flue. This usually happens in the part of the chimney above the roof line (the chimney stack) as that is the most exposed part, so gets all the wind, rain, ice, snow etc.

chimney stack and pot

Throw into the mix the differing temperatures a flue has to endure over the years, from very hot when a fire is burning to very cold, depending on the location of the chimney, and it all means the internal part of the flue can turn very crumbly indeed.

As a result of this, debris can fall down the chimney and into the grate. This tends to be noticed more in summer when chimneys suffer from something called ‘reverse stack effect’. This is when the air outside is warmer than the air inside the house – the complete opposite of what happens in winter. So this can mean air flowing down the chimney from outside into the room, rather than up the chimney, and bringing down soot and mortar.

This can and does happen with any open, old, unlined chimney, whether it has a open solid fuel fire or even an open gas fire attached. Infact, it usually perturbs gas fire owners more as they aren’t used to mess from their appliance.

So what can be done?

Sweeping definitely helps as it will help remove any loose deposits in a controlled manner and allow them to be disposed of safely, rather than you having mess greet you every morning. Regular sweeping also helps prolong the life of your chimney, as it gets rid of the acids and tars which attack the internal brickwork.

chimney soot

A large build up of soot in a brick chimney

If it is a persistent problem, you may want to think about getting your chimney lined if the flue is in use, for safety reasons just as much as anything else. If the flue is not in use, you may consider getting a chimney balloon.

Like I said at the outset, it’s rarely anything more sinister than your chimney showing its signs of age. However, make sure you get a registered, trained sweep in to check it over as every situation is different and they will be best placed to offer you bespoke advice.

Mr Soot Chimney Sweep is a HETAS Approved Sweep, and member of the Guild of Master Chimney Sweeps, offering a professional and friendly chimney sweep and stove service in Wigan, Stockport, Southport, Ormskirk, Chorley and surrounding areas. Book Mr Soot online or call 0800 0541154.

7 thoughts on “Why Soot and Mortar is Falling Down Your Chimney

  1. my chimney was taken down, the man then threw all the bricks down the chimney. l thought that was wrong because now l need to open up the fireplace but fear he has blocked the flu.

  2. Thank you for this article. We only moved in to our 1920 built house last July and have recently noticed quite a lot of brick dust (some little chunks of brick too) coming down on a regular basis. Am I correct in assuming that we need the chimney swept? I am already worried about the age of the house but you have put my mind at rest a little.

  3. I had my chimney swept recently but still have loads of soot coming down the chimney. Should this have stopped or at least lessened?

    • A masonry flue will continue to decay with age. Some flues are more prone to it than others, particularly if they are straight. Maybe ask your sweep for a definitive answer as they’ll be best placed to answer questions about your specific chimney

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