Chimneys


Typical 1920s chimney

Houses built before WW2 were quite different to modern houses – you may have heard of the expression breathable houses. Clay bricks, lime mortar (rather than cement) and lime plaster meant that although the house might get a bit damp sometimes it will dry out. This happens particularly when there may be drafts and big open fires. Problems can arise if the breathability is compromised when modern materials are used during renovation, e.g. cement pointing over the lime or gypsum plaster. Unless they specialise in heritage work, sadly a lot of modern builders will use cement mortar  and gypsum plaster because these materials are cheaper and more pleasant to work with.

This sort of chimney works extremely well when there is a fire in use as the rising heat helps to ward off any damp issues. Normal maintenance that would often not be required very often would include checking the mortar joints, launching and beadwork. If it is well built it could be fine for 50 – 100 years or more. However when the fire below is not used any small amounts of damp that would otherwise dry out may become an issue. This can be made worse if the chimney is capped off and the fireplace sealed in the room below as lack of ventilation means any dampness will not dry out.

This diagram shows a simplified cross-section diagram of a chimney in the Triangle

chimney
Typical 1920s chimney structure

Note that the DPC tray is a single piece of rectangular lead with a hole in the middle and edges sides turned down to form a skirt around the brickwork
DPC tray

Compare this to a modern tray which is turned up on the inside (often to form a spigot for a flue liner). This type of tray stops any water flowing or seeping down internally.

Modern DPCChimney tray

 

Water ingress higher up.

  • Water can obviously enter at the top of the pot. This can be remedied to a very large extend by adding a weather cowl. Chimneys can be completely capped off but should be given plenty of ventilation, e.g. holes in the side of chimney, within the loft space, on the chimney breast and at the fireplace.
  • Water can get in when the flaunching is cracked or disintegrating.
  • Mortar joints between bricks may be compromised, especially when they have been repointed using cement based mortar

Water ingress at roof level

  • Flashing provides protection from water flowing down the roof tiles and forms a gutter behind the chimney. It runs all the way round the chimney but as you can see the gutter is the crucial part. Over years the lead can deteriorate and allow water in.
  • The damp course tray is a horizontal sheet of lead between two brick courses. It is turned down to create a skirt over the flashing and bricks below. Again the lead can deteriorate and get cracks.
  • Below the tray there is often a vertical barrier either of lead or slate tiles around the chimney and over the flashing.
  • Weep holes are often just gaps in the vertical mortar although these days a plastic or metal insert can be purchased. Weep holes allow excess moisture in the masonry to exit or dry out as well as allowing for more ventilation. If you do seal off the fireplace and cap the chimney it is worth adding more of these.

What to do about chimney problems

We’ve gone into quite some technical detail above, but it is really worth having some understanding about the possible issues when looking for a tradesperson. One of the biggest problems is diagnosis because fewer and fewer roofers these days are happy using ladders, and even if they do there is limited amount than can be seen without a full working lift from a scaffold. With any luck there may be glaringly obvious issues such as cracked flaunching and failed mortar joints or obvious issues with the flashing.

However it is possible for there to be invisible problems with either the flashing or (worse) the DPC tray. In this case a lot of roofers will just quote to either replace everything feasible such as launching, pointing and flashing or (in the west case scenario of a failed DPC tray) rebuild the entire chimney. Many of the bigger roofing companies will not guarantee the smaller repairs and only guarantee a complete rebuild which can cost anything between £5000 and £8000. This is because they could potentially do all the smaller repairs, but not guarantee it because they still not know if the DPC tray is sound as it’s impossible to see.

One other issue is that when there are very long spells of rain, the bricks can become saturated – especially with the type of wind driven Southeast winds we get in Southampton. Water soaks down through the bricks and bypasses the tray on the inside. This is something that can be solved by coating with a breathable  waterproofer such as Masonry Creme or StormDry (NOT Thompsons Waterseal or similar that will seal in any moisture already inside the bricks.