How old buildings work
Traditional (Mass) Construction
Old buildings (traditional or mass construction – brick or stone or various types and qualities) account for a significant amount of the UK's current building stock. The majority of builders out there, most very capable and trustworthy, do not understand the way these buildings were designed to function or of the necessity in using the correct materials. They apply modern methods and materials which is often wrong and can lead to greater problems with disastrous effects.
Traditional construction commonly used a solid wall 600-800mm thick containing stone on the outside, stone on the inside, with a mix of rubble and lime mortar internally. This was designed to allow water to enter the structure, by even up to half of it's thickness, and then allow it to evaporate out again - all being well. The materials used in that construction are of huge importance.
Modern (Cavity) Construction
New construction (mainly categorised as post 1919) saw the introduction of cavity walls, where the function of external walls changed to keep water out of the building and the cavity, as a second line of defence, allowing any water that entered the wall to evaporate out of vents or drain out of weep holes - although early cavities were only vented into the roofspace and no other cavity ventilation was provided for. Depending on the quality of the materials, and the standard of the original build, this can be good and bad.
Several problems have arisen with older cavity construction with failure of wall ties (which were designed to keep the internal and external structure together) or breaching of the cavity by sloppy workmanship, causing damp patches in odd areas of the construction.
Lime is hygroscopic - it absorbs moisture but it also allows it to evaporate again once conditions are correct. If this is substituted by using cement, the ability of the wall to allow moisture to evaporate is severely diminished or, in the case of a cement render, lost entirely. In addition to cement preventing the passage of moisture, it also doesn't adhere very well to stone.
Some builders will recommend the use of wire mesh, rawl-bolted into the stone or brickwork to help the cement 'stick'. It doesn't, it only prevents the failed render from falling off, which isn't the same. Moisture will still be unable to evaporate and will be trapped behind the render making any problems worse.
Often the stone is abraded using a scutch hammer (a vicious looking toothed chisel-type hammer) to provide a key for the cement (below). This is a disaster as the wearing surface of the stone is completely lost together with any original finished face detail. This can be replaced but at huge expense and will never be original.
And the reason for this destructive work?
To try to get the cement to 'stick' to the stone - it doesn't work and it isn't any good for the building!
Top 10 Crimes Against 'Old' Houses
I have used the term 'old house instead of 'Period' as most of us don't live in 'period' or 'listed' or even 'important' buildings, we live at home in our modest flat or terraced and semi-detached properties. The same defects afflict all 'traditionally-constructed' buildings where one stone has been built on top of another and no cavity has been provided to prevent water ingress whether it's a one-bedroom tenement flat or a castle.
There are stories of various finds within cavities when they have been taken apart to repair dampness including various tools, lunch boxes, building materials and even a workers donkey jacket left hanging on one of the brick ties (the thin, usually wire, connectors used to attach the inside wall to the outside wall)! No doubt, in the future, we will find lots of mobile phones in there too.
Dampness problems still occur in cavity construction where wall-ties can corrode and fail (now either plastic or stainless steel instead of galvanised) causing bulges and cracking, or where mortar has been allowed to build up on the ties, moisture can pass through from the outer leaf and appear internally.
Hopefully you will realise just how much knowledge of different structure is needed to be able to understand faults and specify remedies for their repair. One size does not fit all!
A cause of damp spotting in cavity construction where mortar has caught on debris or a brick 'tie' and is allowing bridging. The solution? Identify exactly where the bridging is and open up the cavity - otherwise keep the external brickwork in good condition and use the most appropriate (original) materials when repairing so moisture won't affect the cavity and there will be no spotting.
Replastering internally will not make this wall dry - only allow the spread of dampness througout.
http://www.rics.org/uk/knowledge/glossary/top-10-crimes-against-period-houses/ is a link to a document outlining, as it says, common 'crimes against period houses'. Also at the end of the short article, comments from various chartered surveyors about traditional properties.
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