BS7913 sets out good practice for the conservation of historic buildings. As such it includes some very important guidance on the general principles which should be followed when looking after old buildings plus some specific advice on dealing with damp and timber problems.
The standard was put together in conjunction with the IHBC (Institute of Historic Building Conservation) and is intended to give guidance to specialists in the field as well as non-specialist people. It sets out good practice for any works to ‘older, traditional or valued buildings’.
Traditional buildings are generally considered to have built up until the First World War in 1914. In fact much of the UK housing stock would be classes as traditional with many terraced town houses being built at the end of the 19th century. Perhaps my only criticism of BS7913 is that the title may well put off many property owners and professional who do not automatically think a Victorian brick terraced house is ‘historic’. This is a shame since the information in this document is incredibly relevant to them.
The standard sets out the approach which should be taken when looking at some key defects. These are particularly relevant to damp issues and are discussed below.
Dampness and Moisture Ingress
This clearly states that where evidence of water ingress is is found then the ‘source should be identified as soon as possible’. In the case of an old house this is very unlikely to be a suddenly failed damp proof course since the property never had one to begin with. Instead detailed examination should be undertaken to find the real source of the water.
BS7913 clearly states that external repairs, for example to defective guttering, should be carried out before any works are undertaken internally. It is also points out that sealing the built fabric at one point of the building may well lead to damage elsewhere. This brings to mind the liberal use of tanking slurries and cement renders which I have seen over the years, invariably with negative consequences over the long term.
“Dampness is often caused by:
a) the external ground levels being higher than the internal floor level: or
b) the insertion of modern non-porous materials.”
The word ‘insertion’ strikes me as a little odd here… the only thing commonly inserted into a building I can think of is a chemical damp proof course injection. Is this a thinly veiled dig at the damp chemical industry?
Interestingly the standard suggests that possible sources of condensate should be considered when dealing with general moisture in the fabric of a building . When thought through logically this does make sense as provided the building is breatheable, with lime plaster or pointing used, and there are no sources of water ingress, then the only other source can be condensation from the air. A correctly managed old building will not suffer from rising damp due to the wall drying out under evaporation.
According to BS7913 the type and extent of the fungal attack should be identified, and all actual and potential sources of moisture removed. The type of fungal attack is useful to know as some are more able to extend through walls and some stay confined to a relatively small area. Dry rot tends to cause more damage than wet rots so rectification of the issue may be deemed more important.
“It is not necessary to remove timber that has been affected by fungus because fungus becomes inactive when the level of moisture is reduced”
This, of course, does not mean that dangerously weak structural timbers should not be replaced but it should be done sympathetically. My experience is that use of boron in gel and rod format can be useful for certain hard to reach situations, particularly on joist ends etc. Boron is harmless to humans so a great choice.
The standard suggests that analysis by competent persons is the best place to start with this. There have been a number of occasions when I have seen full chemical treatments to a house recommended on the basis of Wood Boring Weevil being found in a damp floorboard. As this beetle only attacks very damp wood the solution has nothing to do with chemical treatment. A competent person would know this. BS7913 does state that the principle objective is to remove moisture sources.
“Insecticidal treatment should only be used as a last resort as it can cause environmental damage and might require licenses for protected species”
BS7913 is an extemely useful guide to the conservation of historic buildings. Just remember that a building does not have to be listed to be considered historic in the sense meant here. If you live in a Victorian or even Edwardian townhouse of the kind found in many town centres then this guidance is relevant to you.
If you have a damp survey carried out on an older building then please ask your surveyor whether they know and understand the guidance given in BS7913. Should you be met with a blank stare then I suggest you politely show them the door.