Question from one of our customers: After the recent inclement weather, damp has begun to penetrate my 200-year old house to an unacceptable level. I am pretty good at DIY and I am considering re-pointing the defective mortar joint and damp-proofing the wall using a silicone-based compound – is this a good idea?

Our answer: We would recommend letting the wall ‘breathe’ to dry out, beforeraking out the cement mortar and replaced with lime to match the original mortar. The problem will no doubt lay in the fact that the original lime mortar is softer than the bricks, thereby allowing water to come through the mortar joints, but this problem is likely to be made worse if you re-point the defective joints.

Research undertaken by English Heritage on old church buildings made of stone has shown that rainwater passing through the walls of older masonry appears to follow well-defined paths, at the end of which it is able to drip out of mortar joints.

Cement re-pointing dams these defined paths so that the only way for the water to escape is through the faces of the bricks, which can lead to a problem known as efflorescence spalling, which is the process through which water penetrates masonry and deposits salts on the surface and can lead to the spread of mould and wood-rotting fungi . This excess build-up of water can lead to frost damage.

A great deal of research has been published on the subject, but it has unfortynately escaped the attention of many honest builders who continue to use traditional methods like the aforementioned re-pointing.

You should also avoid using a water-repellent coating, which might have a waterproofing effect in the short-term, it will ultimately hinder your wall’s ability to dry out by the normal process of evaporation.

At DPR (Huddersfield), we may be roofers but we take a great interest in every to do with construction. If you would like information on damp-proofing your roof or indeed your entire home, please call now on 01484 290087.