Thompson’s Development Chemist, Ross Sellars, offers some advice on tackling chimneys and the importance of waterproofing.
All masonry chimney construction materials will suffer accelerated deterioration as a result of prolonged contact with adverse weather conditions and water. Masonry materials can deteriorate quickly when exposed to the freeze thaw process, in which moisture that has penetrated the materials periodically freezes and expands causing undue stress.
Our research shows that water is the biggest enemy to a masonry chimney and there are many reasons why. When water penetrates into a chimney, it can cause cosmetic as well as structural harm – creating hard-to-fix issues like penetrating damp, erosion and rot.
Brick and mortar can become extremely porous and can absorb large amounts of water – especially following a bout of wet weather. They can act like a sponge and will naturally let moisture in. New masonry will be inherently waterproof, but this can reduce over time and become less effective.
Here are three top tips for professional builders to identify defective masonry and the first signs of water damage:
1. Take notice of the appearance of the chimney and its surrounding masonry
If masonry is noticeably defective, brickwork and mortar joints are at risk and will have a tendency to absorb water. If there are cracks, gaps or missing mortar in chimneys already, these should be repaired before a water seal is applied. A waterproofer can be added directly to the mortar mix to give extra protection.
2. Check for signs of efflorescence on masonry
Increased moisture penetrating masonry surfaces is often a source of efflorescence. For water to carry or move salt to the surface from within the masonry there must be a channel through which to move and migrate.
The denser the material, whether it be brick, stone, or concrete, the more difficult for the water to soak in and transport salts to the surface as the material dries out. If efflorescence appears on a masonry chimney it highlights the need to waterseal the masonry.
3. Odour
Untreated penetrating damp or mould can cause an unpleasant smell, especially when unnoticed for a long period of time. If a homeowner reports problems with odour, defective or damaged brickwork could be the cause.
If there’s no groundwork damp, check the loft for signs of damp or the mould. If the issue can be traced back to a chimney, remember its flues often link to several rooms in the house, so problems can quickly spread.
If left untreated, and in the most serve cases, health problems can occur – respiratory infections, allergies or asthma can be become more common.
What is the best type of treatment to use?
A water-based water seal is best suited to treat and protect large areas, 
covering both brick and mortar. Most formulations offer a unique micro silicone emulsion to stop water penetration – water-based alkyl polysiloxane resin which soaks into masonry to protect from within, not just on the surface.
The benefits of this type of product are that it can be waterproof within hours and can be applied directly onto damp surfaces.
In cold weather, a seal like this also protects against the freeze and thaw process – a common problem which damages a chimneys.
If the problem can be traced to an area of failing mortar, a chemical admixture can be used in the mix when re-pointing the brickwork – giving extra protection in an area susceptible to weather damage.
