There is a submerged village at the mouth of Wexford Harbour, lost to winter storms over 80 years ago. Amid the wave-swept wreckage of homes and a small port, the only recognisable remains are the brick chimneys of the long-gone timber houses. It’s easy to forget that concealed behind the fireplace, painted walls and framed decorations of most period houses lies a massive brick or stone chimney stack, perhaps the most substantial element of the building. The earliest chimneys were walk-in affairs, built both for show and the practicalities of using a log fire to heat a room. Often one of the most noticeable features of ruined period houses, such as Carbury Castle in Co. Kildare, are the long tapering fingers of the chimneys, silhouetted against the sky. As building skills developed and coal became more available, chimneys became smaller and more efficient, serving several fireplaces through separate flues. The size of the flue was determined by the size of the bricks used to build the stack, and the flue was lined with a mortar known as “parging”, specially formulated to prevent gases escaping into the house. Most period chimney stacks have multiple flues, perhaps shared with a neighbour, though some early chimneys are free-standing structures with the house built around them. By the Victorian period, the stack visible above the roof was often a fashionable decorative feature, with moulded or polychrome bricks, and topped by fired terracotta pots.
The most common areas of concern for owners are cracks, leans, moisture, and smoke blowing back into the building. Cracks can appear in a number of patterns, each indicating particular problems. A widespread irregular pattern of cracking suggests that the plaster has lost its bond with the underlying brick masonry, whereas a vertical crack suggests that the flue has cracked either through fire damage or from salt attack. Salts are a common factor in many building problems. Chimney smoke tends to contain moisture and sulphates. Over time, the parging builds up a salt load which acts as a weak acid, and these salts begin to pass into the brick stack – leading to the deterioration of the brick and the mortar. Once the salts reach the surface of the brick, the mortar can be lost resulting in deep “dry joints” and the brick can start to crumble or “spall”.
Repairs to the exposed section of chimney stacks is one of the most commonly undertaken repair jobs. Early intervention is typically the most cost-effective, as once the “skin” of a brick is lost, the underlying material quickly weathers away and the entire brick has to be cut out and replace. It can be very difficult to find bricks to match in size, colour and texture with the originals. However, often of greater concern is that if the repairs are not carried out in time, the top of the stack may have to be taken down and rebuilt. The optimum repair work is repointing the joints only, with no brick repairs, and usually only carried out when a building owner has identified a problem early on. The repointing should always be carried out using a carefully specified mortar as a balance needs to be struck between a durable, structurally strong mortar in an exposed location subject to atmospheric pollutants, and a mortar that’s too strong, which can lead to failure of the surrounding historic brickwork.
Salts and moisture ingress can be a very damaging combination, sometimes extending inside the house, causing brown staining to appear on the plasterwork and wallpaper above the fireplace, and the failure of wall surfaces. The evaporation of salts from the surface of the chimney stack above the roofline can also lead to a perceptible “lean” to the chimney stack. Most leans are tolerable, though on slender stacks or stacks with an extreme lean, the brickwork may have to be dismantled to a secure point and rebuilt. Moisture ingress can be dealt with through a DPC, flashing and “soakers”, and some chimneys with an exterior plaster, or render, may have to be re-plastered. Defective pots and “flaunching”, the mortar which secures them to the chimney stack, are normally straightforward to repair, though finding a suitable match to an ornate terracotta pot can be challenging. Architectural salvage yards hold a certain range of pots, and is it possible to obtain facsimile replacements.
Flues which are used for a range or boiler, or are in constant use as an open fire can show parging failure. There are a number of potential causes including structural defects or excessive condensation and fumes leading to cracking and collapse of the parging lining the flue. Often this becomes noticeable because of a blocked or partially blocked flue, or smoke escaping into other flues or fireplaces, and relining the flue is usually then necessary.
Damp and condensation cause problems however they enter a chimney, whether around defective flashings between the chimney or the roof, through the wall of the stack, or simply directly down the flue. In a well-ventilated flue, air pressure draws out the fumes quickly and the chimney drys out. However, if the flue is very tall, wide or cold, the gases may cool down and condense on the parging, a common occurrence when fresh timber is burnt. Moisture levels tend to rise rapidly if ventilation is blocked at the top or the bottom of the flue, and so disused flues need to be left open with some ventilation to allow drying. Flues in use present a range of other challenges. Often, problems faced by homeowners can be solved without substantial repairs to the chimney, for example by providing a replacement pot designs which increases up-draught and improves the drying of the chimney. The hard part is always to carry out the right repair in the right way, and ideally without altering the character of the building in the process.