Domestic Chimneys & Flues

The chimney and flue system are an integral part of the structure,  function and aesthetic composition  of a traditional building and often  the streetscape in which the building  sits. They are structural elements that require care and maintenance,  although due to the location this is  frequently omitted. This Inform will outline the basic elements of a flue  and chimney system and identify  common faults, their diagnosis and  the principles of how to deal with  them. Washed out joints – fine ashar joints, and thicker joints in rubble chimneys become  washed out over time,  this leads to water  ingress and progressive loss of structural  stability. Repointing and sometimes rerendering is required, as well as checking the condition of the cope stones.

History and Development
Chimneys have long been part of domestic construction, and have come to be part of the  urban and rural archetype. Originally homes did not have chimneys,  and fumes were vented  through the roof, but since the early middle  ages high status building were fitted with  effective fireplaces and flues. Sometime later domestic dwellings adopted flues, although  many Hebridean blackhouses had no chimney  in the early 20th C.  The type of fuel burnt has  also affected the design of flues and chimneys  – generally lower calorific value fuels such as  wood and peat require bigger openings and  flues, different fuels such as coal and anthracite  can burn effectively through narrower bore  flues and chimneys although most types of fuel  will work in most hearths. Often chimneys acquire fittings and fixtures from previous services and installations, especially TV aerials, which, when no longer  in use, should be removed as once loose they  start to rub and loosen the stonework; spalling  ironwork can also lift blocks off their beds  through oxide jacking.

Basic structure of flues
Single or multiple flues were normally built into the thickness of a masonry wall, frequently  on the gable ends. In larger properties with internal masonry walls, flues were run up through  internal walls. In cases where there have been flue problems, or a hearth was added, flues have  been added against a gable, sometimes externally  or internally. The shape of the flue is normally a square or a rectangle, earlier flues from the 16th  C tended to be much flatter rectangles, reflected  in the shape of the chimney head. Much depended on the budget, but in house construction the masons would build an opening  for the fire, and continue the flue up within the  wall. The size and openings of hearths varied with use and status; on plan, earlier medieval  openings were sometimes semicircular, while  most Georgian or Victorian openings are  rectangular or trapezoidal. Internally the flues were lined with smooth plastered masonry  (a layer of lime mortar sometimes called  pargetting). As fuels have become more energy intensive, flues have tended to become smaller  and more effectively lined. Where the builders had to fit a large number of flues within a gable,  such as a tenement, stone slabs were used to  separate flues. The pattern of these slabs can sometimes be seen on gable ends of tenements. Later in the 19th C, ceramic flue liners were used, as well as being quicker to fit and build, they have  also proved more durable, as they seem better  able to resist corrosive flue deposits. In small 18th century rural properties flues were often built against a masonry wall in timber and  clay, sometimes called a “hanging lumb”. Such survivals are rare and if encountered, should be  retained. The clay coating, and the relatively low temperature of the smoke does mean that these  could be safely employed as a flue. Degradation of stone - for reasons of exposure, flue gasses and salts, stones of most  types tend to suffer more erosion and damage  when used on a chimney. On investigation, what may appear to be sound masonry, may  be quite friable and may need replacement,  especially on the hidden inside face. For this reason, and those above, it is likely that  a chimney will need rebuilding every 100  years or so. Close inspection by a building conservation professional is advised before  remedial action, and listed building consent  may be required. Discuss your plans with the local authority before proceeding with any  work. Plant growth can become a problem, and can greatly accelerate the decay of a chimneyhead  and stack. Broadly, if a plant or small tree  has become established, it should be taken  that the roots are some way into a loose and  degraded core, and may indicate that some  rebuilding is required. At the very least, vegetations and stem material should be  removed and the stump poisoned. When carrying out work on a chimney, ensure that any replacement work matches the original, in colour, texture and compressive  strength. If the cope is being replaced it should be done in stone, with the correct drip  detailing on the underside to ensure water is  shed clear. Many traditional buildings have a string course or band of projecting masonry  that throws water clear of the gable. In some cases chimneys have been removed. While re-instatement is favored, it is likely to require consent, and in any event needs to  copy in style, detail and material those in the  same building or terrace. The replacement of a masonry chimney stack with a rendered  brick alternative is not supported, although  was common practice in recent times.

Basic Structure of Chimney stacks
The chimney flue terminates at the chimney head, sometimes called a stack, essentially  the element above the roofline. Likewise of masonry, the chimney could be built of rubble  or ashlar, with bridges of slab or composite  stone if there is more than one flue. The design and disposition of chimneys on a  dwelling are much more than routes out for  smoke - they deliberately conveyed subtle  messages of home and status; historically  builders and architects took full advantage of  this. Capping the chimney is the cope, a single,  or several dressed stones that provide the  level top. Styles of finishing chimney heads have changed over the years, and can provide  indications of the buildings age. Patterns have varied over time, from sloped copes with  rolled edges of the 16th C, to very square  tabulated cornicing and drip details on some  19th C Chimney heads. Banding (raised margins)  on the chimney quoins can give  clues as to the original surface finish of the  chimney and possibly elevation. Sometimes the cope stones secured together with iron or  bronze dogs secured with hot poured lead. Up until the late 18th C, chimney heads were finished with a cope only, but from the 19th  C all but the largest chimneys were finished  with clay pots or cans. They developed in many styles, with some 19th C ones being  very decorative indeed. The pot is sometimes set into a slight recess in the cope, and  mortared in or “haunched” with mortar.

Flues
The flue is probably the harder element to deal with, due to the hidden nature of its  construction. Indications of flue problems are smoke ingress in an upstairs or neighboring  property, fragments of stone or mortar  coming down the chimney, or problems with  the fire not drawing properly or smoking. In more extreme cases render may be cracked  and bulging, and staining is visible on interior  and exterior surfaces. Frequently it is water in the flue that is the driver of such symptoms,  entering through a poorly maintained  chimney or defective gable render. The soot and other deposits that form in a flue tend  to be water absorbing or hydroscopic, and  can hold moisture if inappropriate renders or  mortars are use. Such moisture leads to slight movement in frost, potentially admitting further water. On rubble elevations there may even be holes in the wall into the flue itself.

Ventilation
An important function of the chimney system in a traditional structure is ventilation, even  when no fire is lit. The rising air in the chimney draws new air into the room from  under the floor and behind plastered surfaces,  keeping void spaces and hidden areas dry.

Problems and issues
Due to the heat of the flue gasses, their composition, and the exposed nature of the  chimney, flues and chimneys take a lot of  wear from use and the elements. Common problems, and indicative solutions are:

Chimneys
Chimney Cans. Over time, the haunching washes out or becomes cracked, and the chimney pots, or cans become loose. This will require re-bedding in fresh mortar. Occasionally the can itself becomes cracked and broken and they should be replaced. Invariably styles are limited, but cans should be matched where possible - in height and color as a minimum. If the fire is not used the can should be covered, preferably with a conical galvanized cover fitted with bird netting. Ceramic covers are available, but do limit ventilation somewhat. If there is evidence of damp in the chimney they should  not be used.

Repairs to Flues
Excessive loss of pargetting and internal masonry can make using the chimney  hazardous. Internal timber elements can become exposed, and combustion gases  can escape from the flue into other rooms or  properties. In multiple occupancy buildings, such as tenements, the flue arrangements can  be complex, and the structure of the chimneys  becomes an important safety issue, especially if  the gable is freestanding. Relining or rebuilding of flues is really the only solution in one of the  following ways: • Inserting a flexible metal liner. Normally done for a boiler or wood burning stove, this  is the least disruptive to the household, but  requires correct termination at the top and  bottom. It is advisable to ensure the void within the chimney ventilated. The main benefit of this is that it is reversible, but not  suitable for an open hearth. • Fitting of ceramic flue liners. This is arguably the better long term solution, as it addresses  the internal structure of the flue. However, it tends to reduce the bore of the flue so ensure  a size of liner is commensurate with the type  of fire or appliance to be used. Holes are made in the wall every 1.5 m or so, and a  ceramic liner is inserted and bedded in a weak  lime mortar. This process is continued up the wall until the chimney is reached. The process is disruptive, although if the flue can  be accessed from the outside this is reduced,  but with the consequent need for scaffold. It is also possible to rebuild the flue in the original manner, with stone bridges and slab  lining, although this is more time consuming  and thus expensive. • Poured cement based re-lining. There are many franchises offering this option. Essentially they craft a cement lining, temporarily held within a removable rubber  former. The poured lining fills all voids within the flue, including holes into rooms  where care needs to be exercised. However, this method also reduces the bore of the  flue and due to the cementitious nature of  the poured material, does not allow proper  movement of water vapour within the wall. If the wall and flue are dry or internal, it can be effective, but caution should be exercised  when considering this method for external  gables or exposed chimneys. In the past an additional chimney flue and stack was often build adjacent to the gable. This is not an acceptable solution, as it spoils the appearance of the elevation.

Issues with Smoking
A smoking chimney can be caused by many things, including physical blockage from  failures above. In addition, there is a strict  relationship between the size of the opening  in the room, and the size of the opening at  the chimney head. Normally the proportion is 1/10, but historically much consideration  was given to the optimal shape and design of  an opening. The most well known is Count Rumford’s work of the late 18th C.  If the  flue has been relined for a stove, and if an  open fire is favoured, it will not draw well. There are many advanced thoughts on how to improve draw, that range from the shape  of the hearth to types of cowl on the pot. Other factors come into play, including topography and type of fuel being burnt,  but generally speaking most unaltered flues  should work well.

Sweeping
Where original fireplaces are still in active use flues should be swept at least annually  to remove soot deposits from the inside of  the chimney. Failure to do this will result in a build up of soot, and in the case of a  wood fire, tar deposits. These deposits are potentially flammable and should be cleared  to avoid the possibility of a chimney fire.