Repairs: flues, fireplaces and chimneys

Problem 2: Condensation

Older chimneys can be particularly prone to condensation – especially where the flue is buttressed on an outside wall.

If the chimney is flush with the flank wall of a house, it may be only half a brick thick. This allows moisture to penetrate, causing localised cooling in the flue, thus increasing the risk of condensation together with general air/surface cooling of the masonry, particularly at higher levels of the wall, where air movement is likely to be more pronounced.

Short of adding another skin to the wall, weather protection may be extremely hard to achieve and for many terraced houses, which have 100% site coverage, there is unlikely to be scope to build out beyond the boundary.

Risk of chimney fires

Originally many of the chimneys/flues would have been lined with render smoothed up the inside of the flue. With age, the render will have deteriorated, and frequently the flues are found to comprise soft, powdery, eroded brick when they are broken open for examination. This can lead to a number of problems, in addition to condensation.

As mortar courses deteriorate, ledges can form that might encourage burning soot to build up.

In older properties, the odd joist may have been built into the masonry of the flue/chimney breast, and as the parging cement lining deteriorates from the flue the ends of some timbers can become exposed.

If the fireplace continues in use, first charring and eventually a chimney fire is a possibility.

Another risk factor arises when fireplaces are to be brought back into use with restricted-draught fires such as enclosed wood burning stoves, Parkray™-style solid fuel stoves, and the like. Then, a lining is particularly important.

Note: Modern fires have very high flue gas temperatures which can aggressively attack masonry chimneys. The presence of a flue lining not only protects the masonry, it helps the chimney to draw, by creating an insulated lining that heats up causing convection to draw flue gases efficiently through the height of the flue and chimney.

Solution 1

A number of proprietary systems are available to reline flues, including one method that uses a cast steel ball drawn up the chimney to force cement dropped above it into a circular profile and line the flue accordingly.

Solution 2

Another option is to break open sections of the chimney breast and flue to insert ceramic liners.

Solution 3

Stainless steel flexible lining pipes offer an extremely efficient option to line the flue. However, this is likely to be practicable only where there are very few bends, kinks or distortions within the line of the flue. If that is the case, revert to Solution 2.

If the flue liner is, in turn, insulated on its outer face by sand or mica, its efficiency can be improved and the masonry insulated from any flue gas or contamination. The material can be poured to fill the gap between the liner and the chimney.

Solution 4

A compromise method of strengthening a damaged chimney which is badly eroded on the inside face is described in Chimneys in old buildings (SPAB Technical Pamphlet 3, by GBA Williams), a document the author highly recommends. This has been out of print since the reprinted third edition in 1990: if you cannot borrow a copy, it is listed on the IHSTI subscription website). It also describes methods of mitigating or curing smoking chimneys by adjusting the hearth and fireback positions.

The method uses a stainless steel mesh or grill within the chimney, new flue lining pots inserted from above, and filling with a render or grout to strengthen the hole.

COST SEQUENCE - Flue lining

  1. Assumes chimney height of 1.3m above ridgeline and typical 2-storey building. Floor-to-ceiling heights assumed 2.4m.
  2. For ground floor fireplace, install flexible steel flue lining taken to secured terminal fitting at top of chimney and fixed to register place about fireplace opening.
  3. Assumes that no breaking out of the main chimney flue is required, and the flue liner can be inserted from ridge/roof level.
  4. Pricing to allow for all necessary scaffold access to roof.
  5. Make cost allowance for reconstruction of, say, one quarter of the chimeny stack and for cleaning and removal of soot from the flue prior to starting work.

Chimney flue lining - COST £935 (reusing existing pot); COST £1100 (new pot)

Small works pricing

Since the original price estimating in 2008 the economic turmoil of the last few years has caused some disparities in pricing - if you are reviewing these figures in the London area they may look laughable - if you are located in Hull they might still look a bit expensive. Please treat them with caution and of course always consider obtaining more than one quote against which to test prices. Reference to the BCIS or a small works pricing book such as Spons or Laxton's will help assess costs and the hourly labour rates upon which they are based.