CHIMNEY STACKS

Like a car with poorly maintained brakes, a neglected stack can be ‘an accident waiting to happen’. The prospect of chunks of heavy masonry dislodged by storms crashing onto the roof above your bedroom isn’t a risk worth taking. So if your survey flags up significant defects it’s usually advisable to get them sorted sooner rather than later.

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Things to check

1/    A roof leak around the chimney

The most likely cause of damp is defective flashings at the junction with the roof. Mortar fillets are sometimes applied as a cheap short-life alternative to lead, but are brittle and prone to cracking and are best replaced with new leadwork. Sometimes rain can get around an otherwise perfectly good flashing because in older houses there is no damp-proof course (DPC) in stacks. This can allow water to soak into porous stack masonry and leach downwards into the loft or rooms below. Damp penetration may also be due to eroded mortar joints or damaged brickwork (see Issue 2).

Remedial work:  Defective flashings can be replaced with new lead ones of minimum Code 4 thickness.  It’s a false economy to use cheaper short-life materials.  Traditionally, at the sides of the stack slates and plain tiles have strips of ‘L’ shaped lead ‘soakers’ inserted underneath each one before being covered with the flashing (soakers are not necessary with interlocking tiles).

In some cases the existing flashings may simply have come loose or are inadequately sealed to the brickwork, in which case they may just need to be refixed and wedged into existing joints and pointed up with fresh mortar. Where damp is due to porous stack walls that lack a DPC, eroded mortar joints  should be re-pointed. If all else fails, try fitting new lead flashings that extend higher up and deeper into the brickwork.

2/    Cracked or damaged brickwork, stonework or render

Deterioration is often simply down to the effects of ageing. Over the years, exposure to extremes of weather can result in mortar joints and masonry eroding, small cracks allowing water to penetrate and then freeze, loosening masonry or blowing render. Inside the flue, dampness from condensation of gases on cold surfaces can eventually cause expansion, cracking and chemical erosion of the masonry and mortar. Damage to chimney masonry is also sometimes caused by poorly fitted TV aerials.

Remedial work:     Any areas of defective masonry must be cut out and replaced. If just the upper brickwork has come loose due to soft or eroded mortar then partial taking down and rebuilding may be needed.  Eroded mortar joints can be raked out and repointed. Any loose, cracked, or hollow areas of render can be hacked off and patch repaired.  Internally, installing a flue liner should protect the masonry, prevent leaks and improve thermal insulation, but selecting the right kind of liner for the fire or appliance is important.

 3/ Leaning chimney

A small degree of lean is quite common in old stacks. Many have leaned considerably more than the ‘official’ limit of 1mm in 100mm, but in most cases are still stable. Where there is pronounced leaning a structural engineer will need to confirm whether it is too extreme to be made safe without rebuilding or taking down. When bricks get consistently very wet, any sulphates in the masonry or mortar can react, causing horizontal expansion cracks along mortar joints, indicative of ‘sulphate attack’. If a stack has a large surface facing the prevailing cold wind and rain, constant wetting can lead to expansion of mortar joints, effectively ‘jacking up’ the stack, causing it to lean away from the wind.

Erosion of external mortar joints can be exacerbated by attack inside the flue from  combustion gases. The stack may be colder on its windward side and hence suffer more erosion there from condensing acidic gases and or expansion from sulphate attack. Also check for botched structural alterations, such as removal of chimney breasts without sufficient support to the remaining stack masonry above.

 Remedial work:     Old mortar joints may have become loose or seriously eroded with age, so that repointing is now overdue –  with a sulphate resistant mortar.  Interior erosion can normally be kept at bay by installing the correct type of flue liner.

Where instability is due to the removal of a chimney breast to a room below, the usual solution is to support the remaining masonry within the loft with a steel beam (which will require Building Regs consent). The remedy of last resort in severe cases is to take down and rebuild the stack, or remove it entirely.   In older properties where the chimney is being taken down and rebuilt it’s often possible to re-use most of the original bricks. If the decision is taken to completely remove the chimney, the old opening will need to be covered over with a couple of square metres of replacement tiles or slates.

4/  Damaged pots and flaunching

Long term exposure to extremes of weather can ultimately cause erosion to the supporting masonry at the head of the stack. Cracks to the flaunching (the expanse of mortar that holds pots in place) can develop over time with consequent water penetration leading to frost expansion and loosening. Internally, dampness from condensing flue gases can be a contributing factor that eventually erodes the masonry.

Remedial work:  Damaged pots need to be replaced.  Unstable pots can be taken out and re-bedded. Loose flaunching needs to be completely hacked off and replaced with a suitable mortar mix, waterproofed and carefully moulded so it slopes outwards to disperse rainwater.  Lining flues should resolve internal problems, combined with fitting ‘raincaps’ to each chimney pot to reduce the risk of rain entering flues and to deter bird ingress..

N.B.  This is by no means an exhaustive list, and much depends on the age and type of property you’re surveying

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