Warning on rebuilding chimneys to open fires or solid fuel appliances
Are you planning to rebuild your chimney using timber framing?
Perhaps the chimney to your open fire or wood burner collapsed in the Canterbury earthquakes, or maybe you are intending on remodelling your house and roofline. Whatever the reason; be very careful and seek expert advice!
Chimney Fire within timber framing
Property Check has recently investigated a chimney fire where the original concrete and brick chimney was reduced in height and then extended with a timber structure that incorporated a metal flue and flue shield that extended out from the concrete open fire. The timber chimney structure overheated and caught on fire and while there were no injuries or fatalities the situation was potentially very dangerous.
Lack of ventilation at the base of the chimney
There were a few contributory defects with the design and construction of this particular chimney, however one of the main causes was lack of ventilation within the base of the chimney structure to provide cooling airflow to the flue and flue shield. Further lack of ventilation at the top of the chimney would have also contributed to the cause of this particular chimney fire.
Overheating at the top of the chimney
The lack of ventilation at the base of the chimney caused overheating at the top of the chimney where the air became very hot as there was no cooling air from below. A build-up of gases and creosote formed to the underside of the chimney cap which eventually exploded into flames.
Chimneys must comply with AS/NZS 2918 or C/AS1 - Acceptable Solutions
If you are rebuilding your brick chimney with a lightweight timber frame structure, make sure you engage an adequately qualified person to design the rebuild. You will need a building consent to carry out this work.
Useful links:
http://resources.ccc.govt.nz/files/HowSafeIsYourChimney.pdf
http://www.dbh.govt.nz/earthquake-reconstruction-guidance#aid5