John Miles, Technical and Business Development Manager, Assent Building Control provides a guide to chimney works.
Exposed to the elements, chimneys receive the full force of any adverse weather as well as intense heating and cooling, condensation, and chemical attacks from open fires or appliances running on gas or oil. Work to chimneys varies from simple pointing and maintenance, to rebuilds and removals, with anything beyond maintenance involving a Building Control application and serving a Party Structure Notice under the Party Wall (etc) Act 1996 prior to work starting.
Work that involves lining the chimney, increasing or reducing its height, removing it – or installing one for the first time – requires approval under Building Regulations and must meet the requirements of Approved Documents A on structure, B on fire safety and J on heat-producing appliances and an agreement under the Party Wall (etc) Act 1996.
Building Control surveyors are often required in cases where chimney stacks are being removed, generally occuring in older properties, built from relatively soft, porous clay bricks and lime mortar. In these instances, proper consideration should be given to the existing structure and the neighbouring property.
Chimney removal works
Before starting any job involving the complete or partial removal of a chimney stack, structural calculations are required. The traditional method for removal is to support the remaining chimney stack in the roof space with triangular gallows brackets made of steel and bolted to the wall beneath the stack. The security of stack and brackets alike largely depends on the quality of the brickwork underneath; if this is very soft or loose, it may not be possible to obtain a secure fixing for the bolts.
Another consideration is whether the chimney also supports a neighbouring one. A more secure alternative to brackets is a steel L-section beam that sits on the load-bearing walls of the house and supports the stack above.
This picture shows the ground floor of a property with the chimney removed and an L-section beam installed. The rear lining is visible, and the party wall separating the chimney from that of the neighbouring property is exposed. There are a number of holes in the in the rear of the stack caused by the partial demolition, which will need rectification before the wall is made good and the ceiling reinstated.
The structural repairs are visible and, on the left of the photo, is the steel beam on padstones that supports the remaining redundant stack, also acting as a buttress to the existing party wall. These padstones need resetting to ensure the beam is adequately supported.
Remedial works will be required to make good the party wall where it was damaged during the stack removal, due to the possibility of carbon monoxide or dioxide leaking from the neighbour’s flue should their fire still be in use (see photo 2). All poor brickwork should be replaced and repointed, ensuring that the neighbouring chimney is not adversely affected by the work. A smoke test should be carried out by a Gas Safe-registered engineer on the neighbour’s flue to check for leakage.
Here, a new new ceiling is shown, formed to ensure adequate fire protection for 30 minutes is provided to the structural supports from the room below.
It should be noted that chimney removal works should not be undertaken using the following methods.
- Corbelling should not be attempted into the existing structure as it will not be possible to gain a sufficiently strong bond between the two kinds of brickwork.
- Timber bearers should not be used, as the loads imposed by a chimney are far greater than those of a floor and could lead to collapse of the ceiling or floor.
One of the most common problems with existing chimneys is dampness around them. This could be a result of poor maintenance, or damage to or inadequate flashing to the stack. Such defects often go unnoticed until further works are carried out, and any remedial works should be undertaken at this point to remedy the defect as part of the wider scope of works.
All chimneys should have a capping in place with a drip to shed water clear of the brickwork below, a flashing to prevent rain passing between the chimney and adjacent roof covering, and a damp-proof course in the chimney at the lowest intersection between roof and chimney to protect the masonry.
Assessment of any chimney works start at the plan-checking process, with the proposed height needing to fulfil the criteria of Approved Document A section 2D diagram 20, and those of Approved Document J for positioning and size. Site inspections allow the surveyor to review compliance and identify any potential problems.
Any repairs need to comply with Approved Documents A and C and BS EN 1443: 2003, as well as following the BRE Good Repair Guide 15 of 1998.
For further information on Assent Building Control go to https://www.assentbc.co.uk/