Fireplaces, chimneys and flues

6.8.23All – chimneys

Chimneys shall provide fire protective casing for flues, and shall be capable of adequately supporting the flue liner, while resisting damp penetration and the products of combustion. Issues to be taken into account include:

  1. construction of chimneys
  2. typical construction details
  3. damp penetration and weatherproofing
  4. coring and drying.

Construction of chimneys

Masonry chimneys should be properly bonded to, or supported by, the adjoining walls of the building.

Foundations to a chimney should:

  • be the same depth as adjacent wall foundations
  • be designed to avoid uneven settlement
  • where the chimney forms part of the wall, be a minimum of 100mm wider than the chimney base.

Height (H) of an unrestrained chimney should:

  • not exceed 4.5x the smallest dimension on plan of the chimney (W) (where the density of the masonry is a minimum of 1500kg/m3), or
  • be designed by an engineer in accordance with Technical Requirement R5.

Chimneys which:

  • are of block, brick or stone should have a minimum wall thickness of 100mm, excluding the lining thickness
  • are built in a cavity separating wall should form two leaves, each a minimum of 100mm, between the flue and adjoining building
  • form part of a compartment wall, and are not back to back with an adjacent chimney, should have a minimum wall thickness of 200mm separating it from the other building or home.

Factory-made insulated chimneys should be assembled, erected, anchored and protected in accordance with the manufacturer’s instructions.

Masonry for chimneys:

  • below roof level may be constructed using the same bricks and mortar as used for the general brickwork
  • constructed with hollow or cellular blocks should be suitable for the construction of chimneys and filled with concrete as the work proceeds
  • should be frost resistant above the roof unless protected by a capping projecting by a minimum of 50mm (in Scotland, frost-resistant bricks should be used for all facing brickwork).

Connections between flue blocks and ridge terminals should be made:

  • as detailed in the design
  • using the correct fittings and supports as specified by the manufacturers of the flue blocks, flue pipe and ridge terminal.

Typical construction details

External fireplace recess and chimney

Other alternatives may be suitable, provided they meet the appropriate performance standards.

Damp penetration and weatherproofing

Where chimneys exit close to the ridge of a pitched roof, occasional damp penetration may occur below roof level. In this situation:

  • the roof space should be well ventilated
  • any dampness penetrating downwards should not reach the living areas.

Where chimneys exit close to the eaves of a pitched roof or through a flat roof, trays and flashings should be installed in the chimney so that all damp penetration is prevented.

DPCs, flashings and gutters should be provided at the intersection point of the chimney with the surface of the roof through which the chimney passes. DPCs to the main walls should be carried through the base of chimneys.

Flashings should be made from compatible non-ferrous metal. Lead trays should be bitumen coated where in contact with cement.

In areas of severe and very severe exposure, the following details should be used. In lower exposure zones, the tray upturn may be on the outside of the flue liner. All other details are the same.

Coring and drying

Where a core (e.g. a sack full of loose straw, or similar) is used to prevent mortar dropping into the flue liner during construction, the builder should ensure that it is removed on completion of the chimney.

A chimney should be allowed to dry naturally for a minimum period of 14 days before use.