Fireplaces, chimneys and flues

6.8.7Solid fuel – outlets and terminals

Outlets and terminals shall be adequately separated from combustible material and other parts of the home, enable the satisfactory discharge of flue gases and prevent the ingress of damp. Issues to be taken into account include:

  1. outlet position
  2. terminals
  3. chimney cappings.

Outlet position

The flue will generally function more effectively where the outlet is in a low pressure zone, taking account of prevailing winds.

A low pressure zone generally occurs:

  • on the lee side and at the ridge of a pitched roof
  • close to the windward side of a flat roof.

Where the efficiency of the flue may be affected by adjacent trees or buildings in the ‘low pressure’ zone, the design should account for their effects.

Where down draughts occur, e.g. on hillsides or near tall trees and buildings, the height of the flue outlet may have to be increased or a fan-assisted flue installed.

Table 2: Positions of outlets for solid fuel appliances


1 The weather surface is the building’s external surface, such as its roof, tiles or external walls.
2 A flat roof has a pitch less than 10°.
3 The clearance given for A or B, as appropriate, will also apply.


Terminals should be:

  • purpose-made components
  • built into the top of the masonry to a minimum of 125mm or 0.25x the length of the terminal, whichever is the greater
  • sealed to the flue liner.

An acceptable terminal can be achieved where the top flue liner projects a minimum of 20mm above the chimney capping.

Chimney cappings

Chimney cappings should:

  • be weathered, monolithic slabs
  • be designed to protect the masonry below
  • project a minimum of 50mm, and have a drip to shed water clear of the masonry.

Cappings may be designed as a cover slab supported on piers (to reduce rain penetration into the top of the flue). The height of the supporting piers should be sufficient to allow a free opening equivalent to a minimum of 2x the area of the flue outlet.

Brick chimneys which do not have this type of capping should be constructed using frost-resistant masonry.