Fireplaces, chimneys and flues

6.8.6Solid fuel – chimneys

Chimneys shall incorporate flues capable of safely conducting products of combustion to the external air. The structure shall be capable of supporting the flue lining and shall provide adequate protection to the adjacent structure. Issues to be taken into account include:

  1. separation from adjacent spaces and materials
  2. flue liners
  3. resistance to frost attack
  4. resistance to weather.

Flues for solid fuel appliances should:

  • not serve more than one appliance
  • be of a sufficient cross-section to remove all combustion gases from the open fire or appliance they serve
  • where possible, be vertical (where this cannot be achieved there should not be more than two bends; bends should not be more than 45° from vertical)
  • be a minimum of 4.5m high (measured above the fireplace opening).

Where a chimney is not directly over an appliance or opening, an accessible soot box should be formed.

Factory-made insulated chimneys should:

  • be designed in accordance with BS EN 1856 and BS EN 1859
  • have a minimum operating life of 30 years
  • be installed in accordance with BS EN 15287 or be assessed in accordance with Technical Requirement R3.

Separation from adjacent spaces and materials

Combustible materials close to any brickwork or blockwork chimney (not applicable to floorboards, skirting, dado or picture rails, mantelshelves or architraves) should be:

  • a minimum of 200mm from the inside surface of the flue, or
  • in all areas except Scotland, 40mm from the face of the chimney.

Where the home is of timber frame construction, full details of the separation proposal should be included in the design.

Materials used for chimneys should be capable of resisting fluctuating temperatures up to 1100°C.

Flues should be formed within masonry walls. The walls should be:

  • a minimum of 100mm thick, or
  • a minimum of 200mm thick where separating the flue from another compartment of the same building, another building or another home.

Where there is more than one flue in a chimney, the flues should be separated by a minimum of 100mm of masonry.

Flue liners

Flue liners should:

  • have rebated or socketed joints installed with the socket or internal rebate facing uppermost
  • be installed in accordance with the manufacturer’s recommendations
  • be non-combustible
  • be properly jointed at their junctions with the starter block, or lintel, and the outlet terminal
  • be reasonably smooth on the inside
  • be correctly jointed with mortar (the space between the liners and the brickwork should be filled with weak insulating concrete unless the manufacturer recommends an alternative)
  • have any changes in direction formed using purpose-made bends (cut pipes are not acceptable).

Resistance to frost attack

Where clay brick chimneys are above roof level and are not protected by a capping with an adequate overhang and drip (see Clause 6.8.7c), the chimney should be constructed using F2,S1 or F2,S2 bricks to BS EN 771. They should be bedded in mortar, either:

  • 1:½:4 to 4½, cement:lime:sand, or
  • 1:3 or 4, cement:sand with plasticiser.

Where external chimneys built with clay bricks of F2,S1 designation are rendered, sulfate-resistant cement should be used.

In Scotland, external facing brickwork should be constructed using frost-resistant bricks.

Resistance to weather

In areas of severe or very severe exposure, cavities should be continuous up to roof level. This applies to:

  • cavities below roof level where the stack forms part of an external cavity wall
  • the complete chimney structure, including the fireplace recess.

In areas of severe or very severe exposure, and where the chimney breast is gathered in, the lower projecting masonry should be protected against damp penetration with a suitable capping and cavity trays (see Clause 6.8.28b).

Above the roof:

  • chimney DPCs should link with flashings; where the roof is steeply pitched (where the difference in level between the lower and higher intersection of the chimney with the roof will be more than 450mm) two DPCs should be used at suitable levels
  • plastic DPCs are not suitable
  • face brickwork should not have recessed joints
  • where lead trays are in contact with mortar, they should be protected with a thick coat of bitumen or bitumen paint
  • where chimneys are to be rendered, render should be in accordance with Chapter 6.11 ‘Render’.