NHBC Standards 2006

6.8 Fireplaces, chimneys and flues

MATERIALS STANDARDS

6.8 - M1Materials standards
6.8 - M2Bricks
6.8 - M3Concrete blocks
6.8 - M4Stone masonry
6.8 - M5Mortar
6.8 - M6Flue liners
6.8 - M7Flue pipes
6.8 - M8Flue terminals
6.8 - M9Flue blocks for gas appliances
6.8 - M10Twin wall flue systems
6.8 - M11Flashings
6.8 - M12Damp-proof course

6.8 - M1
All materials shall:
(a) meet the Technical Requirements
(b) take account of the design
 

Materials that comply with the design and the guidance below will be acceptable for fireplaces, chimneys and flues.

Materials for fireplaces, chimneys and flues shall comply with all relevant standards, including those listed below. Where no standard exists, Technical Requirement R3 applies (see Chapter 1.1 'Introduction to the Standards and Technical Requirements').

References to British Standards and Codes of Practice include those made under the Construction Products Directive (89/106/EEC) and, in particular, appropriate European Technical Specifications approved by a European Committee for Standardisation (CEN).

BRICKS

6.8 - M2
Bricks shall be capable of supporting intended loads and have appropriate resistance to the adverse effects of frost and sulfates
 

Bricks should be selected in accordance with BS 6461 and BS EN 771.

In external chimney stacks clay bricks should be of durability rating F2,S1 as described in BS EN 771 unless protected by a projecting capping bedded on a dpc. In Scotland frost resistant bricks should be used for all external facing brickwork.

Reference should also be made to Chapters 6.1 'External masonry walls' (Materials) and 6.3 'Internal walls' (Materials).

CONCRETE BLOCKS

6.8 - M3
Blocks shall be capable of supporting intended loads and have appropriate resistance to the adverse effects of frost and sulfates
 

Blocks should be selected in accordance with BS 6461 Part 1, BS EN 771 or satisfactorily assessed in accordance with Technical Requirement R3. Block density should be minimum 1500 kg/m3, unless designed by an Engineer in accordance with Technical Requirement R5.

Reference should also be made to Chapters 6.1 'External masonry walls' (Materials) and 6.3 'Internal walls' (Materials).

In Scotland, 100mm thick blockwork in chimney construction should have a density of at least 1600 kg/m3. Other suitable masonry specifications for Scotland are given in the Building Standards (Scotland) Regulations.

STONE MASONRY

6.8 - M4
Stone masonry shall be capable of supporting intended loads and have appropriate resistance to the adverse effects of frost and sulfates
 

Stone for masonry should comply with the requirements of BS EN 771 and BS 6461 : Part 1.

Reconstructed stone masonry units should comply with BS EN 771-5 and BS 6461 : Part 1.

MORTAR

6.8 - M5
Mortar shall be batched and mixed to achieve adequate strength and durability to comply with design
 

Mortar should be as specified for areas of Severe or Very Severe exposure in Chapter 6.1 'External masonry walls' Appendix 6.1-B. (1 : ½ : 4 to 4½ cement : lime : sand or 1 : 3 or 4 cement : sand with plasticiser).

Sulfate resisting cement should be used in the mortar where flue gases are liable to affect the masonry, eg above roof level.

FLUE LINERS

6.8 - M6
Flue liners shall be unaffected by flue gases and suitable for their purpose
 

Special fittings should be specified at changes of direction of the flue.

Liners suitable for solid fuel appliances (and generally suitable for other fuels) could be:

  • liners whose performance is at least equal to that corresponding to the designation T450 N2 S D 3, as described in BS EN 1443: 1999, such as:
    • clay flue liners with rebates or sockets for jointing meeting the requirements for Class A1 N2 or Class A1 N1 as described in BS EN 1457: 1999; or
    • concrete flue liners meeting the requirements for the classification Type A1, Type A2, Type B1 or Type B2 as described in prEN 1857(e18) January 2001; or
    • other products that are independently certified as meeting the criteria in a); or
  • imperforate clay pipes with sockets for jointing as described in BS 65: 1991 (1997).

    Liners should be installed in accordance with their manufacturer's instructions. Appropriate components should be selected to produce the flue path without cutting and to keep joints to a minimum. Bends and offsets should only be formed with purpose-made components. Liners need to be placed with the sockets or rebate ends uppermost to contain water and other condensates in the flue. Caulking can be effected with fire cement or refractory mortar. Spaces between the lining and the surrounding masonry should not be filled with ordinary mortar. In the absence of liner manufacturer's instructions, the space could be filled with a weak insulating concrete such as mixtures of:
    • one part ordinary Portland cement to 20 parts suitable lightweight expanded clay aggregate, minimally wetted; or
    • one part ordinary Portland cement to 6 parts Vermiculite; or
    • one part ordinary Portland cement to 10 parts Perlite.

FLUE PIPES

6.8 - M7
Flue pipes shall be suitable for their purpose
 

Cast iron flue pipes should comply with BS 41.

Mild steel flue pipes should have a minimum wall thickness of 3mm and comply with BS 1449.

Stainless steel flue pipes should be at least 1mm thick as described in BS EN 10088 and should be one of the following grades: 1.4401, 1.4404, 1.4432 or 1.4436.

Vitreous enamelled flue pipes should be of low carbon steel coated internally and externally with acid resisting enamel and conform to the description given in BS 6999.

Flue pipes for gas appliances should comply with BS 715.

All flue pipes should be jointed in accordance with manufacturers' instructions.

FLUE TERMINALS

6.8 - M8
Flue terminals shall be suitable for their purpose
 

Clay flue terminals should comply with or be constructed from materials which comply with BS 1181.

Chimneys serving gas appliances should have terminals complying with BS EN 1858. Flue pipes serving gas appliances should have terminals complying with BS 715.

FLUE BLOCKS FOR GAS APPLIANCES

6.8 - M9
Flue blocks for gas appliances shall be unaffected by flue gases and suitable for their purpose
 

Flue blocks for use with gas appliances should comply with BS 1289 : Part 1 (Concrete) or Part 2 (Clay). In situations where the cavity width is reduced by the flue block all insulation and vertical dpms should be in accordance with manufacturers' instructions.

TWIN WALL FLUE SYSTEMS

6.8 - M10
Twin wall flue systems shall be suitable for their purpose
 

Twin wall flue systems should comply with BS 715 or be assessed in accordance with Technical Requirement R3.

FLASHINGS

6.8 - M11
Flashings and trays shall be capable of adequately resisting the entry of moisture into the building
 

Suitable materials for flashings and trays are:

  • milled sheet lead (at least Code 4) complying with BS EN 12588
  • aluminium and aluminium alloys complying with BS 1470 (0.6mm to 0.9mm thick)
  • zinc alloy complying with BS 6561 and 0.6mm thick.

DAMP-PROOF COURSE

6.8 - M12
Materials for damp-proofing shall resist adequately the passage of moisture into the building
 

The following are acceptable for use as dpcs:

  • bitumen to BS 6398
  • polyethylene to BS 6515 (not in the chimney stack above roof level)
  • proprietary materials assessed in accordance with Technical Requirement R3.