Chimneys and Flues

chimneys and fluesFlues and chimneys are ducts in and on buildings, which are intended exclusively to convey the gases from fireplaces to the outside over the roof. The following should be connected to a flue: fireplaces with a nominal heat output of more than 20 kW; gas fireplaces with more than 20 kW; every fireplace in buildings with more than five full storeys; every open fire and forge fire; fireplaces with a means of opening and every fireplace with a burner and fan.

Provision should be made in the foundation plans to support the weight to the fireplace, flue and chimney.

modular flueFlue must have circular or rectangular internal cross-sections ⟶⑦⑧⑨⑩. The cross-section must be ≥ 100 cm², with a shortest side of 100 mm. Brick flues must have a shortest internal side of length ≥ 135 mm, the longer side must not exceed 1,5 times the length of the shorter. The shortest effective flue height ≥ 4 m; for gaseous fuels ≥ 4 m.

The mouth of the chimney should be ≥ 400 mm above the apex of the roof, where the roof slope is greater than 20° and for roof slopes less than 20° this dimension is ≥ 1 m ⟶⑥.

chimney heights above the roofWhere chimneys are closer to structures on the roof than between 1,5 and 3 times the height of the structure, it must be ensured that they clear the structure by at least 1 m.

Where the mouth of a chimney is above a roof which has a parapet which is not closed on all four sides, it must be at least 1 m above the parapet.

Every flue must have a ≥ 100 mm wide by ≥ 180 mm high cleaning opening which is at least 200 mm lower the lowest fireplace connection.

Chimneys which cannot be cleaned from the mouth opening, must have an additional cleaning opening in the flue in the roof space or in the chimney above the roof.

The following material may be used for single skin flues: light concrete blocks, clay bricks, lime sandstone – solid bricks, foundry bricks.

Material for treble-skinned chimneys, with outer casing, insulation layer and moveable inner lining can be formed components in light concrete or fire-clay for the inner lining; for the outer casing, formed components in light concrete, masonry stone, bricks with vertical perforations, lime sandstone, foundry bricks, or aerated concrete blocks.

For the insulating layer, non-combustible insulating material must be used. Exposed outer surfaces of the chimney in the roof space should be provided with a rough cast finish of at least 5 – 10 mm thickness.

Flue walls must not be load-bearing. The chimney can be clad with slates, shingle slates or cement fiber sheets. Zinc or copper sheet can be fixed to the chimney on the sub-structure using dowels (not wooden dowels). Prefabricated cladding are recommended.


This “Chimneys and Flues” is taken from the book Architects’ Data written by Ernst Neufert, whereas this book has become the guideline for architecture college students in the world.

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