Chimney construction †
Typical chimney outlet
Clay bricks † Frost and thaw, severe exposure resistant quality.
Min. density 1500 kg/m^3.
Calcium silicate bricks † Min. compressive strength 20.5 N/mm^2
(27.5 N/mm^2 for cappings).
Precast concrete masonry units † Min. compressive strength 15 N/mm^2.
Mortar † A relatively strong mix of cement and sand 1:3. Cement
to be specified as sulphate resisting because of the presence of
soluble sulphates in the flue gas condensation.
Chimney pot † The pot should be firmly bedded in at least 3
courses of brickwork to prevent it being dislodged in high winds.
Flashings and dpcs † Essential to prevent water which has
permeated the chimney, penetrating into the building. The minimum
specification is 1.75 mm lead, 1.25 mm for soakers. This should be
coated both sides with a solvent-based bituminous paint to
prevent the risk of corrosion when in contact with cement. The
lower dpc may be in the form of a tray with edges turned up
25 mm, except where it coincides with bedded flashings such as the
front apron upper level. Here weep holes in the perpends will
encourage water to drain. The inside of the tray is taken through a
flue lining joint and turned up 25 mm.
Open Fireplaces and Flues