16/11/2022 by am

Defects in chimney stacks

Most commonly encountered defects in chimney stacks

a)Lead flashings loose or poorly applied:

  • Strong mortars with high cement content likely to crack and shrink.
  • Poor workmanship.
  • Lime mortar deterioration.

b)Flaunching crumbling:

  • Sulphate attack.
  • Moisture interchange between brick and mortar.
  • Frost attack.
  • Shrinking and cracking of the cement.
  • Lime mortar deterioration.

c)Unsafe height:

  • Tops should be 1 m or more above the highest point of any window, skylight
  • Ventilation duct or adjoining building/wall, which is not more than 2.3 m horizontally from the top of chimney.
  • The height to thickness ratio of the stack correct? Chimney height (excluding pots ) from lowest point of interception with roof should not exceed 4.5 times the narrowest width.

d)Unsafe pots:

     ● Chimney pots should be sunk into the stack not less than 150mm or one quarter the length of the pot, and flaunched in cement and sand to throw water off the stack.

e)No flashings:

    ● Dampness penetrating through to rafters.

    ● A watertight joint must be provided around the chimney where the roof covering adjoins the brickwork. A gutter and flashing are used to form at top and front edge an apron flashing and soakers and stepped flashings at side

f)Sulphate attack:

Requires a number of simultaneous conditions

  • Water saturation over a long period (including condensation within chimney).
  • Source of sulphate (brick, ground, air or exhaust gases from fires)
  • Presence of tricalcium aluminate (found in Portland cement)

g)Inadequate support for TV antenna:

  • Poor workmanship/fixing
  • Corrosion/Oxidisation of metal fixing components
  • Foundation movement affecting stability of whole building
  • Spalling due to incorrect mortar mix
  • Defective bricks or weak stones incorporated
  • Sulphate attack (see above)
  • Excessive load from antenna/e (ie too heavy)
  • Height to thickness ratio of less than 4.5 is incorrect – chimney inherently weak
  • Chimney stack does not terminate high enough above roof to avoid turbulent air (0.6m minimum) 
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