Calculating Wall Thickness During Building Part 2

Following on from Part 1 of our wall thickness sequence, this article looks at variables that can affect the stability of cavity walls.

For cavity walls in coursed brickwork or blockwork:

What is a cavity wall?

  • A cavity wall is where the wall is composed of two ‘leafs’; an external and an internal wall separating a cavity.
  • The cavity may be filled with insulation or used as a drainage structure. Rainwater and humidity are slowly drawn through walls over time, and the cavity allows this fluid to flow out of the house preventing damp and preserving the life span of the building.
  • Even without an insulating material placed in the cavity, a layer of air between walls also provides insulation to the building, as the air provides a thermal break between the two leafs. This thermal break is caused by the halt of heat energy flowing by conduction out of the building.
  • The two leafs of a cavity wall are held together by internal structures known as ties.
  • It is essential to always contract a reputable builder for your cavity walls as an improper or incomplete installation may result in more harm than the cavity wall is worth.
  • Although the structure can be used to drain water that seeps through masonry, if a blockage develops or is present through careless installation, damp and fungus formation may result, leading to structural building and health threats.

What are the minimum dimensions of a cavity wall to ensure a safe and stable structure?

  • Cavity walls must have a horizontal spacing of 0.9 metres and a vertical spacing of 0.45 metres
  • Wall ties must have a horizontal spacing of 0.9 metres and a vertical spacing of 0.45 metres. Spacing between individual ties must be a maximum of 0.3 metres vertically and within 0.225 metres from all vertical edges of openings, movement joints and roof verges.
  • A roof verge is the edge of the gable wall, which is the triangular portion of a buildings wall between the 2 sides of a sloped roof.
  • If the cavity wall is external (which is most likely given the reasons for constructing a cavity wall), the combined thickness of the two wall leaves plus 10 mm should be a minimum of the thicknesses listed for solid external walls in part 1 of this article series.

BPM Maintenance are builders based in the city of Bath, covering Bath, Bristol and the surrounding areas with maintenance and building needs including wall repairs, replacement and roofing. The company follows all government building regulations to the letter.


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