Building damp-free cavity walls

BRE (Building Research Establishment) is an independent, research-based consultancy, testing and training organisation, operating in the built environment and associated industries.

Building damp-free cavity walls (GG 33 revised) was written by Barry Reeves and Chris Scivyer and published by BRE on 18 March 2015.

In driving rain, water leaks through the outer leaf of most cavity walls, often in quite large quantities. Provided the damp-proofing measures are correctly designed and installed and the wall itself is reasonably free from defects, this water does not reach the inner leaf, but flows harmlessly down the cavity face of the outer leaf until it reaches the footings or is directed out of the cavity via cavity trays or window/door lintels.

However, even though many cavity wall details are long-established and designed with the express purpose that the inner leaf stays dry, dampness is still a common problem. This is generally due to faulty design or construction of damp-proofing measures or the wrong choice of materials.

This 6-page Good Building Guide replaces guidance published in 1999, and offers advice about how to ensure new cavity walls do not suffer from damp. It is intended for architects and other designers, engineers, site managers, house builders and masonry contractors.

Its contents are:

• Introduction.
• Defects in the outer leaf.
• Problems caused by wall ties.
• Defects linked to cavity insulation.
• Where damp-proof courses and cavity trays are needed.
• Installing damp-proof courses.
• Installing cavity trays.
• Weepholes.
• Conclusion.
• References.
• Further reading.

--BRE Group

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