It is estimated that there are over 27 million homes in the UK, of which nearly 20 million are of cavity wall construction. As an industry of property professionals we will be looking at properties with cavity wall construction on a daily basis. Following the recent webinar on Defects in Cavity Wall Construction, here are my simple tips for surveying cavity walls.
Tip 1 – Is it a Cavity Wall?
Might sound obvious and if you are looking at a stretcher bond then the answer will probably be fairly straightforward, but headers do not always mean solid. Early cavity walls, often called ‘hollow walls’, were more often than not held together by bricks rather than more conventional metal ties. The image below from Gwilts Encyclopaedia of Architecture 1899, shows a section through this style of construction.
London bylaws also encouraged the building of a normal 9” solid wall but a thin 4.5” second leaf built internally. The moral of the story here is, do not just go on a visual appraisal, especially for properties between 1890 – 1930s, but check the thickness of a wall using a trusty tape measure!
Tip 2 – Check inside the cavity
Whether intentionally or not, builders have a tendency to leave debris in a cavity. When inspecting dampness at the base of a cavity wall, the cavity should always be inspected. When a damp proof course is being bridged within a cavity, the best long term solution will always be to remove the debris causing the bridge.
Tip 3 – Nothing lasts forever
Wall ties are essential for ensuring the structural stability of a cavity wall, however due to the nature of ties they can be subject to corrosion. BRE guidance states “in the long term, all properties with galvanised ties will probably need to be repaired”. The life expectancy of wall ties will vary greatly depending on a number of factors including the location, and type of tie. However, it is worth bearing in mind that between 1964 and 1981 standards of galvanization for wall ties was reduced, so special consideration should always be given to properties around this age.
Tip 4 – Cavity Tray Best Practice
Trays are often given very little thought, but they are vital to controlling dampness within cavities. Dampness around openings such as a door and windows can often be a result of poor detailing around cavity trays. Below is LABC’s Cavity Tray Best Practice list:
- Cavity trays should be installed over all external door and window openings including bay windows and at roof abutments, both horizontal and pitched
- The cavity tray should be correctly located directly over the window/door head
- Purpose-made stepped cavity trays are best utilised for all pitched roof abutments
- Where natural stone/artificial stone heads are being used. It is advisable to double up the cavity trays, one below and one above the head and ensure that the cavity insulation continues to cover behind the stone head
- Ensure weep holes are provided at each end of a horizontal cavity tray and at maximum 900mm centres
- Ensure a weep hole is provided at the base of stepped cavity trays
- Ensure the cavity trays are kept free of mortar droppings
Tip 5 – Cavity Wall Insulation
The climate emergency is well publicised and if we are to rise to the challenge and become carbon neutral, changes to our existing housing stock is vital. At present Cavity Wall insulation provides one of the most cost effective methods of improving the carbon efficiency of a home. In fact, it is estimated that it has been installed into over 13 million homes. However, the insulation fundamentally changes the way that the wall behaves. The tell-tale signs of retrofit insulation are normally abundantly clear during the course of the survey and must be considered by the surveyor.
One-day Cavity Wall Training Course
In a world where we are seeing growing concern around defects in cavity walls and a rise in companies looking to exploit these growing concerns, it is now essential that all property professionals that inspect buildings understand these defects. That is why the PCA has developed a one-day training workshop that casts a spotlight on understanding cavity construction.
Find out more about the ‘Defects in Cavity Construction’ course by clicking the button below:
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