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BLOG: What do I need to bear in mind when specifying a cavity tray?

With the UK set to face even more rainfall in the coming years, cavity trays are becoming an increasingly vital part of modern masonry facade construction. Whilst, at first glance, these seem like an easy-to-choose product, they are not a ‘one size fits all’ component. Issues such as cavity size, building movement, and combustibility could all impact the choice of product and ease of installation. Let’s take a closer look at this often-under-considered product and the requirements it must meet (as well as at a solution that could help you tick all the boxes!) 

What is a cavity tray? 

Cavity trays perform the job of a damp proof course. They are angled to bridge the cavity in a way that directs moisture away from the inner leaf and enables it to drain externally. This helps to prevent lingering moisture, making sure that any rain that gets through to the cavity does not reach the inner leaf. This reduces the risk of damp which can cause serious problems for buildings and occupants alike. They can be pre-manufactured from a wide range of materials, from heavy plastic to robust stainless steel and aluminium, or foil faced non-combustible insulation. 

For these products to be effective, it is vital to understand what the installation requirements are, and what other considerations there are, such as fire performance, when integrating them with other products, for example cavity barriers and fire stops. 

When and where should a cavity tray be installed? 

For solid masonry walls, Approved Document C (ADC) to the Building Regulations 2010 – Site preparations and resistance to contaminants and moisture – states that cavity trays should be provided and designed to ensure that water should drain outwards in three different scenarios: 
 

  1. Where the downward flow will be interrupted by an obstruction, such as some types of lintel; and 
  1. Under openings unless there is a sill and the sill and its joints will form a complete barrier; and 
  1. At abutments between walls and roofs. 

 
Although ADC states that this guidance is for solid walls, the same principles apply for cavity walls at any point where the cavity between the external and internal leaf may be bridged, for example by cavity barriers, firestops and cavity closures. 

ADC refers to BS 5628-3:2001 for more detailed guidance on alternative approaches to ensuring that precipitation is not carried to the inner leaf. However, this standard has been superseded by the 2005 version, which provides more information on the required configuration for cavity trays, stipulating that they “should step down or slope across the cavity not less than 150mm towards the external leaf and, preferably, terminate in a small drip on the face of the wall”. 

The Approved Document also provides a detailed map of UK zones for exposure to driving rain (Diagram 12 on page 34), and the minimum cavity widths required to deal with the different levels of exposure for various types of insulated masonry wall construction. These range from 50 to 150mm, indicating the scale of different widths that cavity trays may need to cover, and the need for specifiers to take prevailing weather conditions into account early in the design process. 

How can I ensure effective installation? 

As with any detailing, not only is it important to get the design right, but it is also crucial that cavity trays are installed correctly, or else they will not be effective and could even contribute to damp building up within the cavity. Section 6 of the LABC Warranty Technical Manual Version 9 offers detailed guidance on both the design and the installation aspects of cavity trays in 5 different types of external wall construction. The Manual also highlights the fact that cavity trays should be provided over cavity barriers and covers the issue of building movement, which could potentially create problems if cavity trays are unable to accommodate a normal range of movement without loss of integrity. 

What fire performance do cavity trays need to achieve? 

Under current regulations, for high rise residential and other relevant buildings over 18 metres, cavity trays must achieve an A1 or A2-s1,d0 Euroclass rating, unless they are between two leaves of masonry (Approved Document B, regulation 7, paragraph 3a). This means that any cavity trays being installed between a SFS inner leaf and masonry outer leaf must be classified as non-combustible to be compliant. 

How can the Siderise Non-Combustible Cavity Tray (NC-CT) help to meet these requirements? 

The Siderise NC-CT has been created specifically to provide a comprehensive, compliant solution for masonry facades. Unlike similar products on the market, it comprises a flexible aluminium cavity tray with integral non-combustible insulation, creating a single-component solution that is simple to specify and install. This robust patented and factory-engineered design not only provides reliable quality and performance but has been tested to rigorous standards such as EN 13501-1 Fire classification of construction products and building elements, achieving a non-combustible A2, s1 d0 rating. 

Siderise Non-Combustible Cavity Tray

Learn more about this product and the standards it has been tested to or discover the full range of products we have created specifically for masonry facades, including fire stops, cavity barriers and balcony brackets. https://www.siderise.com/masonry