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The use of flexible cavity trays within external masonry walls is common place. A variety of materials are available for this purpose, and polyethylene has often been considered as a widely available and relatively economical option. In recent years, poor installation of cavity trays using flexible cavity tray systems has been an area of claims. Particularly where these products are polyethylene materials, often culminating in water ingress and instability in parapet walls and coping stones due to incorrect detailing and laying being the main contributing factors.
Where flexible DPC materials are to be used as a cavity tray, they must:
Provided as a roll material, most building merchants will stock 100mm, 450mm and 600mm complete with product branding stating a compliance with BS6515:1984 – the latter roll sizes are stocked specifically for cavity tray purposes.
It is a British Standard that relates to the manufacture of polyethylene damp proof courses, intended for use in masonry construction. It covers material composition, thickness, finish and permeability of the material.
BS 6515:1984 contains an Appendix giving the recommended uses for polyethylene DPCs, and notably recommends that these products are not used as a solution against the downward movement of water e.g. cavity trays.
More recent standards, such as BS8215:1991 and PD6697:2019 (a published document sitting alongside BS EN 1996-1-1), offered a variation – in that such materials could be used, provided they were installed, detailed and sealed correctly in line with manufacturers guidance. However, the issue is that polyethylene materials are often produced without any manufacturers' supporting guidance nor do they undergo testing as cavity tray installations, as they are only intended to be used for 'simple' damp proofing situations - so installed to deal with:
A) Upward movement of water e.g. linear DPC at the base of a wall and;
B) Horizontal movement of water e.g. vertical applications at window and door reveals
BS EN 14909:2012 stipulates the characteristics required of flexible sheet damp proof courses for buildings. It details the requirements and test methods to determine conformity of the products.
Any flexible material that is being used as a cavity tray should have supporting evidence in the form of a Declaration of Performance to BS EN 14909:2012. They should also have third-party certification (BBA or similar UKAS accredited body) confirming their suitability for use as a cavity tray.
A material that carries 3rd Party Accreditation will often have supporting ancillaries e.g. pre-formed corner 'cloaks' and adhesive tapes for use as a cavity tray.
Where the material is polyethylene, meeting the requirements of BS 6515 alone is not enough to support their use as a cavity tray.
Manufacturers of flexible DPC materials that can be used for cavity trays will have a multitude of guidance regarding detailing, sealing of joints and installation.
The manufacturer’s literature should explicitly state the suitability of their product for use as a cavity tray and their recommendations for appropriate use and installation should be followed. The advice of the manufacturer will typically be aligned with the guidance of:
A cavity tray that is not laid on mortar, e.g. dry laid, can result in movement in the wall where the mortar bond is not created. It can also allow moisture to track under the material, and where this is above an opening, often lead to visible moisture ingress and damp patches on reveals and wall areas below the cavity tray.
Also look out for laps that are not sealed, poor detailing around corners, wind post penetrations leaving open joints or cavity trays that do not terminate in a stopped end. Ideally a pre-formed stop end but more likely to be a perp joint 'turn-in' of the material should be used to terminate a discontinuous cavity tray installation.
Where flexible, DPC materials are to be used as a cavity tray. They should have supporting evidence in the form of a Declaration of Performance to BS EN 14909:2012. They should also have third-party certification (BBA or similar UKAS accredited body) confirming their suitability for use as a cavity tray.
The Declaration of Performance information should include reference to BS EN 14909:2012 and include one of the 'Product Designation Codes' given within that standard to enable clear determination of the material being used on site.
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Every care was taken to ensure the information in this article was correct at the time of publication (January 2024). Guidance provided does not replace the reader’s professional judgement and any construction project should comply with the relevant Building Regulations or applicable technical standards. For the most up to date Premier Guarantee technical guidance please refer to your Risk Management Surveyor and the latest version of the Premier Guarantee Technical Manual.
Ref: TS-2875