Champions of best practice: slip plane resistance

John Duffin, Managing Director, Keyfix explains:

Brickwork contractors, architects and asset owners are no strangers to the hazards of creating a slip plane when introducing components in between courses of bricks. Slip plane resistance is a challenge for all buildings, and each construction team deals with it in their own way. However, it’s worth questioning whether the most common ways of dealing with slip plane are the best. As champions of best practice, Keyfix believes that the slip plane resistance solution used should deal with the problem at its source, without creating further issues, to result in a safer and more cost-effective building.

The creation of a slip plane between two brick courses is notorious for creating issues that plague construction.  Inserting any element including a cavity tray breaks the adhesion between brick courses, creating a slip plane. This enables differential movement which can create cracks in a building. The cracks are at best unsightly, and at worst, structurally inefficient.

Moreover, another pervasive issue is rising damp which occurs when groundwater rises through the capillaries in masonry to saturate it. This is an extremely difficult type of damp to treat, and when left untreated, rising damp can cause aesthetic and even extreme damage to the structure of a building. Damp in a building can lead to mould internally and algae growth externally, the former causing occupants to suffer from respiratory problems, respiratory infections, allergies, or asthma. The latter can result in unsightly green deposits on the brickwork façade.

Cavity trays are designed to divert water within the cavity out through weep holes in the external skin of the building. At ground level cavity trays can be used in masonry walls to also prevent moisture from travelling upward from the ground into the fabric of the building. 

However, the majority of non-combustible cavity tray products on the market have an inherent design flaw, particularly in how they try to avoid the creation of a slip plane. To do this some manufacturers create a cavity tray with holes punched into the horizontal flange to enable mortar to key through the tray in an attempt to restore bond. However, perforations in the horizontal section of a cavity tray act as a passage for moisture to travel up through the building. Therefore, holes in the cavity tray create a significant problem and the battle with rising damp begins. This problem is not only associated with trays installed at ground level, but the green algae disfiguration on the finished brickwork can occur in and around inset balconies at every level of the building.

The Keyfix Non-Combustible Cavity Tray is completely unique from other products in the way  it resists slip plane. The lower horizontal ‘flange’ of the NCCT incorporates a patented indented surface, providing a physical key within the mortar bed without having any holes punched in it, which resists the passage of rising damp.

This ingenious design is based on technology developed at Keyfix’s sister company, Keystone Lintels. For 30 years, Keystone Lintels has indented the internal surface of a lintel to form a plaster key, therefore ensuring that the indented surface of a tray facilitates a strong bond within the mortar bed. 

Evidently, the design of the Keyfix NCCT provides the ultimate solution as it eliminates the risk of slip plane developing within the masonry outer skin without introducing the issue of rising damp. As a champion of best practice, Keyfix believes that its design effectively deals with the slip plane at its source whilst ensuring that the building is protected against the issues of rising damp and structural movement in the safest way possible. Based on a design that has enjoyed decades of success, the Keyfix NCCT is a highly reliable and safe product that architects can specify, and bricklayers can use with a clear conscience, knowing they are adopting best practice.

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