An introduction to British Standards
When selecting which wall tie to use in a given condition, a number of factors must be considered e.g. cavity size, building type, location and height.
In order to determine the correct wall tie to use, guidance must be sought from several statutory documents. Generally, the purpose of the British (and now European) standards is to ensure a common minimum standard to which all products and services must adhere.
The “old” standard which covers the manufacture of figure 1 (butterfly) ties, figure 2 (double triangle) and figure 3 (fishtail vertical twist) ties.
This standard was concerned with consistency of manufacture for these products, the standard itself goes into no detail regarding the performance or application of the products.
B.S 5628 pts 1, 2 & 3
This standard (which was an updated version of CP110 – code of practice), refers to B.S 1243 and goes some way towards clarifying applications for the three different types of tie, in particular regarding their use in different cavity widths. Also advises on density of ties per square metre and number of ties around openings. The standard also discusses movement joints, their position and construction.
Although only a draft for development (D.D) this standard has generally been adopted as an industry standard for wall tie designs and applications.
The document is a performance standard, i.e. it is unconcerned with the appearance of the tie, but is more aimed at indicating minimum tensile and compressive resistances for the products.
The document grades ties from type 1 (heavy duty) to type 6 (light duty) timber frame ties.
Type 2 being a general purpose tie for domestic and small scale commercial developments, and type 4 being for housing. Since the purpose of a wall tie is to assist the cavity wall in resisting wind load, the document is also concerned with indicating areas of the country with differing average wind speeds which, along with height and building type, affect choice of wall tie.
Density and Positioning of Ties
This varies according to cavity width, generally vertical spacings are 450mm, horizontal spacing is 900mm (up to a 75mm cavity) or 750mm (above 75mm). For estimating purposes the use of 5N° wall ties per square metre is appropriate. In addition wall ties spaced not more than 300mm vertically apart should be provided within 225mm of unbonded openings.
Length and Embedment of Ties
Whilst the British Standards suggest a minimum embedment of 50mm, in order to allow for site variation and reasonable working tolerance, Vista Engineering would suggest a minimum of 63-75mm.
The bulk of Vista Engineering’s product range is produced in Grade 304, 18/8 austenitic stainless steel. Galvanised material (with the exception of internal fixings) has lost favour due to poor anti-corrosion performance.
There is a standard of stainless steel known as 316 (or marine grade) whose corrosion resistance is higher than 304,for information on products in this range please consult Vista Engineering Sales or Technical departments.
Strip products conform to B.S.EN 10099-2.1.4301, wire products to B.S. 1554 1990.