walls these days, a range of longer ties are now available so that cavities of up to 6"(150mm) can be constructed. Types of ties Ties in a cavity wall are typically made of iron, steel, or plastic; though figures are various. Basically, a tie has ring to fasten with mortar on both end like a bow. Ties would be selected by type of masonry, the cavity width, and so on. Typically, ties in a cavity wall are double triangular shape (like a bow), though, depending on the existence of another layer (e.g. insulation sheet) at the cavity, which fills the cavity partially, ties include retaining discs or rods. Tie failure Failure of ties is an increasing problem with cavity wall ties made from galvanized steel. It arises when the galvanizing is not of sufficient quality and the outer leaf of the cavity wall allows water penetration, usually due to porous brick/blockwork. If the tie rusts, the swelling effect may cause horizontal, external cracks to appear in the wall. Frost action can swiftly enlarge these cracks to cause damage. Tie corrosion Since ties in a cavity wall are typically made of metal (iron or steel), they are prone to corrode over time. When metal corrodes, it expands its size, causing ties to lift up from the brickwork. Cracks caused by vertical loads leave parts of buildings vulnerable to corrosion, such as eaves and gable walls above purlin positions, or placed directly beneath openings, where the weight on brickwork is light. Over the time, cracks appear from the top of the wall and extend downward. Cracks due to tie corrosion at the cavity wall are horizontal and tend to occur at the location of wall ties, normally 6 courses apart. The corrosion causes the walls to detach and tilt, resulting in the outer wall snapping outward. The inner wall is held in place by the added support of floor joists. At the gable wall, inner walls without these supporting joists, bow inwards. By avoiding several factors accelerating corrosion, it will last more than 60 years. Acceleration factors of tie corrosion Due to its materiality of the tie, its corrosion is related to the extent of the exposure at acidic substances. The major factors of the corrosion are: Acidic black ash mortar Exposure (coastal or general) Coastal areas (salt in the atmosphere) Industrial areas (acid rain) Cavity wall insulation Cement-based mortar, giving an