The tie in a cavity wall is a component used to tie the internal and external walls (or leaves)—constructed of bricks or cement blocks—together, making the two parts to act as a homogeneous unit. It is placed in the cavity wall during construction and spans the cavity. The ends of the tie are designed to lock into the mortar. Also incorporated into the design of the tie is means of preventing water transfer from the outer to the inner leaves. In flat ties, this can be a twist. In wire ties, this can be corrugations formed in the wire or again a twist. Background Cavity walls often have insulation in the cavity which may either partially or fully fill the cavity. Partial fill insulation systems require specialized ties or clips to keep the insulation in position. A vapour barrier may be necessary on the inner wall to prevent interstitial condensation. This is often incorporated into the cavity wall insulation system. The spacing of ties is laid down in building regulations, though there may be variations with specialised blocks. Additional ties are used around window and door openings. Improper installation may lead to water damage or fungus formation within the cavity, leading to structural and health hazards. Ties are exposed to water and chemical attack from cement. They were traditionally made of galvanized steel, the fishtail tie being the most common. On high quality work, ties were occasionally made of bronze. In the mid-twentieth century, wire ties were widely used, again made from galvanized steel wire. As time has passed, many galvanized steel ties have deteriorated due to moisture in the outer leaf of brickwork. The corrosion may force apart the cement joints and even result in the collapse of walls if no remedial action is taken. Any cracks appearing in cavity walls dating from the twentieth century need to be investigated before irremediable damage ensues. Horizontal cracking is especially suspect. Failed ties have to be isolated and substitute specialist ties installed by drilling through inner and outer leaves from outside the building. The replacement ties may be fixed mechanically or with special adhesives. Galvanized steel ties are no longer in use for this reason. For a brief period, plastic ties were used but were not satisfactory. Modern practice is to use stainless steel ties. Cavity walls were traditionally spaced 2"(50mm) apart. Due to the need for thicker insulation in exterior