Since the early 1900s buildings with outer walls of Cavity construction have become common. A Cavity Wall consists of two leaves of masonry connected together with Cavity Wall Ties. Very early types of Wall Ties included slate, cast and wrought iron. Mild Steel Cavity Wall Ties have however been the most commonly used Wall Tie and since around 1910 the Steel has been Zinc coated to protect from corrosion.
Typically, Cavity Walls are constructed with an outer skin of brickwork, a 50mm wide cavity, and an inner skin of brickwork or blockwork, the average total thickness commonly being 10.5 to 11 inches overall.
The two skins are tied together with Cavity Wall Ties, usually 900mm apart horizontally and 450mm vertically. Extra ties are installed around window and door openings.
The Zinc coating on older Wall Ties was relatively thin and proved be inadequate to stop the Wall Ties from corroding. A British Standard came into force in 1945 which regulated the thickness of Zinc coating applied to Mild Steel cavity Wall Ties. The standard has been revised over the years and in 1981 revisions more than tripled the requirement for Zinc Coating for some types of Wall Ties.
The ends of the Cavity Wall Ties are designed to lock into the cement or mortar bed. Also incorporated into the design of the Cavity Wall Tie is a means of preventing water transfer from the outer skin to the inner skin. In flat ties this can be a twist. In wire ties this can be corrugations formed in the tie or again a twist in the wire itself.
The Zinc Coating on wall ties does however break down, the time this takes is dependent on a number of factors which commonly include the buildings exposure to the elements and the materials used in construction. The Building Research Establishment has predicted that the Zinc Coating may have a life expectancy of 12 – 26 years for Wire Cavity Wall Ties and 25 – 46 years for Vertical Twist Cavity Wall Ties. The resultant unprotected mild steel Cavity Wall Tie will corrode readily and eventually break down completely.
There are two principle Structural problems created by corroding Cavity Wall Ties.
1. The ties can corrode in the cavity and break, such that the outer and inner skins are no longer tied together. If this occurs, there is a distinct risk of the outer skin collapsing, especially under high winds which cause suction on the face of the wall.
2. The more common problem is when the part of the Cavity Wall Tie embedded in the outer skin corrodes. This tends to occur on South and West facing elevations which are exposed to driving rain and which are often wet. Rust occupies a greater volume than the original steel and therefore the expansion of the corroding tie in the bed joint forces it to crack. Cracks running along the bed joints, every 450mm up the wall, are a good indication of Cavity Wall Tie corrosion.
The exact condition of the Wall Tie is determined by using a metal detector and then either removing a brick or the mortar or drilling a small hole and using an endoscope to examine their condition. If ties are corroded or if there are insufficient ties, a suitable type of replacement tie must be installed. Corroded ties will need to be cut out of the outer skin to prevent further cracking, either by complete removal, bending back into the cavity or by isolation.
It is essential to have a professional Cavity Wall Tie inspection carried out of older buildings, not least as the volume of material used in Mild Steel Wire Cavity Wall Ties is usually insufficient to cause bed joint cracking due to expansion. This can give a false appearance that there is not a Wall Tie problem in existence when in fact, the Cavity Wall Ties may well be at an advance state of decay.
Protectahome carry out Cavity Wall Tie inspections and have a wide range of remedial strategies to choose from in the event that Wall Tie replacement is deemed to be required. Our remedial Wall tie Service usually carries a long term Guarantee form the company and may be subject to the issue of further insurance backed guarantees (Guarantee Insurance) if required.
With offices in Bristol, Bromsgrove, Cardiff, Cheltenham, Leamington, Llanelli, London, Newport, Plymouth, Stockport you will never be too far away from a Protectahome Surveyor. For advice or to book a Survey please Contact Us on 0845 601 1980 or alternatively email us via email@example.com