The brickwork over door and window openings in buildings constructed with cavity walls (Cavity wall construction became commonplace from the 1920’s) is usually supported by a lintel. Prior to the use of combined ‘Catnic type’ lintels (which became popular around the late 1960’s to early 1970’s, and are used almost exclusively nowadays) the inner leaf walling to the cavity wall was supported by either a timber or concrete lintel. Whilst the outer leaf was supported by either a self supporting brick arch, a stone lintel, a flat steel bar or a steel angle (angle iron). Sometimes, no lintel was used to support the outer leaf of brickwork, the brickwork being built directly on top of the window head. This was usually not a problem as the windows were constructed of a thick timber profile which was capable of supporting the load. The steel lintels were sometimes protected from corrosion by a coat of bitumen or ‘red lead’ paint but more commonly, the steel lintel had no protective coating applied. In good quality work, a cavity tray was installed above the lintel however, this was often omitted. (A cavity tray is used to prevent water from entering the cavity and running down the inner face of the outer leaf of brickwork causing dampness internally above the window).
Unfortunately, the protective coating applied to the early steel lintels, (if applied at all) breaks down over time allowing the steel to corrode. As the steel corrodes, it expands in size. This expansion can actually lift the brickwork above the lintel causing bulging of the walling and cracking of the mortar bed joints. This cracking allows more water to enter the cavity causing further corrosion of the lintel until ultimately the lintel can corrode completely through.
Cavity trays should be installed above any type of lintel in a cavity wall be it an arch, stone or steel lintel to prevent internal dampness. However, these trays were often installed badly with the tray in the wrong position, damaged, lacking stop ends or weep holes (Weep holes allow any water that has collected on the tray to safely drain to the outside) or the tray may have been completely omitted.
Where the brickwork above an opening is built directly on top of a timber window or door frame problems usually occur if the existing timber frames start to rot and decay or when these frames are replaced with modern upvc frames which are not designed to support any brickwork loading. When the existing frames are removed the brickwork above can drop causing telltale stepped cracking. If this does not happen immediately, it can develop over time, especially if the new frame is not packed tightly to the brickwork and/or where the frame is unable to support the loading. Common signs of this are bowing of window heads especially on large windows or the sticking of window openers.
The solution is to replace the existing defective steel lintel, or in the absence of an existing lintel insert a new one.This lintel is usually a steel ‘angle iron‘ type lintel, the size of which depends upon the span of the opening and the loading above it. To prevent any future corrosion this lintel should be hot dip galvanised to BS EN ISO 1461.
The new lintel should have a minimum end bearing of 150mm each side of the opening and a new cavity tray installed above the lintel. The cavity tray can either be preformed or fabricated on site using roll damp proof course material. Whichever method is chosen, it should be dressed/sealed to the inner leaf and stop ends should be fitted or formed. Weep holes should be provided at 450mm intervals with a minimum of two per opening as per the NHBC (National House Building Council) or at each end of the cavity tray and at a maximum of 900mm centres as per LABC (Local Authority Building Control). These weep holes can be of an unobtrusive design such as the “Ryton Rytweep” cavity weep.