Hard durable stone, such as granite, resists natural deterioration well, but softer stone, such as limestone, can be significantly weathered away over time. Stone is also vulnerable to defects within it, and to fractures occurring either naturally, or as a result of loading or thermal movement. For layered stone, such as some limestone, the direction of the layering in the lintel can significantly affect its weathering and strength.
If it is kept dry and free from wood boring insect and fungal decay it can last indefinitely. However, it is sometimes used on the external face of a wall and can decay much sooner than the wall it is supporting. Timber is also subject to initial drying and shrinkage after the tree has been cut down, and this can result in the timber ceasing to provide support for the masonry above it after the wall has been built. For this reason only well seasoned or kiln dried timber should be used for lintels.
Timber lintels are light in weight and easy to cut and fix. They also provide easy fixings for items such as curtain rails and have a relatively good thermal performance compared to stone or concrete lintels.
These can be made on site or in a factory and are reinforced with steel reinforcing bars (or rods). Common problems with this type of lintel are:-
a) Poor quality concrete found in the site made lintels
b) Misplaced reinforcement. If the reinforcement is not in the correct position within the concrete the performance can be greatly reduced. Lintels with the reinforcement close to the surface of the concrete i.e. with less than the specified cover to the reinforcement can fail within a few years of installation.
Metal lintels are made of mild steel or, more recently, of stainless steel as well. Mild steel lintels can be hot dipped galvanised after manufacture or made from pre-galvanised steel which has a much thinner galvanised coating. Some lintels have an applied finish to the steel, such as paint.
Large openings may be spanned with a steel channel or joist/beam section supporting the inner skin or face of a wall with a steel plate welded onto the bottom flange of the section. The plate extends out to form the support for the outer part of the wall.
The corrosion protection of the lintel is particularly important if it is to last for the design life of the building, normally 60 years, without the need for replacement. Lintels made from pre-galvanised sheet may last as little as 15 years in some areas unless additional protection is provided to the zinc. Additional protection may also be required if, for example, the mortar contains substances which are aggressive to the zinc coating.