Traditional Buildings & Issues with rubble filled cavity walls

When restoring or addressing defects to a property, it is essential that the surveyor undertaking the survey understands the original building systems, such as lime-based plaster, pointing, and the structure of the walls and floors. But can also advise and work with the client and the local conservation officer, to provide an end result which is best for all parties including the traditional building itself.

Structure

First let us look at the building material of a typical traditionally constructed property in the Saddleworth area of Greater Manchester. The walls of traditional properties are typically constructed of two leaves of stone with a rubble infill, the original mortar being of a lime base and the internal plaster, if applied, also being lime. Through stones were installed to tie the two wall structures together and improve stability.

Defects

This photograph showing damage caused to stonework through the incorrect use of an OPC cement-based strap pointing. The pointing to a wall is considered as sacrificial to the stone and should therefore always be softer. In this situation lime pointing should have been applied. Lime pointing is more vapour permeable (Porous) and more flexible, allowing faster evaporation and drying of the structure. It also has the ability, to some extent, to reseal small cracks, so preventing water ingress which results in frost damage.

Problems can also arise from loss of structural integrity, allowing water ingress into the wall structure resulting in penetrating dampness. Common causes are settlement or displacement of the rubble core, particularly where this was not consolidated at the time of construction or where there have been changes to the original structure. Particularly where new openings have been formed without structural stabilisation being undertaken, by injecting a grouting system to consolidate and bond the infill before undertaking opening up works. As the rubble infill will inevitably fall away leaving voids, which cannot be effectively filled due to the restrictions of the external leaves. The usual practice of providing "needles" to support the wall above an opening is unsatisfactory, as rubble falls from the sides of the opening, leading to instability in the area adjacent to the new work.

Rubble in-fill material is very susceptible to damage from water penetration and frost action. Failure can occur where rainwater has been allowed to penetrate the inner material. The water inevitably percolates downwards, removing fine material and leading to consolidation at lower levels. Fill material can sometimes fall from its original upper position leaving large voids. Water in the core can also cause washing out of mortar from joints between facing stones, further weakening the wall.

Repetitive cycles of freezing and thawing of moisture over the years within voids in the material, leads to expansion in the size of the voids and increases the pressure on both inner and outer leaves of masonry. Freezing and thawing also breaks down stones within the inner core, producing further consolidation. In severe cases, loss of material from upper levels within a wall can create large voids, reducing the inner and outer leaves to independent structures.

Consolidation of rubble fill has also been observed where walls have been affected by the close proximity of road traffic, particularly that of heavy goods vehicles. Ground-borne vibration from vehicles is transmitted to the rubble infill material and has a similar effect to water percolating through the core, but without the additional problems of expansion and contraction produced by freezing and thawing.