Masonry Buildings
Masonry buildings – those constructed with stone or brick – are more resilient to long periods of flooding or continuous flooding than earthen structures or timber-framed buildings. This is because they are less likely to experience component failure unless extreme force is asserted, generally via debris.
Lime plasters, mortars and renders are naturally able to disperse excess moisture via a large, connected pore system. However, masonry walls can be damaged by inherent soluble salts and by salts absorbed from coastal waters. When masonry is saturated, the salts dissolve. But when the water evaporates, the salts are carried nearer to the surface, where they crystallise and can appear as a powdery white residue called efflorescence. Although unsightly, this is not usually harmful, and it can be brushed, vacuumed or washed away. Where heavy levels of salts are present, particularly in buildings previously used to house livestock, removing salts may require desalination water.
Problems arise if the salts are trapped behind a relatively impermeable coating, such as a water-repellent sealant or an oil-based or acrylic paint. The salts crystallise within the substrate and then expand, pushing off the surface of the brick or stone in a process known as 'spalling' or 'exfoliation'.
England predominantly experiences its rainfall during the cooler months. This means that external facades are vulnerable to freeze/thaw cycles, drying times are slower, and humidity levels are higher. Understanding how porous the masonry is will help establish how susceptible it is to freeze/thaw. Structural features regarding the type and distribution of pores will influence the extent of weathering that will occur. The presence of small pores (micropores) means the stone is at a greater disadvantage then those with larger pores (macropores). Disintegration via freeze/thaw is attributed to how the pores interconnect to the other particles in parallel layers, and their resistance to water. It is not as simple as determining that all limestones will respond in the same way as they will have a variety of differences, and it is important to assess the moisture properties and suction forces of the pores in connection.
The spalling of soft brick or stonework usually occurs on a building exterior when the temperature drops below freezing and moisture is still trapped within the walls. Water expands when the temperature falls below 4°C, turning to ice crystals and causing spalling on wall faces. Lime mortar joints can also become weakened by spalling due to the long-term seepage of contaminated water through masonry. Impermeable heavily cement-based mortar will exacerbate deterioration during a flood, because it holds the moisture in the masonry for longer and causes salts to crystallise within the historic materials. Removing cement mortar completely and replacing it with weaker porous lime-based mortar will help the walls dry out.
Masonry structures that have a rubble infill are very susceptible to damage from water penetration, because the water will dislodge the finer materials. This can cause the infill to fall from its original position, thus creating large voids and consolidation at lower levels.
Another consideration is the geology of the stone that has been used to construct the building. Sandstones and limestones that are made up of a higher percentage of clay deposits are more vulnerable to water penetration than stones from igneous and metamorphic rock. The water penetration can result in the clay particles expanding and contracting from cyclical wetting and drying cycles.