Solid Wall Construction & Conservation Issues

“Managing old buildings in a modern world”

The walls of old buildings were built differently to their modern counterparts: they were constructed as a single connected body, rather than, as seen today, two discrete structures separated by a cavity; they were thick – typically 2′ to 2’6″ – which provided stability and helped regulate the temperature within the building; and they were constructed with permeable mortars that allowed moisture to escape harmlessly, where it might otherwise accumulate to form damp, mouldy conditions. These are characteristics which continue to be useful to this day

They were also constructed from simple materials – stone, lime, sand, clay, and wood – all still available as low impact, relatively sustainable and locally sourced materials today. Of course, problems can arise with old buildings (as with anything in daily use), but timely maintenance with like-for-like repairs will generally keep these to a minimum; most complex difficulties with old buildings arise from the use of modern materials with radically different properties to the original fabric.

Traditional solid wall construction

The principle construction techniques of stone-built solid walls are essentially the same as those used in dry stone walls, but with the addition of mortar. Two faces are built up and supported in the centre with stones and mortar; through-stones are incorporated at regular intervals to distribute the weight evenly and tie the faces together. Except in areas with very flat or very soft stones, the stones are brought together so that they have good contact to carry the load of the wall above, whilst mortar is used to cushion the stones and prevent movement where the shape of the stones makes contact impossible. The stones, whether coursed or randomly placed, are laid flat – for greater stability – and where possible, brought lengthways into the wall.

Most importantly, the structural properties of the stonework omit the need for a hard setting mortar. This enabled builders to utilize materials that were nearby or easy to obtain, but not necessarily strong. It also enabled those materials to perform a more important function – namely, to control moisture.

Understanding water movement in walls

Water moves through walls (rather than staying in one place) because most materials are hygroscopic. A water droplet spreads out across a piece of paper because the water molecules near the edge of the droplet but not touching the paper are attracted to the unwetted surface outside. The water spreads through the paper, because paper is porous and the water molecules are similarly attracted to the unwetted surfaces within the paper. Thanks to some clever natural processes to do with drying fronts and capillary action (which are well explain in this link). hygroscopic, porous materials also dry out efficiently.

These same processes occur with traditional solid wall buildings and explain why, somewhat paradoxically, materials like lime which are porous and easily wetted are so good at keeping buildings dry. As the most vigourous drying front is normally the external wall surface (where the wind whisks away water molecules as vapour), water within the wall is drawn away from the interior. This is also why applying standard masonry emulsion – which has low porosity and is generally hydrophobic – prevents walls from drying out. Drying out such walls water to be lost through the building (usually by heating), which draws moisture towards the interior walls.

Movement in old buildings

Old buildings, with often shallow foundations, move with the seasonal heaving and drying out the ground beneath. The processes of construction (discussed above) are designed to accommodate most of this movement. Yet structural defects can occur.

Unlike drystone walls, which are expected to run for indefinitely without lateral support, the walls of buildings require support. This support is provided by the building itself – by adjacent walls; by floor beams; and by roof struts. Most structural cracking is due to failure in these supporting elements, and if corrected soon enough, further movement can be arrested.

Movement can also occur if drainage around a building changes, allowing the softening of the ground beneath. Over time this can lead to subsidence, which if not uniform across the building, will result in stress cracks to appear. Again, rectifying the problem will arrest further movement without recourse to major repairs.

In some cases, additional support may be required (e.g. in the form of tension rods and spreader plates), but these should be carefully planned to avoid restricting beneficial movement elsewhere.