Mitigating dampness in historic buildings: when ancient techniques and science come together

In the second of our posts from SEAHA students, as they prepare for the 2nd International SEAHA conference on 20-21 June, we hear from Lucie Fusade, SEAHA Doctoral Student at the University of Oxford, School of Geography and the Environment on how ancient techniques and science can come together to mitigate dampness in historic buildings.

Mitigating Dampness in Historic Buildings: When Ancient Techniques and Science Comes Together

By Lucie Fusade, University of Oxford, School of Geography and the Environment

This research looks at the use of traditional materials such as lime mortar and additives in order to mitigate driving-rain ingress to historic buildings. It aims to develop a repair mortar for specific conservation issues while gaining a scientific understanding of the behaviour of the materials. This is why this research has been co-created with the University of Oxford, Historic England Building Conservation and Research Team and the Churches Conservation Trust (CCT), as part of the EPSRC SEAHA centre for doctoral training. This collaboration allows the research to undertake a combination of laboratory experiments and field trials in order to fully assess the properties of the mortars tested.

North HuishP1000119
St. Mary’s Church, North Huish, a church under the care of the CCT

Lime mortar is a traditional building material, which technology has been extensively developed by the Romans. A mortar can be made with many combinations of binder, aggregates, water and additives. In most of England’s historic buildings the binder traditionally used is lime, sand usually used as the aggregate. Lime, a breathable and soft material, is the material produced for the burning of limestone. Most of the repairs to historic masonry require the use of mortar. Although cement, an impermeable material, was used extensively since the late 19th century, lime-based mortar is now mainly used.

In England, many historic buildings of architectural, national or local significance, are particularly exposed to harmful environments, especially wind and driving rain. Rain water and moisture are one of the key factors in the deterioration of building materials, leading for instance to biological growth and dampness. To ensure preservation of historic masonry, rain penetration needs to be mitigated and moisture drawn out of the wall. These are the main roles of masonry joints and especially of the surface mortar, called pointing mortar. The joints indeed contribute to take harmful soluble salts away from the historic stones. Practitioners said that, when done correctly, repointing intervention is the most significant repair to assist waterproofing of a wall. Dampness can easily occur in masonry that has been repaired with cement rather than lime and more damage to the masonry units done.

Repointing intervention

Developing a suitable mortar, using traditional material, for repointing intervention is therefore necessary in order to preserve the historic masonry. The main requirements for a lime-based mortar to be used in pointing intervention on buildings exposed to dampness are that the mortar should be softer than the surrounding masonry, porous, highly permeable, quick at drying and especially able to cure under damp conditions.

Considering these criteria, it is necessary to enhance the properties of the lime mortars commonly used. They have a tendency to be too strong for the surrounding masonry after several years and sometimes not able to dry out quickly enough. Thus, additives, such as wood ash or crushed stones, can be added to the composition to enhance its different mechanical or physical properties. The repair mortar is then designed to meet specific conservation issues.

mortars samples
Samples of lime mortars made with natural-hydraulic lime and different concentrations of wood ash


The first phase of this research focuses on the use of wood ash as additives. Wood ash has been used in the past, by Romans for instance, to moderate water ingress. However, a scientific understanding of its effect is lacking. Previous research done during an MRes has demonstrated that wood ash added to lime mortar has a clear impact on the performance of the material in increasing the hardening process of the mortar, called carbonation, as well as the permeability and the porosity. The positive impact of wood ash on the permeability of lime mortar seems to be due to the high number of fine particles it contains, enhancing the micro-porosity.

The research aims to characterise the physical and chemical roles played by wood ash in lime-based mortars. Working with an experienced mason, a series of mortar samples was made with different concentrations of wood ash, 0, 10, 20, 30, 40, 70 and 100 percent, as part of the aggregate, and mixed with natural-hydraulic (NHL 3.5) and non hydraulic lime (lime putty). Prior to on-site investigation, the research involves laboratory experiments at the Oxford Rock Breakdown Laboratory (OxRBL). Properties such as the water absorption coefficient, the open porosity, the vapour permeability, the degree of carbonation and the flexural and compressive strength are assessed using gravimetric tests, thin sections microscopy and scanning electron microscopy.

Based on the results obtained in laboratory, some mixes will be applied on test walls outside at Wytham Woods in order to evaluate their performance when exposed to driving-rain. Later on, selected mixes will be applied to damp historic buildings cared for by the CCT and in collaboration with expert practitioners from Historic England. Combining ancient materials and workmanship with scientific investigation contributes to sustainable and targeted conservation interventions.

lab experiment
Experimental setting to determine the open porosity of lime mortars


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