The Maintenance of Garden Walls

A Practical Introduction

Jonathan Taylor

Proper protection from the elements is particularly important when the garden wall is made of chalk, as here in the Wylie Valley on the Salisbury Plain.

The use of hard cement mortars has caused more damage to historic stone masonry, including garden walls, than any other type of alteration in the past 150 years. This article provides an introduction to maintenance issues and the use of lime mortars in pointing for owners of historic gardens.

Before building materials could be readily distributed by canal and rail, walls were built with the material to hand. Construction methods changed according to the durability of the materials as well as according to local traditions, and the result is the rich variety of colour, texture and form which changes not only regionally but often from one village to the next. Garden walls of stone and brick are common, some dating from the Middle Ages when walls were essential not only for privacy but also to keep out livestock, and some garden walls have survived long after the original building was pulled down. All are vulnerable as many traditional skills are now rare and the materials and techniques now used are often harmful to their structure as well as their character.

In some parts of the country the stone was extremely soft and was rendered over to protect it from the elements. In other cases it was so durable that not even mortar was used. In all cases, the most vulnerable point is the head of the wall, and a variety of different details emerged according to the durability of the masonry. One of the most common is a simple coping of flat stone slabs laid across the top of a stone or brick wall, often with a groove on the underside of the leading edge called a 'drip' which prevents rainwater from running back and down the face of the wall. In London and parts of the South-East, brick walls are more often topped with a course or two of projecting clay tiles, known as creasing tiles, immediately beneath a 'soldier course' of bricks set on edge. In other areas no additional protection is required at all.


Because garden walls are exposed to wind and rain from two sides and from above, they are much more vulnerable to decay than the walls of a building. Saturation can lead to frost damage as freezing moisture expands within the pores of the stone, causing the surface to disintegrate. A similar effect may also be caused by salt crystallisation within the pores as moisture evaporates from the surface, leaving behind the salts carried there from other parts of the wall. This is most usually a problem where the wall has been coated with an impervious paint or a consolidant. Damp walls also enable plants to take root in cracks and joints, causing further damage as they grow, displacing mortar and weakening the structure.

To keep the wall free from damage by excessive water penetration there are two options: prevent water penetration by applying a completely waterproof skin or create a porous structure which dries out quickly. Completely waterproofing a masonry structure is generally impractical because all paint finishes and modern sealants are vulnerable to erosion by wind-borne dirt, and tend to crack as a result of oxidation and the constant thermal expansion and contraction of the substrate. Inevitably moisture finds its way in and is trapped. In the case of a garden wall, moisture also rises from the ground. Evaporation concentrates wherever cracks occur, causing salt crystallisation problems. For this reason, waterproof coatings should always be avoided. Where used historically - for example, where a wall has been painted with an impermeable lead paint or rendered with a dense mortar - there should be a presumption in favour of retaining the original finish, but if the wall or structure is deteriorating as a result, serious consideration should be given to alternative solutions, including its removal and replacement with a more permeable finish.

The best solution is to control penetration of the rain by ensuring that the head of the wall is intact, replacing any missing copings or other weathering details and, if necessary, modifying the upper surface by, for example, adding a lead flashing. Limewash may be used to consolidate and protect limestone as it is extremely porous and will allow the wall to dry out.


A 17th century brick wall at Ham House, Surrey, neatly repointed and repaired by the National Trust

Masonry walls are constructed by bedding the masonry in a soft mortar which is then finished off at the joints to form a neat, slightly recessed edge. For structural purposes the mortar needs to be nothing more than a well graded aggregate consisting of particles of stone or brick with smaller particles of sand to fill the voids and transfer loads evenly from one stone to the next. However, to prevent the mortar being washed out by rain a binder is added. Traditionally this was lime, but ordinary Portland cement (OPC), which was discovered in the early 19th century, slowly took over and it has been the predominant binder in use since the First World War. OPC's great advantage over lime was that it arrived on site ready for use, whereas lime had to prepared on site by slaking quicklime with water and allowed to mature before being used. OPC sets rapidly and, as the product of an industrial process, it is relatively uniform and reliable.

It was not realised until later in the 20th century that mortars rich in OPC were incompatible with traditional structures, and by then the art of using lime had all but died out. The cement made the mortars too hard, too inflexible and too impervious for traditional construction techniques and they were entirely unsuitable for repointing walls which had been originally constructed with a lime-rich mortar, causing considerable damage to softer stone in particular. The problem is partly caused by differential movement. As lime mortars do not set as rigidly as those rich in OPC any movement in the wall, for example due to thermal expansion, can be absorbed by joints filled with a soft lime mortar. However, if a dense mortar rich in OPC has been used in later repointing, the surface of the wall becomes rigid while the core remains flexible. The slightest movement leads to stress at the surface where the joints have been repointed, and structural failure commonly occurs across the arrisses (or corners) of the stones or bricks. In some instances, the entire surface of some stones may disintegrate.

As well as being relatively more flexible than a cement-rich mortar, lime mortars have the advantage of being significantly more porous, helping to ensure that the wall dries out quickly after a shower of rain. Although building contractors and consultants who specialise in the repair of historic buildings are now well aware of the need to use lime mortars when repointing these earlier walls, there are still far too many non-specialists who do not understand the issues - including professional consultants.


Walls of clay and earth are found in many parts of the United Kingdom, hidden beneath a lime render. The survival of this cob wall in Devon has been jeopardised by the introduction of a hard cement render.

Over the past 25 years or so the revival of lime mortars has tended to focus on the use of nonhydraulic lime - lime which sets gradually by carbonation, a reaction with carbon dioxide in the atmosphere. Mortars made with this type of lime are too soft for very exposed conditions and in cold weather (below 10șC) they set very slowly.

Non-hydraulic lime is best obtained as lime putty, a smooth white gelatinous porridge supplied in air-tight tubs. For most repointing work a mix such as 1 : 3 lime putty to aggregate is all that is required. The aggregate must be well graded from fine to large particles (up to 6mm) - fine building sand alone will not do. Once mixed together, the mix, which is then referred to as 'coarse stuff', tends to stiffen up when left and may appear quite dry. One of the mistakes made by inexperienced builders is to add more water. However, coarse stuff rapidly becomes more pliable as it is worked, and should be knocked up thoroughly just before it is used. Additional water is rarely necessary at this stage.

Lime putty is essentially the same as the dry hydrated lime sold in most builders merchants. Both are made by 'burning' pure limestone or chalk (both forms of calcium carbonate) in a lime kiln to form quicklime (calcium oxide): this material is then slaked with water. In the case of lime putty an excess of water is used and the material is left saturated to mature, protected from the air under a thin layer of water. In the case of dry hydrated lime, only the minimum amount of water is used to slake the quicklime, which is then ground to a fine powder and placed in bags. Because lime hardens by reacting with carbon dioxide in the open air to form calcium carbonate once again, the dry hydrated lime supplied by the ordinary builders merchant will have started to carbonate from the moment it was made and its performance may vary unpredicatably according to the degree of carbonation that has occurred. The best results can be achieved with a traditional lime putty.


There is a growing body of opinion that in some areas of the country most mortars used for the construction and pointing of walls were made with lime that was at least feebly or moderately hydraulic. Where hydraulic lime is used, an initial set takes place through a reaction that requires water, not air. The mortar therefore gains strength much sooner, before it has dried out. Further hardening of the mortar then occurs subsequently by carbonation. For repointing and other repairs to garden walls, a hydraulic set offers the advantage that the mortar will resist a shower of rain within hours rather than days. Hydraulic lime is made by 'burning' limestone which contains some clay, then slaking it on site, usually with the aggregate. Where the limestone available was not naturally hydraulic, an hydraulic effect can also be achieved by adding small quantities of certain highly reactive materials known as 'pozzolanic' additives such as brick dust and other ceramic materials which have been fired at high temperature.

Today many specifiers recommend the addition of cement to a non-hydraulic lime to gain a suitable set (for example a 1 : 1 : 6 lime putty : OPC : aggregate mix). However, adding cement is likely to make the mortar less permeable and could cause problems later. For garden walls constructed of sandstone, it is best to use a feebly or moderately hydraulic lime (or a non-hydraulic lime modified with a pozzolanic additive) because it is believed that lime may leach from ordinary lime-rich mortars, causing certain types of sandstone to deteriorate.


For grass to colonise the head of this brick wall and fine pier, the structure must be permanently damp. Repointing may help, but the head of the pier may need a lead flashing to prevent further deterioration.

Lime mortars most often fail as a result of poor preparation work. Joints need to be raked out carefully before pointing to remove loose mortar and any hard cement in particular. The depth should be greater than the width of the joint so that the new mortar will not fall out, and the back face should be square to provide a good key. Sound lime mortar should not be touched on the principle that historic fabric should never be altered more than is necessary. Before repointing, the joints should be dust free and well wetted to reduce the amount of water soaked up by the wall out of the fresh mortar.

Contractors sometimes damage the edges of the stones when hacking out the mortar. This not only changes the appearance of the wall, but also causes problems when they come to repoint, because the joints should be slightly recessed from the face of the wall. Extending the joint across the face of the stone or brick creates a weak point in the mortar (see diagram) which readily disintegrates. Good pointing can transform a garden wall, not only visually but also in terms of its performance, as it sheds water readily, dries out quickly after a storm, and ceases to provide niches for plants to colonise. Today all the materials required are available from specialist suppliers, including natural hydraulic lime of varying strengths graded from NHL5 or 'eminently' hydraulic lime (the strongest and least permeable) to NHL2 or 'feebly' hydraulic, a variety of pozzolanic additives, ready mixed coarse stuff, aggregates, limewashes and pigments.

This article is reproduced from Historic Gardens, 2002


JONATHAN TAYLOR is Executive Editor of The Building Conservation Directory and the BCD Special Reports. He studied architectural conservation at Heriot-Watt University, Edinburgh and is a member of the Institute of Historic Building Conservation.

Further information


Lime Mortars and Renders: The Relative Merits of Adding Cement

Lime Plaster and Render Reinforcement

Traditional Lime Plaster: Myths, Preconceptions and the Relevance of Good Practice


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