BUILDING BRIEFING: 

Stonework

- common problems and defects

This article looks at the most common defects in stonework and how to maintain, repair or restore stone elevations.  By Derek Worthing, Principal Lecturer in Building Surveying at the Faculty of the Built Environment, University of the West of England, Bristol.

Background

Stone has been used as a material for constructing and facing the external walls of houses for centuries and, although there are regional preferences, has vied with brickwork as the dominant walling material. Stone is valued for its aesthetic
and symbolic appeal but is rarely used nowadays for new housing because of its high cost - unless there are strong local traditions of stone building and/or there are specific planning requirements.

Sandstones were formed from components derived from the breakdown of a variety of other rocks but the essential material is quartz. Quartz is largely inert and the cement largely determines the weathering resistance of sandstones

Stone dressing

 One way of defining stone walls is by reference to the way in which the stone is dressed. Ashlar is the name given to squared stone which is dressed to a smooth face. Ashlar consists of precise blocks usually of identical dimensions and separated by very thin joints to produce a ‘classical’ appearance.

 Rubble was a popular approach in the Victorian era. It was used because it was cheaper than Ashlar but, in many instances, it was deliberately chosen because of its appearance.

Some of the common defects in stonework

Inherent characteristics

Pore structure has a significant effect on the durability of stone because it influences the amount of water entering and moving through the stone. Also salts can be transported by water and accumulate in the pores.

Some stones, however, have specific composition characteristics that may affect durability. For example, where the stone has layers with different characteristics such as so called 'soft beds', differential rates of erosion can result and produce horizontal 'furrows' in the stone. Also small fissures (cracks or faults) in the stone can cause problems, particularly on finer carved features.

Acid rain

A significant cause of damage is the interaction between limestone and the products of pollution in the form of 'acid rain', which results in the production of soluble gypsum (crystals).

The resulting pattern of decay of the limestone is determined, essentially, by its exposure:

• On exposed areas of the building rainwater will wash away the gypsum resulting in a gradual weathering of the stonework.
• In sheltered areas of the building more severe damage may occur. In such situations a relatively impermeable skin will start to form which often contains 'dirt' from the atmosphere.

The crystallisation (and re-crystallisation) of salts will usually occur beneath the skin, which may stay in place for a long period, or it may form blisters that burst to reveal a powdery decayed sub surface.

Salt crystallisation

Salts in solution pass into the stonework entering the pores of the stone. As the stone dries out, the salts are deposited at the surface or within the stone. Crystallisation within the pores will exert pressure, often resulting in damage. The potential for
damage is increased because of the repetitive nature of the mechanical action caused by the cycle of (re)dissolving and (re)crystallising.

The damage usually shows as a powdering of the surface, but it can look more dramatic by causing splits in the stone as the mechanical expansion works against an existing weakness - such as incorrect bedding or a fissure.

Some of the salts will absorb water from the air which will also cause damage even if there is no direct wetting from the more obvious sources such as rain, rising damp or plumbing/drainage faults.

Frost attack

Frost will tend to be a problem in stonework which gets both excessively wet and subject to freezing. Frost damage is not very common in sheltered plain walls, except perhaps at below damp proof course level. 

The pore structure of the stone is a significant factor in determining susceptibility to frost attack.

Where it does occur, frost damage can dislodge quite big pieces of stone either in vulnerable areas, such as parapets or copings at the top of the wall, or where the stone has been damaged by other agents such as salts.

Design and craftsmanship

Incorrect bedding

A common defect arises due to incorrect bedding. When sedimentary stone is placed in a wall the general rule is that it should lie in its natural bedding position - horizontally, in the manner in which the stone was originally formed. The stone is stronger in this position and is also less vulnerable to defects.

If the stone is face bedded (the layers are vertical), it is more vulnerable to damage through crystallisation of salts and/or frost action. The mechanical actions involved can more easily dislodge the bedding layers as there is no restraint from the adjoining stones.

However copings and cills for example should be edge bedded because they are exposed on their top side.

Incorrect pointing

As with brickwork, an inappropriate mortar mix can cause problems in both the mortar and the stone itself. The main problem is usually related to having too strong a mortar. Strong (dense) mortar allows less evaporation so any moisture in the wall will have to evaporate through the stone. This may increase the likelihood and severity of salt related defects. Stronger mortars are relatively brittle and they are more susceptible to shrinkage cracking. This increases the likelihood of rain penetration, which again can lead to salt related defects. It also increases the likelihood of frost attack.

Making the jointing stand proud of the face of rubble stonework, sometimes known as ribbon pointing, is another common mistake. Ribbon pointing is usually formed with a very strong mortar. Aesthetically it is unattractive, and technically the ribbon pointing performs poorly because it shrinks and allows water ingress, it is brittle and so easily cracks and falls away.

Expansion of metals

Iron and steel cramps have traditionally been used as fixing devices in stonework and when they rust, they expand and can fracture the stone.

Failure of 'protective' coatings

Coatings are often applied to stonework in an attempt (usually misconceived) to protect it. Whilst there may theoretically
be good reasons for applying a silicone based water treatment to stone in some circumstances (i.e. where penetrating damp is occurring), it should not be used to try to arrest the effects of stone decay, and there is evidence that the rate of decay is actually accelerated.

This is probably because moisture takes longer to evaporate when a coating is applied. Salt crystals are often trapped behind the treatment and this can cause greater damage to the stone. Eventually, the forces exerted by the salts will probably break down the coating. Likewise, painting stone work will usually increase the rate of deterioration.

Tips on maintenance and repair:

• Check the condition of the pointing every 4-5 years.
• Routine maintenance should include inspections of brick and stonework, gutters, roofs and drains and any existing surface/paint finish.
• Use binoculars as an aid when inspecting walls on a property over more than one storey.
• Damp accelerates the deterioration of stone.
• The stonework should be clear of vegetation. Plants should be carefully managed to ensure that roots are not pushing between the stones or that moss and creepers are not encouraging damp by holding water against the stone.
• The deterioration of the stone may be due to the need to re-point the wall, or that an inappropriate type of mortar has been used.
• The purpose of pointing is not to ‘hold the bricks together’ (a common misconception), but to help keep the stone dry by absorbing moisture, and to give the stone room to expand during warm weather.
• In most cases pointing work should be finished with a flush or slightly recessed joint with the mortar being brushed back off the face of the stone.
• Turn the ‘bricks’ around if they are generally sound and damage is cosmetic.
• Lime based putties and mortars should be used for all re-pointing work.
• Paint and render can damage the stonework by trapping water and causing damp, or making existing damp problems worse.
• Hire builders or contractors who have experience in working with stone buildings and can provide local examples of good re-pointing.

Further reading:

The Conservation of Building and Decorative Stone, by John Ashurst and Francis Dimes.