Repairing External Cladding: Stone

Many period or period-style homes are constructed from natural stone, and in some areas of the country it is more prevalent than others. Stone will weather over time and when dealing with any deterioration it is important to proceed with care, as it is all too easy to make the problem worse through lack of understanding of the nature of the problem. It is also important to remember that maintenance and treatments will vary depending on the type of stone you are dealing with.

How to fix it

The most important thing to remember when carrying out stone repairs is that specialist knowledge is usually required to avoid further damage. Stone that is delaminating can be secured by stainlesssteel pins, resin or grout, whilst cracks can be filled with lime-based mortars.

Crumbling stone can be strengthened through the use of chemical consolidants, although these should only ever be applied by an experienced tradesperson; DIY versions should be avoided.

Where complete areas of stone are missing, restoration work can be carried out using matching stones (which will weather to blend in over time) and lime-based mortars. In severe cases, whole blocks of stone may need to be replaced, but care must be taken to match the coursing, joint width, profile and surface finish of the original stone.

Causes of Problems

The causes of stone decay can be down to natural weathering, structural problems within the building or due to the use of inappropriate materials or treatments.

Natural Weathering

Acidic rainwater is one main cause of the weathering of softer stones, such as limestone. It leads to the gradual erosion of the stone and is particularly noticeable on decorative, carved areas. On sheltered areas of the building, such as under porches, calcium sulphate can form, building up to show a dark crust which over time blisters and leaves behind a soft, powdery stone — which eventually disintegrates.

Frost can cause problems. If stone is allowed to become completely saturated with water, when frost sets in it can cause the complete shattering of the stone. Flat copings and cornices are most at risk.

Excessive plant growth can cause damage when roots open up mortar joints and dislodge blocks of stone. Even bird droppings – being a concentrated source of acid – can cause decay, particularly to limestone, if left unchecked.

Structural Problems

Structural defects in a building, such as subsidence, can manifest themselves in the form of fractured individual stones, cracks and bulging areas of walling. Although some movement in buildings is just seasonal, if you notice any of the above symptoms affecting your home you should seek advice.

Inappropriate Materials and Treatments

The most common cause of this type of problem is the use of hard cement mortars for repairing stonework. Traditional stone walls need to breathe to allow water to easily evaporate. Most cement mortars are harder and less porous than the stone they hold in place, so any evaporation occurs through the stone. This means soluble salts contained within the moisture crystallise in the surface of the stone and not in the mortar, so the stone crumbles away rather than the pointing.

Soft lime mortars allow moisture movement and salt crystallisation to take place in the pointing in preference to the stone — and it is far cheaper to repoint a wall than to rectify decayed stone.

Repairing Cladding:

• Brickwork
• Render
• Weatherboarding

Further reading:

• A Guide to External Appearances
• Choosing Stone: Natural or Manmade?
• Repointing Old Walls