Cracked render on old buildings is a familiar sight, but what does it mean and should you be worried about it?
Render is the skin which is laid on top of the wall substrate, and is intended to offer protection from the worst of the weather.
On old (pre 1900) buildings, the render may often be seen to be cracking and pulling away from the walls. This is usually due to the render being formed from cement rather than lime, which is a material that does not work well in the long term with materials such as cob and stone. A good lime render applied by an experienced lime mason is a much more durable and healthy option.
Why Use a Lime Render Rather Than a Cement Render on an Old Building?
There are two main reasons why cement render does not work well with cob and stone buildings - the first is that unlike lime it is hard and brittle and does not have the flexibility to deal with the tiny movements that are inherent to period properties. The second reason is its low breathability; it does not readily permit the passage of moisture vapour. The result of this is that any moisture entering the walls becomes trapped and may saturate the wall substrate.
When it comes to old buildings, cracks in cement render may often be explained by the characteristics of cement.
Being mortared primarily with clay, old Cornish cob and stone buildings tend to experience tiny movements depending on the local environmental conditions. This movement often causes hairline cracks in brittle cement render.
Once these tiny cracks form, the door is open to more moisture penetrating the wall by means of wind driven rain. As moisture that has penetrated the wall has no means of evaporating away, this may develop into a vicious circle. Water penetrates, it becomes trapped, and the wet substrate loses the key that is holding the render to the wall.
A good lime finish such as render or pointing allows moisture to evaporate away rather than becoming trapped.
What Issues May Arise from Blown Cement Render and What Traditional Techniques Can Be Used to Correct Them?
On stone buildings moisture build up may only result in rotten lintels and damp living conditions, but is much more serious on cob buildings when left untreated.
Cob is very susceptible to prolonged exposure to moisture. When wet, it becomes pliable and when it is exposed to repeated wetting and drying cycles it loses its strength and crumbles, necessitating cob block repairs.
Cracked render on a cob building should be addressed as soon as possible, as the longer it is left the more expensive repairs will become.
What's So Good About Using Lime?
Lime mates very well with cob and stone. It has been used for hundreds of years to weatherproof buildings. Lime is both (relatively) flexible and highly breathable, meaning that is less likely to develop hairline cracks. If such cracks do develop, moisture is not trapped behind the render and the vicious circle is less likely to occur.
Re-rendering a cob house with lime in the traditional manner is not a quick or cheap process and should only be carried out by a specialist lime mason.
Before rendering can take place, structural repairs often have to be carried out using cob blocks and lime mortar.
Lintels which have sat behind cement render may have to be replaced, and window and door reveals rebuilt.
What is the Best Practice a Lime Mason will use when Applying Render?
A good lime render will go on in four coats. First, a scat coat is hand cast onto the wall to drive into all the nooks and crannies and provide a good key. This should be followed by two well-haired base coats to give the render tensile strength.
Finally, a top coat of fine sand is applied to give an attractive aesthetic. At least a week should be allowed between the last three coats to allow for shrinkage and curing to take place.
Then a good breathable coating such as a mineral silica paint should be applied to assist the new render to shed water. A lime render applied using best practice should last a lifetime provided the rainwater goods are maintained.
Here at Muddy Mortars we are specialist lime masons who are experienced in all aspects of diagnosing and treating cob and stone buildings. We are skilled in finishes, replacing lintels, and cob block repairs. Please call us today if you feel that your building would benefit from our services.