Historic Buildings Dance : A Building Surveyor’s Analogy
Posted by GLM on Oct 2, 2020 in Building Surveying
I am a fan of analogies. I use them all the time. I see them as very handy to illustrate lots of situations when speaking to work colleagues, clients and contractors. As English is my second language, they help me to easily communicate technical information in a more friendly way. Here is one of my favourites:
You may have heard people saying: “I’d love to know what stories that historic building could tell, what secrets it knows” or you may have heard people talking about how buildings “breathe”. What you might not have heard anyone say, until now, is that historic buildings “dance” as well. Yes, you read it right, historic buildings dance.
Construction materials such as timber, mortars, or metals typically move rhythmically in response to changes in temperature through the different seasons of the year. This phenomenon can be described as the “expansion-contraction rate”. The difference between the hottest and the coldest temperature in an area or region also determines the amplitude of this volume oscillation. Different materials have different “coefficients” of thermal expansion and contraction so, when they come together to form a building component, their volume oscillation results in differential movement.
When materials and buildings components are initially combined in a building, it is expected that cracks or other defects resulting from differential movement will appear during the first years of the building’s life. Careful choice of materials and thorough detailed design helps to accommodate this differential movement and allows materials and components to dance together.
Some materials are better “dancers” than others. Thanks to their plasticity, they can act as buffers that accommodate the differential movement between more lively components. This is especially the case of lime mortars, for example. When a lime mortar is correctly specified, its plasticity will allow some of the movement of the masonry elements to be absorbed. This contributes to maintaining the integrity of the masonry. In a similar way, sand mastic, when used to seal the joints between external masonry and windows or doors, contributes to the tightness of the joint, as masonry and joinery elements don’t typically dance to the same rhythm.
There are, however, some “twinkle toes” materials, such as cement. Cement is very stiff and prone to cracking. Rather than helping to harmonise differential movements, it can disrupt the rhythm of the dance.
Once components get familiar with each other, they usually settle down, reaching a point of equilibrium. From then on, they usually move together in harmony, so they dance.
Building materials and components begin to dance together from the earliest days of construction of a building, to the rudimentary beat of the hammers and chisels and to the cutting of saws, and this choreography of construction continues for decades.
The key to keeping buildings dancing to the beat of the changing seasons is determining the right repair strategy and selecting the right materials. Introducing inappropriate materials or components will disturb that harmony. This can compromise the integrity of the whole ensemble. Modern materials that may be perfectly harmonious in new construction, when introduced in historic buildings can be discordant partners.
It is like a stranger who comes late to a party where people have been dancing together for hours. The people there are a very closed group of people where they have all known each other for a long time and like the same music. They are “in their element”. Then the newcomer wants to play different music for the others to dance to, but now they don’t want to. The dancing stops and the party is over.
So, when colleagues are immersed in specifying a building material or in deciding the most appropriate way to repair a defect in a historic building, I would ask them to think about this and always remember that historic buildings dance!
Chartered Building Surveyor