2. Use your eyes, not your calculator
Solving the puzzle of an existing building comes down to detective work. Petticrew learned to pick up clues from watching building demolitions and learning from experienced engineers on site visits. Heavy rainfall will reveal the path of chimney flues in a brick wall, as the thinner sections dry out first; spiders like to spin webs on dry, stable surfaces. In the UK, buildings that predate the opening of the Forth Bridge – Britain’s first major steel structure – are almost certainly iron, whereas buildings completed after the 1890s are probably steel. “If you know when those thresholds in time are, you can make assessments about how a building was put together with a high degree of certainty, so you don’t have to pull it apart or carry out tests that could damage it.”
World War I is another important threshold in Europe, because so many mature forests were felled. Before the war, buildings would have been made from the more resilient heartwood at the centre of an older tree; later timber structures are almost certainly made from the softer, more insect-prone, sap wood around the outside. This is why older timber buildings don’t always make sense in the context of modern structural engineering calculations, which are based on design codes optimised for sap wood.
Cracks are clues, but not all of them are important. Petticrew can tell from close examination what kind of movement has caused them, for example from the shape and whether the edges are jagged or clean. “You can make the wrong choices if you’re only focused on the cracks themselves and you don’t realise what they’re telling you. It’s the significance of the crack that matters – is it in a component that’s on the load path? Then that’s interesting and we need to know whether that component is going to become compromised.”
For heritage buildings, Petticrew often has little more to go on than a rough date and the evidence of his own eyes. But close observation is equally important on more modern buildings, for which you might expect there to be more information. “There still might not be any drawings and even when there are, there’s no guarantee that’s how something was actually built. They’re only ever useful information. My default position is always to trust what I see in front of me.”
3. Follow the water
One of Petticrew’s tried-and-trusted rules of thumb is to look very early on at the key locations where water could enter: for example, gutters, rainwater pipes, parapets and roof valleys. “It doesn't matter whether the building’s 500 years old or 50 years old, water is always the biggest danger,” he explains. “If it’s going to be bad anywhere, that’s where it will be worst. Conversely, if the areas where water is likely to get in are in a good condition, then so are the places where it isn’t.”
Moisture is another reason why we can’t just apply today’s low-carbon solutions to older buildings. Modern walls are built with a cavity, which collects water and redirects it out through weep holes. Older walls are thick and heavy, and work by absorbing and releasing moisture. If this process is not understood, with construction details arranged accordingly, there is a risk of unintended consequences that can damage the wall and cause damp and mould inside the building.
Similarly, when cracks in breathable lime mortar are repaired with modern cement, the result can be to weaken rather than strengthen a structure. “If the water can no longer get out through the lime mortar because it’s blocked by the cement, it’s forced through the brick or stone, and that makes the brick or the stone crack and spall and come off, making the problem worse.”