Retrofitting: What Can We Learn From Heritage Projects?

Retrofitting: What Can We
Learn From Heritage
Projects?

In a net-zero world, retrofitting becomes the default choice for all development.
What can conservation specialists teach us about making older buildings work
like new?

Global | Monday, August 14, 2023
Reading Time : 8 minutes

To reach the global goal of net-zero emissions, we need to significantly reduce the carbon associated with the built environment. That means improving the energy performance of our existing building stock – and that retrofitting must become the default approach for the majority of developments, at least in established markets. We can’t afford to squander the embodied carbon in existing structures, no matter how ill-suited they may seem to current needs.

In this context, even outdated and unloved buildings become a precious resource to be conserved. So, who better to turn to than the designers who specialize in extending the useful lives of heritage assets, for whom repurposing has always been the default?

WSP structural engineer Matthew Petticrew has focused on conservation for more than 25 years, and his projects have spanned the ages – from post-war offices to a 900-year-old castle, and even the archaeology beneath the site of the prehistoric Stonehenge. “Older buildings work in a different way to new ones and you have to have a different mentality to deal with them,” he says. “If you've never taken apart an existing building before, you wouldn’t necessarily know where to start.”

As the construction industry gears up for a new era of adaptation, what could it learn?

1. The existing structure is the solution, not the problem

Petticrew got hooked on conservation early, when he was part of a team investigating cracks in a newly completed concrete building. They quickly found the problem: it had been constructed with half the necessary amount of steel reinforcement. But what really intrigued him was how the building was still there at all.

“I worked out that the structure had found its own way of standing up by an alternative load path, and that opened my eyes to a whole new world. When you put up a structure, it can stand up in any way that it chooses. When you apply a code of practice, you’re just demonstrating that there’s at least one way it might be standing up.”

The first, crucial question with an existing building is not how it should stand up, but how it actually does. “Until you’ve worked that out, you don’t know what will happen if you change it. If we want to preserve existing structures, we need to recognize that they’ve been doing their job successfully for years, even if they may not comply with the modern standards we’re used to.”

Older buildings work in a different way to new ones and you have to have a different mentality to deal with them.

Matthew Petticrew Technical Director, WSP in the UK

Matthew Petticrew Technical Director, WSP in the UK

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.”

Planting grass on top of a 900-year-old castle wall absorbed the rainwater and stopped it travelling down into the wall, but didn’t stop it breathing. That’s a very simple, low-energy solution, rather than pumping concrete into the wall to stabilise it.

Matthew Petticrew Technical Director, WSP in the UK

Matthew Petticrew Technical Director, WSP in the UK

4. Be opportunistic

So how do you improve the energy performance of a building that is permeable and draughty by design? Typically, solutions for existing buildings tend to be a lot more bespoke. Before he joined WSP, Petticrew worked on the adaptation of an 18th-century school to become a BREEAM Outstanding university building. It was category A-listed, the highest grade, which meant that any alterations had to be sensitive to its character.

There, the team created ventilated cavities at the back of the thick walls, using wood fiber insulation that would allow the building to breath. “That created an air pathway that sucked moisture from the existing walls and vented it at high level. But that created another puzzle we had to solve, because fire and smoke could travel up the cavity and spread to other floors. You have to block the cavity to prevent that – but then it doesn’t work for ventilation.” Their solution was to add intumescent strips at the edge of every floor, which swell when in contact with hot gases, closing off the cavity.

At another old university building, the challenge was where to put new building services, without damaging the finishes, floors or walls. “We discovered that we could stick the air conditioning units into the fireplaces, and we used the old flues to draw air in and out. You have to get creative and exploit the opportunities that the old building presents to you. Sometimes we need to relearn old strategies and use them again. It's not always new technology that gives you the answer.”

At the 900-year-old castle in southern England, water leaking into the walls was dissolving the lime holding the rubble and stone together. “We planted grass on the top of the wall, so it absorbs rainwater and stops it travelling down into the wall, but doesn’t stop it breathing. That’s a very simple, low-energy solution, rather than pumping concrete into the wall to stabilise it.”

5. Remember we’ve been here before

Carbon has only recently become a parameter for building design. But the need to use materials efficiently has waxed and waned throughout the history of construction. After World War II, materials and expertise to rebuild European cities were in short supply, but there was a glut of unskilled labour as demobbed soldiers returned to civilian life.

“That had a profound influence on the way that buildings were created and designed from the 1950s to the 1970s. The industry started doing a lot of offsite prefabrication, and we designed formwork that took a long time to build but used as little material as possible.” This is the origin of the concrete waffle slabs that have become the star of some of London’s most desirable adaptive reuse offices. This feature is now being reprised on experimental structures designed by algorithms and built by robots to place material only where it is needed.

From the 1980s until the early 2000s, labor became expensive and materials cheaper – resulting in simple, thick slabs poured on site. “We need to turn the clock back to that post-war period and revitalizing some of those methods.”

This perhaps goes to the heart of what older structures can teach us: carbon may be a new design parameter – but making the best use of limited resources is a problem as old as building itself.