Conservation Insights
A series exploring the history, significance, and technical care of historic buildings, sharing the challenges and solutions that preserve architectural heritage for the future.

03. Syon House Facade: Understanding the Hidden Causes of Stone Façade Failure

Syon House is perhaps best known for its extraordinary Robert Adam interiors, but much of what we see today from the exterior dates from a later chapter in the building’s history. In the 1820s, the 3rd Duke of Northumberland embarked on a programme of major works at Syon. Thomas Cundy senior was commissioned to undertake works to the House, including the re-facing of its exterior in Bath Stone, while Charles Fowler designed the large conservatory to the north. The Bath Stone façade therefore represents an important layer in the history of the building: a substantial 19th-century transformation of an earlier house.

Syon remains in the ownership of the Dukes of Northumberland and is the largest suburban London mansion still in private ownership. It is open to the public, while also continuing to accommodate family life, events and filming. Maintaining the building therefore means balancing the conservation of a highly significant historic structure with its continued use. It was the concealed construction of the Bath Stone façade, however, that became the focus of a major programme of conservation.

The Conservation Challenge

The Bath Stone is approximately 100mm thick and was fixed to the underlying structure using wrought-iron cramps. These relatively simple pieces of metal perform an important structural function, tying the stone veneer back to the building.
Iron cramps were widely used in historic masonry construction, particularly during the 18th and 19th centuries as masonry cladding became more common. The problem is that iron does not remain inert when concealed within porous masonry. Over time, moisture can reach the metal, causing it to corrode. As the corrosion products expand, they exert considerable pressure on the surrounding stone. This process, known as rust jacking, can fracture and spall the masonry around the concealed fixing. Importantly, the cramp itself may remain invisible until significant damage has already occurred.

At Syon, the façade had consequently received piecemeal repairs over many years. While these interventions addressed individual areas of damaged stone, they did not remove the underlying cause. Each new failure could require complex access and disruption, while the location and extent of the next failure remained uncertain. For a building of Syon’s scale and significance, this presents more than a conservation challenge. Unpredictable stone failure creates risks to people, property and the continued operation of the House.

Looking Beyond the Visible Damage

The project therefore presented an opportunity to move away from a cycle of repairing individual failures and address the underlying problem. The approach was to remove the affected Bath Stone completely, remove the defective ferrous cramps and replace them with modern stainless-steel fixings before reinstating the stonework. In doing so, the work addresses both the visible damage and the concealed cause. The intention is not simply to make the façade look repaired, but with the intention of providing a durable solution for the next 200 years and beyond. This approach also creates opportunities to investigate and repair elements hidden behind the façade, including concealed window construction, and to rationalise or renew external surface-run services such as soil vent pipes where appropriate.

Getting the Stone Right

Replacing historic stone is not simply a matter of finding something that looks similar. The replacement Bath Stone has been sourced from Stoke Hill Mine outside Bath, selected for its compatibility with the original stone in colour and texture. Samples were supplied and assessed before 2m x 2m mock-up panels were constructed to test the proposed jointing, finish and overall appearance. The process then moved from the mock-up panels to a trial area on the building itself. This allowed the team to test the actual methodology at full scale and refine the approach before committing to the main phases of work.

The attention to the stone extends to its preparation. The original stones were likely honed in situ to accommodate a substrate that was not perfectly flat or plumb, contributing to the remarkably narrow joints and almost seamless appearance of the façade. Reproducing this character with new stone requires considerable precision. Rather than relying on extensive in-situ honing, the current approach minimises this through detailed surveying, careful cutting and dressing of the replacement stone.

DJA’s involvement has extended beyond specifying a material: David visited the quarry to understand the stone at source and how it is extracted, while the replacement stone is being hand-worked to recreate the character of the original façade. The specialist contractor was also an important part of the approach. The selected contractor is based in Northumberland and has previously undertaken work for the estate at Alnwick Castle, bringing relevant experience of working on significant historic buildings for the Estate.

Testing Before Committing

The trial area was particularly valuable because it allowed several aspects of the work to be considered together: the stone itself, the fixing methodology, jointing, finishing, craftsmanship, access and the practicalities of working on an occupied and operational estate. This is especially important on a project that will be delivered over a number of phases. Phase 1 was completed in early 2025, Phase 2 is being delivered in 2026 and the project is now in Phase 3, with further phases planned over the coming decade. The phasing reflects not only the considerable scale of the House, but the need to manage funding, minimise disruption to events, family life and filming, coordinate the availability and travel of specialist contractors, and manage the quantity of new stone that needs to be quarried and stored. The courtyard façades will ultimately require recladding too, extending the programme beyond the principal elevations. The logistics of conservation are therefore as important as the technical methodology. Temporary accommodation has been created within the grounds for the specialist contractors, while a carefully planned logistics strategy allows the estate to continue operating as far as possible during the works.

Conservation at the Cause, Not the Symptom

Syon House demonstrates why understanding how a historic building was constructed is fundamental to conserving it. The visible problem is damaged Bath Stone. The underlying problem is concealed ironwork. Replacing a piece of stone without understanding why it failed risks simply beginning the cycle again. By understanding the construction, identifying the cause of deterioration and testing the proposed approach before progressing to the main works, the project seeks to replace a pattern of reactive repairs with a long-term conservation strategy.

Conservation Insight

Effective conservation begins with understanding that relationship between construction, material and failure. Addressing the root cause can require more intervention than simply repairing what is visible, but it can also provide a more durable and sustainable outcome.

Learning from London’s Buildings: Summer Visits Across the Capital

As summer gets underway, members of the Dannatt Johnson Architects team have taken the opportunity to step away from the drawing board and explore a number of remarkable places across London. From major cultural projects under construction to temporary architecture and local maritime heritage, these visits offered valuable insights into design, conservation and the stories that shape our built environment.

Looking ahead to the New London Museum

We were delighted to join a hard hat tour of the new London Museum at Smithfield, where work continues on one of the UK’s most significant cultural projects. Although we are unable to share photographs from the visit, it was a privilege to see the progress being made behind the scenes and to hear directly from the client, architect and contractor teams about the vision for the museum. Learning more about the layered history of the Smithfield site, alongside the ambitious programme of adaptation and new intervention, reinforced the importance of balancing conservation with contemporary public use. We look forward to seeing the museum open its doors later this year and experiencing the completed transformation.

Exploring the 2026 Serpentine Pavilion

We were also pleased to accept an invitation from Wienerberger to visit the 2026 Serpentine Pavilion, a serpentine, designed by Mexico City-based LANZA atelier. Marking the twenty-fifth Serpentine Pavilion commission, the design takes inspiration from the traditional English crinkle-crankle, or serpentine, wall, using its undulating geometry to create both structural efficiency and richly varied spatial experience. Constructed predominantly from brick, the pavilion establishes a thoughtful dialogue with the adjacent Serpentine South Gallery while demonstrating how a familiar material can be reinterpreted in a contemporary architectural language.

The visit provided an excellent opportunity to reflect on how architects respond to an open design brief, and how materiality, structure and landscape can work together to create an engaging public space. It also prompted interesting discussions about permanence and temporality in architecture, and the ways in which a temporary intervention can make a lasting contribution to architectural discourse.

Discovering the Golden Hinde

Closer to home, members of the practice joined a private tour of the Golden Hinde, organised by Better Bankside. Situated just a short walk from our office, the Golden Hinde is a familiar local landmark, but the tour offered a fascinating insight into both Sir Francis Drake’s original sixteenth-century vessel and the story behind the faithful modern reconstruction that now occupies the site. The visit also provided an opportunity to meet fellow Better Bankside members and strengthen connections within our local community. As architects working in Southwark and across London, understanding the history and character of the places around us is an important part of our work, and these opportunities to engage with our local context are always greatly appreciated.

Staying Curious

Whether exploring a nationally significant museum before completion, experiencing one of the world’s best-known annual architectural commissions, or rediscovering a local historic landmark, each visit offered a reminder that learning extends well beyond the studio. Engaging directly with buildings, places and the people behind them continues to inform our approach to design, conservation and the stewardship of historic environments.

Conservation Insights
A series exploring the history, significance, and technical care of historic buildings, sharing the challenges and solutions that preserve architectural heritage for the future.

02. Laurie Groves Baths: Looking After the Roof That Protects a Century of Social History

For generations of south Londoners, Laurie Grove Baths was far more than a municipal swimming pool. Opened in 1898 under the Public Baths and Wash-houses Act of 1846, the Grade II listed building was conceived as an investment in public health, providing swimming pools, slipper baths and laundries for a rapidly expanding industrial community. At a time when many households lacked bathrooms of their own, it offered something we now take for granted: the opportunity to wash, swim and maintain personal dignity.

Designed by local architect Thomas Dinwiddy in a confident Jacobean style, the building has continually adapted to changing needs. Its pools have hosted dances and concerts, welcomed the Anglo-Caribbean Association during a period when many venues excluded Black Londoners, and today provide studio space for students at Goldsmiths, University of London. Few civic buildings encapsulate so much of London’s social history within a single structure.

Yet, while the richly detailed façades attract attention, it was the roof that presented the greatest conservation challenge.

The Conservation Challenge

Historic roofs rarely fail because of one dramatic event. More often, deterioration begins gradually through blocked gutters, slipped slates, failed flashings and areas that are simply too difficult to inspect safely.

Laurie Grove Baths is no exception. Behind the prominent street elevations lies an intricate roofscape of pitched slate roofs, glazed lanterns, patent glazing, lead valleys, parapet gutters and concealed drainage channels. Over more than a century, routine maintenance had become increasingly difficult, leaving vulnerable areas inaccessible and allowing small defects to develop into larger maintenance issues.

Rather than focusing solely on repairing individual defects, our approach considered how the building could be better cared for over the decades to come.

Our Approach

The project combined urgent fabric repairs with carefully considered improvements that would support the building’s long-term maintenance while respecting its listed status.

Works included repairs to slate roofs, leadwork, asphalt coverings, roof lanterns, patent glazing and rainwater drainage systems, alongside the repair of defective flashings, blocked outlets and weathered detailing. Existing dormers were re-clad using traditional lead detailing more appropriate to the character of the building, while improving thermal performance.

Perhaps less obvious, but equally important, was the introduction of permanent maintenance infrastructure. New access ladders, roof walkways and discreet fall protection systems were designed to allow future inspection and cleaning of gutters, valleys and rooflights without risking damage to fragile historic fabric or compromising the building’s appearance. Located away from public views and finished in dark colours to minimise their visual impact, these interventions demonstrate how contemporary requirements can be integrated sensitively into historic buildings.

Like many successful conservation projects, much of the work is almost invisible to visitors. That is often the measure of success: preserving historic character while quietly improving performance, safety and longevity.

Lessons from the Project

One of the enduring challenges of conserving historic buildings is recognising that maintenance is itself a form of conservation.

A well-designed maintenance strategy can prevent the need for far more extensive interventions in the future. By improving access, simplifying routine inspection and addressing the causes of water ingress rather than simply its symptoms, projects such as Laurie Grove Baths help ensure significant buildings remain both usable and resilient.

For us, conservation is rarely about freezing a building in time. It is about understanding how it works, respecting its significance and making thoughtful interventions that allow it to continue serving new generations. Laurie Grove Baths has already lived many lives; as a public bathhouse, community venue and university building. Sensitive conservation helps ensure it is ready for whatever its next chapter may be.

Conservation Insight

Historic buildings rarely deteriorate because of a single failure. More often, defects begin with routine maintenance issues: blocked gutters, slipped slates or failed flashings. Designing buildings so they can be safely inspected and maintained is one of the most effective forms of long-term conservation.

David Johnson recently travelled to Kotor, Montenegro, as part of his ongoing work with the Organization of World Heritage Cities (OWHC). Accompanied by OWHC Secretary General Mikhaël de Thyse and Michal Krazucki, Director of the Warsaw Heritage Protection Department, David took part in an advisory mission focused on strengthening disaster risk management for Kotor’s UNESCO World Heritage Site.

The workshops, held on 18–19 June, brought together local stakeholders to review and update the city’s risk assessment for threats including fire, earthquakes and overtourism, and to develop a practical action plan. As Consultant for the OWHC’s Assistance Program in Case of Emergencies, David contributed to discussions that will support the implementation of a pilot project to create a strategic map and emergency evacuation plan for St. Trophimus (Tryphon’s) Cathedral and the Kotor Museum; the first project of its kind within the OWHC network.

The progress of this pioneering initiative will be presented at the 18th OWHC World Congress in Marrakech later this year, helping to share best practice in protecting historic cities and their cultural heritage.

Conservation Insights
A series exploring the history, significance, and technical care of historic buildings, sharing the challenges and solutions that preserve architectural heritage for the future.

01. Tollgate House, Hampstead Heath: Preserving an 18th-Century Landmark

On the eastern edge of Hampstead Heath, opposite the iconic Spaniards Inn, stands a small square building that quietly marks a piece of London’s transport history. The Tollgate House, a late 18th-century Grade II listed building, was originally the gatehouse where travelers paid tolls to enter land owned by the Bishops of London. Its painted brick walls, brick dental course, and ceramic tile hipped roof, along with a vaulted lower ground floor, reflect its age and functional origins.

The building appears on John Rocque’s 1746 map of London alongside the Spaniards Inn, showing a dotted line across the road representing the gate that barred passage until tolls were collected. Travelers heading into the Bishops’ land (from what we now call Hampstead to Highgate) would pass through the toll and immediately find themselves on a better-maintained road. The road to the east widened shortly after the gate, a detail that survives in the road layout today.

A Narrow Road Through Time

Despite the increasing traffic over the centuries, the Tollgate House has remarkably survived. Its location at a narrow bend opposite the inn has made it the focus of multiple proposals for removal, dating back to a 1922 report in the Hampstead and St John’s Wood Advertiser. Later, in 1961, the London County Council proposed demolition to improve traffic flow, and the debate even reached the House of Lords in 1966, when Lord Colwyn questioned whether the gate should be removed to ease congestion. Ultimately, the toll gate survived, and in 1974 it was granted Grade II listing, securing its protection under the care of Camden Council. A commemorative plaque now marks its historical significance, and a black-and-white bollard installed in 2008 protects the building from passing traffic.

Our Conservation Approach

By the time we were commissioned, the Tollgate House was unoccupied, unserviced, and suffering from significant damage. The number of vehicles passing had caused deterioration to the high-level brickwork, rainwater goods, and roof, allowing water ingress and fabric decay. Our work focused on returning the building to a weather-tight, structurally consolidated state, while respecting its historic character:

External Repairs: Brickwork repairs, repointing, and decoration of all elevations and chimney.

Roof and Rainwater Management: Structural consolidation of damaged roof areas, replacement of roof tiles to match existing, and a more robust gutter and downpipe system designed to withstand potential vehicle impacts.

Internal Conservation: Repointing and crack repairs to internal brickwork, including the vaulted basement ceiling and floors, and repair of timber doors, windows, and shutters. Low-level snorkel vents improve natural ventilation to aid drying of historic fabric.

Drainage Improvements: Installation of perimeter channels to manage groundwater ingress.

The majority of the work was like-for-like repair, with sensitive upgrades to ventilation and drainage ensuring durability while preserving the building’s historic character.

Impact and Legacy

The project achieved more than structural consolidation. By returning the building to a weather-tight state and improving internal conditions, the Tollgate House was removed from the Heritage at Risk register and Camden Council was able to let the building commercially; helping to ensure its continued use and preservation.

Tollgate House reminds us that heritage conservation is not just about preserving bricks and mortar; it’s about protecting stories, patterns of daily life, and landmarks that have shaped the way Londoners move through the city for centuries.

Conservation Insight

Historic buildings often deteriorate because the original causes of damage remain unresolved. Effective conservation is not simply about repairing historic fabric, but understanding why it failed and designing discreet interventions that prevent the same problems occurring again.

David Johnson, as a part of his ongoing work with the Organization of World Heritage Cities (OWHC), recently had the opportunity to travel to George Town, Malaysia for the ‘We Are Site Managers’ conference, where he was a key note speaker for Session 13: ‘Resources for Site Managers: Disaster Risk Preparedness and Dealing With Emergencies’. He introduced attendees to the ‘Emergency Programme’ developed by OWHC which aims to assist member cities and their site managers to identify risks, explore methods on natural and man-made disaster prevention and longer term management with a view to protect heritage sites worldwide.

We recently presented our proposal for the transformation of the Nautical Museum in Castletown on the Isle of Man to the Tynwald and various stakeholder engagement days. Interviews with Project Lead, Jon Parry, together with our animated walkthrough can be found here and here

The project combines conservation and repair of the entire historic Nautical Museum site with an architecturally distinct, sustainable and sympathetic design for a new-build display boathouse for the historic ship, The Peggy. The completed project will transform access & interpretation, combining historic collections, vibrant new interactives and displays to provide a world class, all weather attraction for Castletown, offering engaging and enjoyable experiences that will embrace and inspire visitors of all ages and abilities.

The proposal combines the highest standards of conservation and adaptation of existing historic buildings together with innovative and accessible approaches to exhibition design, visitor interaction, understanding and enjoyment.

Dannatt Johnson Architects lead a design team including Steensen Varming and AECOM, with Project Management from Fraser Randall. This team delivered our recent award-winning collaboration at the Imperial War Museum Second World War and Holocaust Galleries. Exhibition Designers Real Studios have also worked along side us, following our previous work together for the National Museum of the Royal Navy.

We are proud to be working with Manx National Heritage on this prestigious and significant site, and look forward to the exciting possibilities and opportunities that this complex and transformational project will provide. The project is currently on hiatus, while funding applications and fundraising activities are undertaken. In the meantime, we are pleased to share some of our team’s concept images for the project, presented to the Tynwald and stakeholders in recent days.

We recently enjoyed a tour of The Palace of Westminster (Houses of Parliament), kindly escorted by our ex-colleague who, having worked at DJA on projects including at Chiswick House and Dover Castle, is now involved in Strategic Estates at the Palace. It was a wonderful opportunity to hear about the large projects undertaken and overseen by her team, such as the recent refurbishment of the Elizabeth Tower; and the history of the site, as well as appreciating the variety of architectural details throughout.

DJA are proud to be one of 200 businesses taking part in the Mayor of London’s Business Climate Challenge (BCC) 2023. This programme is funded by the Mayor and works with London businesses to help them prepare for the green transition and future proof their organisation, and help them become net-zero by 2030. By committing to decreasing their energy usage by at least 10 percent a year, we will receive technical support to help us operate more efficiently, reduce energy consumption, and play our part in tackling the climate emergency.

As a practice, sustainability is already at the heart of what we do - adapting existing buildings, rather than building new is one of the greatest contributions we can make within the construction industry. We also strive to reflect this within our workplace: all of our energy is from renewable sources; all staff commute via public transport, on foot or by bike; and we operate a zero-to-landfill initiative. Having access to the support and resources provided through the BCC will push us even further in our commitment to sustainability and protection of the environment.

You can see a full list of participants and learn more about the scheme here.

We are delighted to announce that our recent competitive bid to deliver the transformation of the Nautical Museum in Castletown on the Isle of Man was successful.

The project combines conservation and repair of the entire historic Nautical Museum site with an architecturally distinct, sustainable and sympathetic design for a new-build display boathouse for the historic ship, The Peggy. The completed project will transform access & interpretation, combining historic collections, vibrant new interactives and displays to provide a world class, all weather attraction for Castletown, offering engaging and enjoyable experiences that will embrace and inspire visitors of all ages and abilities.

The proposal combines the highest standards of conservation and adaptation of existing historic buildings together with innovative and accessible approaches to exhibition design, visitor interaction, understanding and enjoyment.

Dannatt Johnson Architects lead a design team including Steensen Varming and AECOM, with Project Management from Fraser Randall. This team delivered our recent award-winning collaboration at the Imperial War Museum Second World War and Holocaust Galleries. Exhibition Designers Real Studios are also joining us following our previous work together for the National Museum of the Royal Navy. We also look forward to working alongside Island based architects Horncastle:Thomas and quantity surveying practice Magee & Co.

We are proud to be working with Manx National Heritage on this prestigious and significant site, and look forward to the exciting possibilities and opportunities that this complex and transformational project will provide in developing a new all-weather heritage visitor experience for the island. In the meantime, we are pleased to share some of our team’s concept images for the project which formed part of the tender submission.