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Structures | Refurb at Kew Gardens

Temperatehouse

The world’s largest Victorian glasshouse requires an extensive and delicate £34M restoration.

Each year nearly 2M visitors visit Temperate House, located in the south west corner of Royal Botanical Gardens in Kew, west London.

But it took three directors of the gardens a four-decade-long fight to finally build the world’s largest surviving Victorian glasshouse.

Temperatehouse

Temperatehouse

Aerial view.

First proposed by Sir William Hooker, initial director of the gardens, in the early 1850s, the finished glasshouse did not open to the public until 1899.

Made up of three sections, north, south and central glasshouses, the entire design was an ambitious 191m in length and up to 18m tall.

But money troubles set the project back: construction of the central glasshouse started in 1860 and the scheme quickly burned through £10,000 from the Treasury – equivalent to about £1.2M in today’s currency. Work ground to a halt in 1863, £19,000 over budget.

temperate house restoration 3x2

temperate house restoration 3x2

A tent runs the length of the glasshouses.

Years passed before the north and south glasshouses were added, finally making Hooker’s idea – and renowned Victorian architect Decimus Burton’s design – a reality in the late 1890s. From start to finish, it had taken 39 years. Once open, the ornate cast iron and steel structure housed rare and exotic plants from around the globe.

Time had since taken its toll on the glasshouse. Creepers wound around pillars and pushed against the glass roof. The cool and moist atmosphere required for the rare flora caused delicately carved timber to rot. Outside, gutters became damaged, spilling water on to metal,  corroding intricate Victorian metalwork.

In 2012, Temperate House was placed on the Heritage at Risk register, which identifies important buildings in danger of being lost to decay. Major restoration work was needed to save the Grade I-listed structure.

temperate house 1115 017

temperate house 1115 017

The Victorian structure features ornate steel work.

Construction company ISG was tasked with rescuing Temperate House in 2014. Respect for heritage was key during the £34M restoration.

“What they wanted from us was someone who would…treat it with the TLC that it required,” explains ISG project director Shane Mason.

The challenge was significant. Trees that could be easily damaged by construction work surround the glasshouse. Inside, while most plants could be removed and re-bedded, several trees would need to be protected. As a Grade I-listed building, scaffolding was not allowed to touch the structure, making it difficult to remove and replace the nearly 70,000 components, a range of steel, iron, glass and timber sections, which needed to be taken offsite to be restored.

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temperate house 1115 028

The ground floor view in the central glasshouse. Smaller indoor tents protect the remaining trees.

To further complicate matters, a 1970s structural and aesthetic refurbishment had seen sections of the structure selectively replaced. The refurbishment had happened before the building was given listed status, meaning the 1970s alterations were also protected.

Protection

Lead paint covering the structure caused a pollution risk to the surrounding gardens. To mitigate this, a watertight tent encased each glasshouse, under which work could safely take place. This was held up by birdcage scaffolding underneath, which could support the tent without placing any pressure on the glasshouse.

But the trees inside were still exposed. It was decided that they should be covered in their own mini tents, complete with heating systems and monitors so the Gardens’ horticultural team could keep an eye on them.

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temperate house 1115 030

The massive tent requires extensive scaffolding.

During restoration, the trees’ needs posed their own set of challenges. Restoring part of the structure which could not be removed involved spraying a crushed garnet sand, causing material to settle on top of the interior tents and block the light for the trees.

“When you spray this sand all over the steel structure at high level, it falls down onto that tent and you’ve got to clean the tent off,” says Mason, explaining that a specific level of light needed to be maintained for the trees to stay healthy.

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temperate house 1115 036

Rare skills

Working with traditional materials of the Victorian age, ISG had to source skilled workers such as joiners, painters and experts in lead work for the laborious restoration.

“I don’t think we would’ve been allowed to [use modern techniques] because of the listing,” says Mason.

The primary aim of restoration was to conserve the building, so as the team kept as much of the original material as possible.

temperate house 1115 035

temperate house 1115 035

Water and sand blasting was used to strip components.

However, some rotten timber beams had sections which could not be saved. Instead of removing the whole beam, as little as possible of the damaged wood was cut off and joiners created replica sections designed to fit in with the existing material unnoticed.

“No-one will ever know [the woodwork has been restored] but it’s nice to know that it has been done properly,” says Mason.

But some damage required a more typical construction solution. In the north and south blocks, corrosion damage was discovered to the original Victorian steelwork, encased in masonry. To stop any further damage, cathodic protection was used.

1970s section

During a three-year refurbishment in the late 1970s, parts of the original building were removed and replaced with modern materials, such as aluminium window frames. But as the structure was later heritage listed, the team were restricted to using a light restorative steam clean, rather than a full replacement using traditional materials.

The glazing was different. None of the original Victorian glass remained when ISG began in 2014. Some of the 1970s glass was retained; vertical panes in clerestory windows in the north and south blocks were removed and restored. But much of the glass was beyond restoration, and 5,653m² of new 4mm clear float glass was installed across Temperate House.

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temperate house 1115 037

Each component had to be carefully labelled and logged in a database.

Repainting

Starting at the top of the north block, 69,151 components were removed from the glasshouse and taken offsite to be restored.

On the cast iron and steel segments an ultra-high pressure wash was used at 35,000psi to strip several layers of paint, accumulated over many years, while not damaging the metal underneath.

Although an initial survey had been carried out, cleaning revealed the extent of porosity in some sections. Following repairs where appropriate, the metal was given a light blast with crushed garnet – which is heavier and produces less dust than other blast abrasives – just before the primer coat was added, to make sure the material was in the best condition to receive the paint.

Two more coats were applied for protection before a fourth was added for colour. Once restored each component had to be carefully returned to the exact location it was taken from.

temperate house 1115 051

temperate house 1115 051

Logistics

Two offsite restoration hubs and one storage facility held the many thousands of building components while they were repaired. Sizes ranged from wrought iron rafters weighing several tonnes each, to 12mm fixings. As a result, managing each piece proved to be a logistical challenge.

“Control has been tight and very well managed I would say,” said Mason, explaining how each piece was individually tagged and tracked during its time offsite.

“We’ve had a database which has maintained a register of where every single piece of the building is at any one time, so whether it was offsite, whether it was in our storage area, whether it was onsite or whether it had been fixed on the building. An awful lot of it has now been fixed onto the building; we’ve still got that register running.”

Bringing materials onto site has also posed a challenge. As a working attraction open to the public seven days a week, restrictions were inevitable at Kew Gardens, which has its own constabulary.

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temperate house 1115 061

Timber sections are restored offsite.

“Anything that’s going to affect the gardens we do pre-10am,” says Mason, who gets up around 4.30am to come to site.

Small entry gates prevent articulated lorries from accessing the site, meaning suppliers have had to give more consideration than usual to what it brought to Kew.

Despite these barriers, Mason says communication and relationship building with the 150-strong team proved to be the biggest challenge.

“On top of our team here we’ve probably got as big a team in terms of all the leaders of all the various supply chains as well,” he says.

Project completion

The contractor is on track to leave Kew in December, giving the horticultural team plenty of time to bed-in the plants before Temperate House reopens to the public in May 2018.

Although the team will be onsite until December, the restoration will not be under cover for much longer. The protective tent structure has already been taken down from north and south blocks, and the central block will soon follow.

“We’re unveiling something now, aren’t we?” says Mason. “There’s thousands of visitors coming here [Kew Gardens] every day; fairly soon they’ll be able to see the whole thing.” 

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