There are, says English Heritage chief structural and civil engineer Terry Girdler, three fundamental principles by which he and his 12 strong team evaluate proposals for work on listed buildings and scheduled monuments. 'Minimum intervention, maximum retention of the original, and reversibility, ' he explains.
'It's not always possible to achieve all three at the same time or at an economic cost, of course. And you have to be practical - the most reversible and cost effective solution to a sagging beam is usually an Acrow prop, but that's hardly ever the best option.'
Usually evaluating applications for approval of works on listed buildings is dealt with at local level within EH's nine regions, by EH's historic inspectors. The Savile Row team normally only gets involved when major engineering is involved.
And for the last five years, it has been responsible only for the more sensitive Grade II* and Grade I buildings. Girdler also has responsibility for the buildings and monuments actually owned by EH, such as the Wellington Arch (see box), which recently underwent a major change of use.
Around 1,000 proposals a year cross his team's desks, of which 30% involve EH-owned properties. Economic viability is taken very seriously, says Girdler, which means that sometimes 'change of use' actually means demolition. 'It's rare for us not to find a technical solution, ' he adds. 'But sometimes the only possible technical solution just costs far too much and demolition may be the only answer.'
A typical scenario might involve a developer wishing to add a lift motor room to a listed building to convert it into a hotel or retirement home. This could overload the foundations, which would need underpinning, and render the whole project uneconomic.
On the whole, EH prefers to 'conserve as found', unless it transpires that a later insertion is threatening the original fabric, or that an earlier repair was done badly. Some conservation projects from 20 or 30 years ago are now highlighted as examples of how not to do it, hence the current stress on reversibility.
The most significant technological development in the last few years, says Girdler, has been the refinement of the diamond core drilling, tie insertion and grouting package which has successfully stabilised many decaying structures. 'What I would really like to see, ' he adds, 'is the development of a 'PropertiesOmatic'meter which you hold against a structure and it gives you all the parameters you need for analysis.
'Even better would be the Star Trek version, which would carry out a virtual load test for you.'
Making the grade Statutory lists of buildings of 'special architectural or historic interest' are compiled by the Secretary of State for Culture Media and Sport on the recommendations of English Heritage. There are three grades Grade I: buildings of exceptional interest Grade II*: particularly important buildings of more than special interest Grade II: buildings of special interest, warranting every effort to preserve them.
Buildings can be listed because of their age, rarity, architectural merit, history, or method of construction. Currently around 500,000 buildings are listed, of which more than 90% are Grade II.
Joining the cast
Despite its official preference for traditional materials and methods, the EH engineering team is prepared to adopt the very latest technology on occasion.
One such example is the world-famous Iron Bridge in Shropshire. When cracking was discovered in the parapet railings of the 18th century structure, the centrepoint of a World Heritage Site, the first move was obvious - find out just how safe the railings were.
The Transport Research Laboratory was commissioned to carry out a series of full scale load tests on a replica section of railing, accurate down to the exact composition of the cast iron itself. The results indicated that in perfect condition the railings were remarkably strong, but past fracturing and earlier unreliable repairs had taken their toll.
It seemed that the posts supporting the railings would not be able to take maximum likely crowd loadings, even though these practical maximums were significantly lower than those specified in the relevant British Standard.
Making them safe without radical changes to the original structure would not be easy.
Cast iron is notoriously difficult to weld insitu, even with the latest nickel welding technology. Instead, EH turned to the aerospace alternative.
Carbon fibre reinforced epoxy plates only 3.8mm thick were bonded to the inside faces of the posts. Once painted, these strips were virtually invisible, leaving the structure looking much as it did more than 200 years ago.
Wellington Arch revisited
The reopening last year of London's Wellington Arch after a £2M transformation into a visitor centre marked the completion of the second most expensive restoration project ever undertaken by English Heritage. EH chief conservation engineer Terry Girdler says it is the perfect example of the maxim that the best way to preserve a listed building is to find a proper use for it.
'Its last use was as London's smallest police station, but after this closed in the 1960s the Arch suffered from neglect and began to decay, ' Girdler reports. 'A survey carried out in 1997 found it be in a very bad state, so something obviously had to be done. But turning it into a visitor centre had its own problems - not least the question of disabled access.'
Historical records showed the structure had had a chequered past. Started in 1827 as a grand entrance to Green Park and designed by Decimus Burton, it was still unfinished due to lack of funds when in 1838 a committee seeking to commemorate the victory of Waterloo virtually hijacked the project. Amid much controversy, eight years later the world's largest equestrian statue - featuring the Duke of Wellington - was erected on top of the Arch, where it remained until 1883.
By then traffic congestion at Hyde Park Corner had reached saturation level.
The arch had to be moved to make room for a major road widening and realignment. The controversial 40t bronze statue - described as stiffly composed and oddly proportioned - was banished to Aldershot, and the arch re-erected on its present site looking down Constitution Hill.
The last major change was the 1912 strengthening of the roof to take the 38t weight of the largest bronze sculpture in England, Adrian Jones' monumental Peace descending on the Chariot of War.
Like its sister, the Marble Arch to the north, the Wellington Arch had served as a police station and section house since the 1850s.
Both were finally abandoned when road improvement schemes in the 1960s isolated both arches on islands surrounded by seas of traffic. But by the time Girdler and his team began their conservation project in 1999, new pedestrian crossings at Hyde Park Corner had turned the Wellington Arch island into a key element of the route between Hyde Park and Green Park, and made a visitor centre a logical function for the arch itself.
EH's plans included the creation of three floors of exhibition space and viewing platforms over the porticoes. Girdler's main concerns centred on the lintels spanning the central openings. He explains:
'Obviously these couldn't be built in stone without support or reinforcement, the spans were far too great.
'We had to find out how they worked as structures. An ultrasonic survey produced interesting but inconclusive results.
In the end we took one section apart to get the answers.'
What they found was a tribute to the ingenuity of 19th century structural engineers. Inside the lintels were iron beams, with (good in compression) cast iron top flanges and webs and (good in tension) wrought iron bottom flanges.
Much to the EH team's delight, these beams turned out to be in excellent condition. 'The wrought iron hangers which transferred the weight of the stonework to the beams were quite badly corroded, ' says Girdler. 'But luckily these could all be repaired insitu, so intervention was minimal.'
Some 21st century engineering ingenuity was needed to tackle the other major structural worry, an alarming diagonal slip plane crack in one of the arch's Corinthian columns.
'We looked at around 30 repair options before deciding on a system of stone stitching developed in-house, ' Girdler explains. 'Portland Stone plugs along the slip plane solved the problem without compromising the appearance of the column, a solution that is entirely in keeping with our basic conservation philosophy.'(See diagram) And the provision of disabled access, which at the outset had seemed like a major challenge, turned out to be relatively simple.
An hydraulic lift was shoehorned in, making it possible to reopen the arch last year with an exhibition that featured the Iron Duke's own original Wellington boot.