C O N T E X T 1 2 4 : M A Y 2 0 1 2 9 and that sprockets are part of a roof construction as well as in the gearbox of a car. People dealing with old structures need four things: a good understanding of how old buildings were put together; common sense; experience; and the patience to spend a great deal of time inspecting and researching the problems in hand. Each of these elements leans heavily on the other three. For example, common sense tells us that if pointing falls out, something harder will last longer. Experience tells us that it is rather more complex than that. Sadly, common sense is not all that common. Another part of the training for me and my five fellow student draughtsmen was three months on a building site.The other five went to new construction sites. I was sent to Number 2 Carlton Gardens, a then bombed-out John Nash house of around 1828 (now listed Grade II*) facing on to the Mall in London. I seem to recall being rather peeved by this. But how valuable that experience turned out to be in the long run. The decaying building, which had stood completely open to the elements for some 17 years, had massive outbreaks of dry rot, several levels of bonding timbers and the remains of a stone cantilever staircase. Cantilever staircases have fascinated me for many years, and this early introduction to them was invaluable. Although the bottom couple of treads were no more than stubs in the wall and the next few treads were damaged, the contractor’s staff used the stairs quite regularly. The built-in bonding timbers (if you do not knowwhat these are, please investigate further as you never know when you might come across them) were totally rotted. As they are completely enclosed by the wall material, they cannot be seen but being timber they provide a veritable motorway for dry rot to spread around the building.These much-decayed timbers had to be removed in the fashion of an underpinning exercise, short lengths at a time, and the resulting holes bricked up. The floors had mostly long since disappeared, so they were replaced in reinforced concrete, as was the stone staircase.There has clearly been a change in conservation policy since those far-off days. I do not recall what was done to the missing roof structure. I suspect that I had been sent back to the office by that stage. The dry rot was treated by flooding the whole of the wall construction with appropriate fluid. Even though the outbreak was so widespread, I doubt that such drastic treatment would be carried out today. Introducing concrete into historic buildings is generally not a good idea except for underpinning. Using modern materials in historic fabric can be risky if the long-term effects are not fully understood. I am reminded of a new wonder reinforcing material for concrete that I investigated in the 1970s. It worked well for about 10 years, then disintegrated suddenly because of some defect that no one had anticipated. I prefer to use the tried-and-tested technology of days gone by whenever possible. If we can copy something that worked well for a very long time, we know we are safe. Resins should be used sparingly and with great care in historic building work. In timber there is a risk that a flexible structure may be stiffened. Many of us are concerned about the longevity (remember we are thinking hundreds of years) of resins used as timber adhesives. A couple of years ago my wife and I went to a village church in Scotland for morning service, as we do whenever we are in that area.A newminister was leading the service, and as we left my wife asked her how she was settling in. She replied that she was settling in well but (even though she had no idea who I was) she said that there were major dry rot problems with one of her other churches and that repairs were going to cost £250,000. As the sermon had been about the various ways in which God speaks, I felt obliged to ask where they had got that advice – feeling that I already knew the answer. The upshot was that I visited the building, saw that there was a problem needing attention and advised that they seek advice from an expert timber surveyor. This was done and the work completed for about £15,000. Lessons 1.Simple techniques are usually the best when dealing with historic structures. Calculations often prove that the structure should never have stood up in the first place. This shows that the calculations are wrong or, more likely, that the wrong calculations are being done. If a structure has worked satisfactorily for many years, that is a guide as to how it should be maintained and repaired. 2.Understanding how the building is put together helps understand what makes it stand up and what might be wrong with it. 3.We never know where our lives are headed and when experiences will become valuable. 4.Do not let anyone tell you that cantilever stairs do not work and need to be demolished, or that you need calculations to prove they work. Meaningful calculations can not be done. 5.Dry rot outbreaks should be investigated by expert consultants rather than by a company with a product to sell. Ian Hume teaches conservation engineering. He was chief engineer at English Heritage from 1987–97 and a conservation engineering consultant until 2008. The remnants of a cantilever staircase at Penicuik House, Lothian, Scotland A fine example of a cantilever staircase (location forgotten)
RkJQdWJsaXNoZXIy MjgyMjA=