I’m not a builder, I'm an architect. But I've always been fascinated by building construction. I'm also not a writer, but I have a burgeoning interest in words and how they fit together. Both of these subjects have been on my mind recently as I wrestled with a problem involving a roofless Georgian farmhouse. Until recently, it did have a roof, but by the time I joined the project, it had been removed; the timbers condemned as too rotten to be retained. Over the weeks that followed my introduction, many more building defects were discovered, and the project that had started as a simple roof repair grew into a complete restoration.
The causes of the defects were many, but there is no denying that much of it resulted from poor construction. Inadequate foundations had caused movement of the three-storey building above. The walls had deformed for other reasons too: mortar joints in the brickwork had disintegrated, lintels were under-sized, and unnecessary timbers built into walls had suffered from both insect attack and wet rot. Had the hipped roof structure still been present, it would also have been misshapen, partly as a result of the movement of the walls and partly because of its deteriorating condition.
Now, as the team attempted to put the house back together again, we faced the challenge of constructing a new roof on the existing contorted structure. Should the new roof follow these contortions, or should it be built to a perfect shape regardless of its imperfect support? If the latter, how would the different geometries be satisfactorily resolved? And would anybody notice the difference? To answer those questions, we needed to know the extent to which the building was deformed.
First, we looked at the level of the top of the walls. The word 'level' in building construction is a noun, an adjective and a verb. It comes from the Latin word 'libra' which refers to a unit of weight or a pair of weighing scales. You can easily see the connection if you imagine the connecting arm of a set of scales being horizontal as a result of equal weights placed to either side. Using a builder’s level, we found that around the building, the wall level varied by up to 6 inches (or two brick courses). In other words, it varied very noticeably.
Next we wanted to know about the plumb of the walls. 'Plumb' comes from 'plumbum' which is Latin for lead. It refers to the lead weight at the end of a plumb line used to measure the depth of water or the verticality of a structure. The word has associations with measurement and precision and therefore turns up in everyday phrases such as 'plumb in the centre'. We found that the walls were out of plumb by as much as 4 inches, sometimes bowing outwards and sometimes bowing inwards. The new roof would, therefore, have to negotiate this snaking geometry.
Finally, we wanted to know whether our building was true. The word ‘true’ in building construction refers to alignment and shape but finds its root in ancient words that mean ‘honest’, ‘loyal’ and ‘trustworthy’. In this case, we wondered, if the building was a perfect rectangle with opposite sides of equal length and square corners. Using string lines to join the four corners of the farmhouse, and across the diagonals, we established that the building was, at least, a reasonably true rectangle.
Of course, even without the movement that can occur over time, it's impossible to construct a building with perfect accuracy. The extent to which a structure is built out of its specified position is referred to as the construction tolerance. Depending on the situation, the accuracy of construction should be within a tolerance of a few millimetres, say five or ten. But due to the movement of the building, our new roof structure would have to cope with a supporting structure being out of position by much larger margins than normal.
It would be theoretically possible to construct a roof that followed the irregular wall geometry; afterall we can build a roof that resembles almost any organic form if we are so minded. But our subject is a Georgian farmhouse, and it would appear odd if the roof were anything but a regular shape. Following the deformations of the walls would cause each rafter to be a different length, and their ends would have different profiles. It would also mean that the roof planes would be warped, the fixing of the roof slates would be more complicated, and the gutters may not fall in the right direction.
As such, we decided to adopt the alternative approach of constructing the roof to be level and true; the potential visible differences between the geometry of the roof and that of the walls being dealt with in the connecting details. Such are the challenges of a working architect in balancing the practical or economic concerns of construction, with the aesthetic impact of the finished building. Knowledge and experience of those things that draw the eye are required in order to derive satisfactory solutions that mitigate the issue. The word 'tolerance' is derived from the Latin 'tolero' which means to endure or bear pain and hardship. Feelings we hope the viewer will not suffer when the works are complete.
This article was written by Andy Foster for the July ‘20 edition of the Sherborne Times.