Scotch
The Scotch
All roofing carpenters will celebrate with me the wonderful ‘birds mouth’ which neatly clips over the wall plate and is twice skew nailed into position. It was always a joy to pop the roof on a well-built building, especially in the summer. I can see now how departmentalised my thinking was and, that there was a time when the roof was naturally integral to a modular structure. Remarkably, it was only 4 years ago that I encountered my first scotch, but before I lose you in reminiscence over your last fine malt, I’m referring to the medieval carpenters seat joint of the rafter on the wall plate. When I first saw this joint my immediate thought was ‘there’s something wrong here’, I thought ‘the housing in the wall plate could be subject to pocketed water retention leading to rot’ although surveys of timber framed building have not found this to be the case. It also appeared extremely precarious ‘without fixings how would it contribute to the prevention of spread and movement’. By now you will be realising that firstly I am no structural engineer and secondly this is just the typical meeting between two entirely different methods. The traditional timber frame building is able to sufficiently withstand weight and movement and to such an extent that if maintained they last for many hundreds of years. The modern roof is dependent upon components, fixings and ironmongery (that used to oxidise and corrode but have been improved by the introduction of stainless steel) which provide restraints that combat movement for an estimated duration. As with the fixings for tilers lath It is often the components that fail before the timber. To provide context to this subject, I’d like to Tardis you back to a time to a pre- steel industrial era when iron fixings were hand wrought and an expensive commodity. It’s simple really the vernacular timber framed roof/building was designed with a holistic interdependence of evolved joints and frames that required only quality English oak and skilled craftsmen. There you have it , the scotch joint requires no fixings and the foot of the rafter is held into the housing of the wall plate by the weight of the roof. If you’ve found this article interesting I would recommend an invaluable introduction ‘Discovering Timber Framed Buildings’ by Richard Harris for the princely sum of £6.99 used from £1.79
