GLENFARG OAK TRUSSES & PURLINS

Design Brief

Our brief was to construct a roof structure which would be both capable of supporting a traditional slate roof and, as it would be fully visible from the inside be aesthetically pleasing. In addition the room height was to be visually higher than the normal 8 feet found in the rest of the property. The roof pitch, span and wall-head heights were predetermined as it had to match other softwood trusses which were already designed. The length of the room dictated that the minimum number of trusses required would be four and to get a higher visual room height a raised collar truss would need to be adopted. The preferred timber at this point was to be oak.

The Process

We designed our trusses with all the relevant criteria taken into account and after structural analysis and consultations with our clients we finished with an oak structure with a raised collar tie, two arched braces below and two above. The section size worked out at 275mm x 180mm with the centre two trusses having steel brackets fitted for additional strength. These were fitted in such a way as to be virtually invisible from the ground. Once the design had been fully agreed with both the engineer and the client, we ordered all the required oak which in this case came from an estate thinning in Haddington, East Lothian. It turned out that our client knew of the estate, having spent time there in the past, adding to its authenticity.

Due to the wet summer of 2011 we decided to set up a marquee tent for the duration of the project, allowing us to work unaffected by the weather. Once we had the oak in our yard we set up the beams on trestles, sanded each beam with a belt sander before cutting all the required joints. We used 25mm kiln dried oak dowels to secure the frame, all of which we turned. Turned dowels are much stronger than traditional cleaved dowels and being dry they absorb moisture from the green oak beams, creating a tighter finished frame. Our clients were interested in contributing in some way to the frame so agreed to come along to the workshop for a turning lesson. They each produced a finished oak dowel, which hold in place the centre arch braces of the most prominent truss. Once the truss was complete we finished them with a couple of coats of Danish oil which enhanced their appearance. We repeated the process for each of the 4 trusses and then prepared the six purling sets. The purlin’s also in oak 175mm x 150mm were notched into the top chords of the trusses with a shaped support bracket at each joint. They were all stop-chamfered, (a detail which we also used on the main truss sections) sanded and oiled.

When all the component parts of the roof were complete we arranged for them to be transported to the site at Glenfarg. A crane was organized by our clients and with the help of the site joiners all the trusses were lifted into place and secured. Each purling was then lifted into position drilled and secured with a long wooden dowel. The whole structure was erected in around four hours, after which a small oak branch was screwed to the apex and everyone involved toasted the great oak (as is tradition) in this case with a glass of champagne. A traditional softwood roof was then constructed on top of our frame, completely enclosing it and leaving fully exposed inside.