Attic Truss Design for Old Cottage

[mondo45]

G'day! We have a house in the country, some 2 hours SW of Sydney, the old part of which was built, we think, some time in the 1880s. The original cottage was three rooms - bedroom, living room, kitchen with overall dimensions of 9.9m by 4.6m. This is now one room.

The construction of this part is post and beam. The posts were originally tree trunks that were planted 4 feet into the ground, and squared off above ground level to 200mm by 200m. There were 200 by 200 floor level beams as well around the perimeter. Joists were round tree logs, adzed flat to take the floor. There were stumps supporting tree log bearers as well. At 2200 mm above the floor, there are 125 by 125 beams with a 20mm slot cut into the middle of the lower side. The walls are ironbark (or at least a very hard red hardwood) slabs that vary in width from 160mm up to 290mm. They are rough hewn, probably adzed, although there are signs of circular saw marks on some of them.

At some time there were additions made to the back of the house. The roof over the oldest part was a simple pitch roof with gable ends, 9:12 pitch. The ceiling in the old part is 'truncated'. That is, next to the side walls, the t&g ceiling boards are fixed to the underside of the rafters. At 30mm or so up, the ceiling reverts to flat.

The roof over the newer part is a wide gable which will have to come down. We are planning to put in an attic truss roof system that spans the whole width which is 8220mm. We want to preserve truncated ceiling in the main room, which means that we would prefer to use the existing 9:12 pitch for the truss design, and use the 125 by 125 beam as the support for the truss on the northern side, with a standard plate bearer on the southern side.

We are planning to use recycled hardwood and bolted construction since that is more in keeping with the style of the building than modern gangnailed pine trusses. We can use what will be the middle wall (the southern wall of the old part) as a solid structural support for the middle of the truss.

My question relates to designing the truss. If we build the supporting frame making the horizontal joists of the truncated ceiling (adjusted for section/span requirements), then the 9:12 pitch and 8220mm width give an internal height at the peak of 3100mm, and the cross beam tieing the truss would be at 2100mm above the floor. It gets a bit tricky if we lower the truss design to incorporate the truncated ceiling, since we lose the 300mm of the truncated ceiling.

If we were to use 1.2m side walls, I can get useable width of 5000mm. We would like to put dormer windows in to bring light, and for aesthetic reasons.

I have tried to access the building code requirements for attic rooms on the net, but would have to buy access, and in any case I don't know if it would answer my question. How do I access design expertise for a truss in this situation? I note that the providers of gang-nailed trusses would probably welcome my enquiry if I wanted to use their product, but since I want to use recycled hardwood/bolted construction in the old fashioned sense, I don't feel that I should ask them.

Appreciate any comments.

[joe greiner]

You're probably right that the building code might not address this question. Building codes usually concern more normal situations, with allowance for fully-engineered variations.

I doubt you'll find qualified specific advice on the internet for free. Very few professionals would stick their neck out far enough. I'm not trying to drum up business for my brother engineers, but I think you need to engage one. As you've already established your general concept, fees for professional advice shouldn't be too dear.

Joe

[DvdHntr]

What Joe said. Structural Engineers are the way to go. You could contact frame and truss companies or even one of the three nailplate suppliers and ask for their engineers.

Another solution that would get them interested would be to cover the nailplates with plates (ply or steel) and glue on the bolt heads/nuts. It could end up much cheaper as the factory would run them all through and making them by hand may be quite time consuming.