Monday September 14 / Tuesday September 15, Wednesday September 23
Rafters or trusses?
The original idea for 60k House was to use rafters with collar ties (a horizontal tie beam that ties the rafters together) – the traditional method for a timber framed roof that gives you extra clearance inside. After discussing with both the engineer and the builder we decided that trusses would be cheaper, easier and a quicker method to build.
The overall shape of the trusses are unusual: most trusses have the ends of the bottom chord sitting on the top plate of the wall. That’s the most efficient and cost effective, but it doesn’t allow for increased volume inside the building. Because 60k House is modest in size I want the overall volume to be a bit more generous. Standard ceilings in modern houses are 2.4m high (this is the minimum they are allowed to be for a habitable room under the building code). 60k House has 2.7m high external walls, raking up to a 3.1m high flat ceiling – this is the underside of the bottom chord of the roof truss. Generally you want higher ceilings in the tropics (to allow warm air to rise and be further away from you) and lower ceilings in colder climates (to keep the warm air close to you and make heating easier). With all of the windows to be double glazed and the extensive exposed thermal mass within the building I’m confident of easily being able to heat the volume if required to achieve a pleasant thermal comfort all year round. The overall proportion of the building, with the higher ceiling, also feels ‘nice’.
All exposed timber in 60k House is hardwood (Tas Oak – one of three eucalyptus species); the rest is softwood (pine). Because of the desired roof overhang (to protect the building from weather and provide appropriate shading to north-facing glazing) the top chords of each truss (similar to a rafter) are 190×35 hardwood – that’s much bigger than usual. The bottom chord and ‘webs’ (other members joined together with nailplates to form triangles) are made from pine, except for the trusses that will support a future solar panel array – these have hardwood bottom chords.
The roof trusses were designed and built by the truss manufacturer before being delivered to site. The trusses are spaced at 1200 centres. Purlins are the horizontal roof structure (similar to battens) that sits on top of the trusses and will support the roof sheeting. Purlins are larger and sit on their edge to span an increased distance; battens are smaller and lay flat. We used green (unseasoned – not dried) 75×50 hardwood purlins for cost effectiveness and to achieve a cantilever at each end. The size of each purlin was a balance between the spans of the trusses and the minimum height required for the fascia and gutter (see below). On top of the purlins metal strap bracing ties all of the roof together. There is also a timber tie that connects the bottom chords of the trusses together.
It’s bizarre how the perceived size of a building can change as the building progresses. You start with a bare patch of land = BIG. You dig the footings = small. Pour the footings = big. Lay the blockwork = small. Pour the slab = big. Put up a wall = small. Put up all the walls = big. Put on the roof trusses = small.
Once the roof trusses went on it was possible to gain a sense of the volume of the building. The addition of horizontal structure (noggins and purlins) or sheeting (brace ply or roofing) increases the sense of enclosure, and with it a perceived reduction in the overall size. The space, which used to be limitless, now has definition and boundaries – inside/outside.
A fascia is a board that usually covers the ends of the rafters/trusses and carries the gutter. The design for 60k House has some of the trusses with extended top chords longer than the others to provide extra cover at the front door and over the verandah. That means the fascia and gutter won’t run in the usual position – instead it will be located next to the last purlin. Fascias tend to be vertical (or plumb) for fixing the gutter to.
This is an example of resolving a detail for the project on site, with the builder, as the construction progresses. Greg produced a 1:1 scale drawing of his suggestion; we looked at a couple of other options before some discussion saw us agree on a simple solution – to ‘rip’ (cut length ways) a seasoned hardwood fascia that would be fixed to the last purlin.
Costs: trusses – $2,900; purlins – $718; verandah beams and hardwood posts – $390; fascias – $238; builder (framing walls and roof) – $6,370
Total cost of building to this stage: $37,044 including materials and labour
Disclaimer: Any advice contained within this blog is of a general nature only and cannot be relied upon. Details provided are in good faith and relate specifically to this project. Any author will not be held responsible for advice or information presented.