Truss details
February 2nd, 2008 - Posted in eco-building - by Sarah|
Unfortunately when our trusses were delivered we were still working on plumbing and fixing the walls so we weren’t able to have them lifted directly onto the top plate by the truck. Plumbing is still done using the old fashioned plumb line (a string with a weight on the end) as over the height of the wall this is far more accurate than a spirit level. We plumbed each of the corners in both directions and then nailed them together. Then had a go at straightening the top plates by running a taut string from one end to the other and ensuring that the distance between it and the top plate remained the same all the way along its length. Where it wasn’t we used props connected to pegs to push or pull it to where it should be. The sledge hammer came in handy occasionally too! Once everything was straight we tightened up the metal bracing straps on the walls and were at last ready to raise the trusses.
We have got three different kinds of truss – one plain gable end truss at the back, nine standard trusses in the centre and one structural gable end truss at the front.
The south gable end truss with its vertical struts has a bottom chord only 100mm high as it rests on the top of the rear wall all along its length.
The trusses are made from H3 treated pine because the sprocket ends that form the eaves poke out 900mm beyond the walls and will remain exposed to the elements when the building is finished. The council are only happy for Lawson Cypress to be used in exposed situations where they are not structural eg board and batten cladding. The quality of the wood in the trusses is pretty awful, especially when compared to our wall timber but there’s not much we can do about it. The house will have rafters (ie a framed roof) rather than pre-fabricated trusses and as these will be completely enclosed we will be able to use untreated Lawsons.
We lifted the trusses up onto the roof and let them hang point down. Then we moved them into position one at a time and lifted them upright. We made sure that they were sitting directly over the wall studs and were vertical before attaching them to a temporary purlin near the peak. The purlins are long pieces of timber that run the length of the roof at right angles to the trusses. Then we used CPC40’s ( L-shaped metal purlin cleats) to attached the ends of the trusses to the top plate.
A CPC40 connecting the truss to the top plate.
We have also had to put tie downs (yet more metal straps) onto the top of each stud that a truss rests on so that a strong wind catching the roof cannot rip the top plate from the wall. Brendhan has suggested that we keep a running tallly of the fixings that we have used in this building as each piece of metal requires at least 9 nails. When someone asks what the building is made from it is very tempting to answer “nails”.
