I'm contemplating designing/building an attached garage for a 70's brick veneer house. There is currently a flat-roof carport in front of the main bedroom, and I'm thinking of replacing this with an extension of the house roofline. The house is a hip roof with concrete tiles and traditional stick framing (no trusses). For various reasons, I want the roofline to be continuous, i.e. not having a box gutter in between house and new garage. To accomplish this, I basically need to convert the existing hip end above the main bedroom into a gable end. As far as I can tell this should be a fairly simple procedure using prefab trusses, but I want to make sure it's okay to do this structurally.

Here are my main concerns/questions:
  • With a gable roof, obviously all the roof load is placed on the side walls and eventually down to the brick piers. Do the side walls need reinforcement? The hip is relatively small (about 5.8*5.8 m in plan area, and about 3.2 m from eaves to apex). I will probably be placing a ribbon top plate around the side walls where the new trusses will go so that I don't have to position them over studs. The side walls where the roof load will be going are parallel to the floor bearers, if this makes any difference.
  • Does the end of the ridge beam need to be supported by a vertical strut going down to a strutting beam? I'm not sure if it already is, but even if there is already a strut it may need to be moved further back from the end of the ridge so it doesn't interfere with the new trusses.
  • Should the end wall (below the gable) be treated as a bracing wall? And if so, how should I do this? I guess this is normally done using a trimmer and blocking/brackets like a normal internal bracing wall, but since the bottom chord will be directly above the top plate I'm not sure how to brace it. Should I try to locate the end truss just behind/in front of the parallel top plate so that there's room to anchor a slotted bracket to the top plate? Or is there some slotted plate connector which can attach to the side of the top plate and link it to the bottom chord? Btw there are no internal walls under the section I'm planning to replace with trusses.
  • When attaching diagonal bracing to the new trusses, should this continue over the old rafters as well?
  • I'm not entirely sure, but it's possible there is a hanging beam under the hip to support the ceiling joists. If the hanging beam is parallel to the new trusses that's fine and I'll just work around it, but if it's perpendicular to the trusses I'd have to remove it as it would interfere with the bottom chord and possibly the webbing. In this case, should I temporarily brace the joists whilst the trusses are being stood and then anchor them to the new bottom chords? I don't really like the sound of this as the bottom chord will deflect over time and cause cracks in the ceiling. I guess the other option is to install a few rows of trimmers between the joists (if they aren't there already) and put in a couple of new hanging beams parallel to and in between the trusses. Any opinions on the best way to go about this?


Thanks in advance for any advice you can give me. I've never done a major structural renovation like this before, and I expect I'll have to consult a structural engineer before going to the truss manufacturer (at least to satisfy council's approval). But any indications of what to expect beforehand would be appreciated. I can post photos/plans if it would help.