Roof/Rafter question

[yarnfly]

I am building a 12 x 20 cabin.  I am going to build with 2x6 Rafters on 16" centers versus trusses.  8 feet of the 20' length will be a loft and thus have horizontal ties from wall to wall.  The remaining 12 feet I wanted to leave cathedral.  Can I do this?  Do I need collar ties at the top?  Should I run a larger beam across the center of the 12' section to keep my walls tied together?  I am not certain what common practice is for this and want to do it right.  I don't want my roof load pushing my walls out sideways.
My roof will have a 10/12 pitch (39deg) due to pretty substantial snow loads in my area.
Thanks for the input.

[Amanda_931]

You could always make your  collar ties decorative, e.g. on both sides of the rafter, with scallops on the bottom, something like that?

(Or, do a whole bunch of research and find details about the--Swiss, I think--method that allowed for movement in the rafters and walls with snow loads)

My guess is that you ARE going to need them.

[VaCottage]

As I understand it, you can double the ties over the cathedral area (ie attach a tie to each side of every other rafter) at the same height as the ceiling/floor joists.  Then add a decorative trim piece (1xwhatever) to hide the bottom of the ties & gap between them caused by the rafter.  So if you have rafters 24" o/c, you'd have the "beams" (dbl ties) 48" o/c.

[Bart_Cubbins]

The collars ties (near the roof peak) can probably be replaced with plywood gussets or metal strapping across the tops of the rafters.

As for keepiing your walls together, I think you have three basic options...

1. Use a ridge beam to support the tops of the rafters and eliminate the spreading effect. Either go gable to gable or gable to a post at the loft edge.

2. Install rafter ties as VaCottage suggests.

3. Beef up the sidewall top plates so they become in effect beams whose ends are tied together through the gable wall and through another tie at the loft edge.

Option 2 is probably the most straightforward, but 1 and 3 give you a completely open cathedral. Of those two, I think 1 would be easier, provided you can easily support the ridge beam (i.e. no "wall of windows" in the gable).

Whatever option you choose, you will of course have to get a local professional to review your plans and size the structural members and fasteners according to your local snow loads and other requirements.

Bart

[John Raabe]

Nice review Bart. Good clear analysis.

[yarnfly]

Awesome feedback gentlemen.  Very informative.
Big thanks.  Nice to know my options.