Are you familiar with a 'Larson truss'.
It is basically external insulation added onto the outside of a timber frame building while all the structural elements are within the thermal envelope.
http://www.builditsolar.com/Projects/SolarHomes/LarsenTruss/LarsenTruss.htm
Failing that, is there any way you can but a thermal break between the hull and the super-structure.

Or ditch the timber and go for a GRP Foam GRP sandwich.

could you isolate using compacfoam ( edit: or Puren )? It's meant to be a material that has similar bearing properties to wood but insulation value is higher, material is expensive, but maybe you only need a small ammount. i.e. could you could cut squares, or strips to support rafters and fill the rest with with EPS/XPS or something?

clever video here for windows where they only use a small ammount and then infill with EPS

https://www.youtube.com/watch?v=GUUrd7dbBzU

Also
What is your worry with thermal bridging?

Right - just got in from work -

WOW! So many thoughts and suggestions - thanks to all who've posted!

Now, where to start...

I'm taking a good look at the Larsen Truss info - lots of reading there (!) - thanks Steamy Tea.

To give a better idea of project, this is a steel barge so basically the roof rafters will bridge across the original open section of the vessel (where the goods would have been loaded), connected (for want of a better term) to the steel structure (so no actual 'deck' area). The rest of the barge will be insulated with solid 'Celotex' type insulation between the outer steel structure and internal wood framing (so a good thermal insulated barrier between outside and inside).

Going on from the suggestions by Gyrogear and Silky I remember from when I worked in the steel industry that we had to specify Farrat thermal breaks for things like balconies so the cold wouldn't travel from the outside structures through to the internal building frame (very similar to the Compacfoam) - this type of solution also fits in well with how I would like the rafters to sit into the opening.

I'm off to do some reading and roof designing....

Thanks again for the input!

DT

Posted By: discotrollThe rest of the barge will be insulated with solid 'Celotex' type insulation between the outer steel structure and internal wood framing

Not an expert, but I thought that people used sprayed on insulation, especially with steel hulls, to eliminate condensation forming on the hull and collecting in the gap between other forms of insulation and the hull?

I'd be surprised if you didn't want to still be able to walk over the top of whatever roof you build, effectively creating a deck.

Perhaps there might be a way to suspend the rafters (or whatever supports the insulation and deck) from above (i.e. the cold side) rather than from the ends.

Posted By: gyrogear

(but it's your project)
gg

Hahaha - read that as 'Your *problem*'!

Yep - a double-hull with wooden inner shell is the basis of what I'm hoping to do...

I'd like to avoid spray foam insulation just because it becomes such a pain if you need welding work undertaken in the future (though the potential condensation issues to the inside of the hull needs to be properly adressed).