Roof Spread - Vaulted Hip Roof

[J4mes]

We are having a block built double skin garden room built. Its under 30sq meters so doesn't require building regs. Carpenters have said they can vault the ceiling no problem when building the roof structure, i would like this but am a bit worried about roof spread as the tiles are the heavy concrete ones. 

Any idea if we could / should be adding rafter ties? I was thinking something like the below and leaving them exposed which would give us an area in the middle that's fully vaulted but would still tie the walls together? Or do we need them both ways? Any advice would be welcome! Thanks 

Building is 4.8m x 3m. 

 

[markc]

Ties where you indicated are not doing anything, a hipped roof will be fine as long as the 4 corners cannot spread. You need a substantial and properly connected wall plate to tie the bottom of the 4 hip rafters in place.

[J4mes]

Thanks @markc

 

I have now spoken to the roof framer. He said the best bet to gain a small amount of extra headroom whilst keeping the structural integrity would be to have ceiling joists on every rafter as usual and collar ties but that he can lift the ceiling joists up 200mm above the wall plate giving us the additional head height we need. He is quite experienced and seems to think this would be more than sufficient to stop the walls spreading.

 

Given we only need a small amount of additional headroom, i think this would work! 

I hadn't really thought about what happens when you fully vault a roof and take away all the supports, but it makes a lot more sense now.