Roof spread - Hip Roof Construction

[J4MES99]

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.

 

[Deluks]

Ask them how they will design it to resist the roof spreading.
It's probably possible over a 3m span, but your solution would see it spreading out in the middle.

Are they fitting collar ties or gussets? Any steels involved?

 

[J4MES99]

Ask them how they will design it to resist the roof spreading.
It's probably possible over a 3m span, but your solution would see it spreading out in the middle.

Are they fitting collar ties or gussets? Any steels involved?

Thank you for the reply.

They are fitting collar ties higher up but i don't believe that's enough to stop the spread. No steels. I did read somewhere that dragon ties at each corner on hip roof's stop the wall plates spreading out but in all fairness i have no clue.

I wonder if we added more Rafter ties like the below and just left a slightly larger gap in the middle so we have some extra head height at that part (it's going to be a gym so wanted a bit of extra height in the middle)

Also lots of American literature on rafter ties / ceiling joists being ok to be riased higher than the wall plate as long as its no higher than 1/3 of the overall roof height. This might give us the extra height without needing to be fully vaulted?

 

[Deluks]

You're on the right track about the raised ceiling joists. A bit of platform storage is valuable too.

But why not get a structural engineer to give you an optimal design rather than guesswork and the word of 'some bloke off the internet'.

if you're builders can't explain it then how can they build it? ...and if they can explain it, get back to us with what they say, so you get 5 different opinions back on why it's wrong (or right!)

 

[noseall]

Probably need a double cranked beam running centrally lengthwise.

 

[freddiemercurystwin]

Or a king truss at each end of the main roof ridge.

 

[noseall]

Or a king truss at each end of the main roof ridge.

A pair of decor (visible) King trusses would not be cheap either.
All faffy, including a cranked beam.

 

[J4MES99]

Thanks for the replies @noseall & @Deluks

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.

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.

 

[J4MES99]

Excuse my very basic sketch.... Do you think this set up would work to stop any spread..... Rafters ties installed 200mm above the wall plate. Gives us a little extra headroom.

Overall height from wall plate to ridge is about 1.2M so this would be in the bottom 1/3 of the roof.

 

[cdbe]

I was under the impression you could raise the ties up by a third without issue so I think you'll be fine with 200mm. You can get something called glide shoes which allow the rafters (or rather raised tie trusses) to move out a bit when loaded with roof, but won't be necessary.

 

[noseall]

I was under the impression you could raise the ties up by a third without issue so I think you'll be fine with 200mm. You can get something called glide shoes which allow the rafters (or rather raised tie trusses) to move out a bit when loaded with roof, but won't be necessary.

Glide shoes won't work with birds-mouthed rafters, fixed to a WP.

 

[robinbanks]

You only need an extra 200mm. Is there something restricting you from making the walls 200mm higher, or the floor 200mm lower?

 

[J4MES99]

You only need an extra 200mm. Is there something restricting you from making the walls 200mm higher, or the floor 200mm lower?

Yes sadly, the building is on sloping ground about 300mm difference length wise, we spoke to the planners who said they measure from the lowest point of adjacent ground! Meaning we had to keep the walls slightly lower than planned and reduce the wall plate height.
Neighbour objected to the planning so to avoid issues we are trying to keep it all very tight and not a cm over the approved height!

 

[J4MES99]

I was under the impression you could raise the ties up by a third without issue so I think you'll be fine with 200mm. You can get something called glide shoes which allow the rafters (or rather raised tie trusses) to move out a bit when loaded with roof, but won't be necessary.

Thanks that was my understanding, there seems to be lots of build codes online or literature from America / Canada which states this.

 

[robinbanks]

not a cm over the approved height!

No external party will care if you have the floor 200mm lower.

Convenient (for them) that planning said they measure from the lowest point. It's more usually the highest point