Vaulted ceiling construction

[iep]

I'm installing a new ceiling in a room that has been opened up to the full height of the roof (45 degree pitched roof). Architect's drawings show 100mm of celotex between rafters and then a continuous run of insulated plasterboard (65mm celotex) under the rafters.

This all seems fair enough to me.

To improve sound insulation though (and possibly to make the plasterboard installation easier) I was thinking of adding a layer of 11mm OSB below the rafters before adding the insulated plasterboard.

So, construction would be a sandwich of 100mm insulation, 11mm OSB, 65mm insulation and then a layer of plasterboard bonded to the 65mm insulation.

Does this sound daft/sensible/unnecessary?

Cheers,

ip

 

[theoldun]

You should be asking your architect these questions.
Out of interest, to form a vaulted ceiling you need to remove ceiling joists. Is this what you have done?
Also has your roof purlins?
Regards oldun

 

[iep]

I will be asking the architect but he is on holiday this week. Just thought I'd canvas for opinion.

Yes, ceiling joists will all be removed once the ridge beam is installed.

No, we don't have purlins so a nice flat surface to work with.

iep

 

[tony1851]

If the roof is at 45deg., and the ceiling joists are being removed, what prevents the rafters pushing out on the walls?

 

[stuart45]

If the ridge beam is correctly installed it will stop the roof spread.

 

[tony1851]

If the ridge beam is correctly installed

Depends what you mean by "correctly installed".
It's not so much the beam itself that is critical, but how the rafters themselves are secured at the apex.
You could have a massive 24" deep plated steel girder at the ridge, but if the rafters themselves are not securely fixed either to the beam, or each other, there will still be the tendency to spread.

 

[iep]

Ridge beam will be installed as per the structural engineer's drawings. Roof spreading has been accounted for and will not be a problem.

Got in touch with architect today He's pretty sure that the combination of 165mm of celotex and 12.5mm of plasterboard should be more than enough to keep out exterior noise.

Cheers,

iep

 

[catlad]

Well iep I am interested in how your engineer has accounted for roof spread keep posting.

 

[iep]

I don't have the diagrams to hand but the fundamentals are:

1. Ridge beam running length of area where joists (acting as ties) have been removed. Fully supported at both ends.

2. Rafters bolted together at apex using m12 bolts and those star timber joiner things.

3. Rafters notched just below apex so the ridge beam is slightly inset into the rafters and they rest directly on it.

4. Joist hangers used to tie the ridge beam to the rafters above.

5. Ties between the rafters directly below the ridge beam.

Total length of roof without the original joists/ties is only 4m and rafters are also attached to beams of wood running the length of the wall head which are, in turn, bolted to the wall head. Wall is 1m thick and only 2.8m high.

Cheers,

iep