Board under vaulted ceiling

[grahama]

Hi all,

extension coming along now, and I am thinking about the inside. I am having a vaulted ceiling to the single story extension with roof lights. (ie the roof rafters form the basis for the ceiling and it will be the roof shape).

The plans I had drawn up originally said after 125mm insulation inbetween rafters I put 25mm over the whole ceiling area. Fine with that.

What I am unsure about is the 40mm douglas fir plywood he has stated is then put over all the insulation before plasterboard.

When the building inspector passed the roof structure he spoke about the 125mm and 25mm over that but suggested using 25mm insulated plasterboard instead of separate insulation and plasterboard.

This suggests to me that the 40mm ply isn't required, is this so ?? If it is what is it for ??

many thanks as always,

Graham

 

[catlad]

Sounds unusual to me it would normally be insulated plasterboard under the rafters.

 

[dann09]

Ply is typically specified for framed roof dormers. The ply sheathes and firms-up the framing against racking, for instance.

Perhaps, your vaulted framing/cut roof design requires sheathing to firm it up, but surely it could be done on the exterior and with 22mm ply, unless its a "dropped vaulting" arrangement?

40mm is used as unframed doors or in commercial work. Sheets are extremely heavy and expensive.

Maybe you could post a drawing of the roof/ceiling proposal? I've previously noticed that the term "vaulted" can mean different things to people.

 

[tony1851]

As per Danno, the ply is probably to diaphragm the ceiling to provide some lateral stiffness (have you taken out some walls downstairs).

But 40mm is way over the top - 18mm would do just as well.

Insulated plasterboard is too expensive; go with the 25mm ins. plus plasterboard.

 

[RogerBoyle]

Ply is typically specified for framed roof dormers. The ply sheathes and firms-up the framing against racking, for instance.

Perhaps, your vaulted framing/cut roof design requires sheathing to firm it up, but surely it could be done on the exterior and with 22mm ply, unless its a "dropped vaulting" arrangement?

40mm is used as unframed doors or in commercial work. Sheets are extremely heavy and expensive.

Maybe you could post a drawing of the roof/ceiling proposal? I've previously noticed that the term "vaulted" can mean different things to people.

Good explanation and agreed 100%

 

[tony1851]

OSB is just as good as plywood for this type of application, and a lot cheaper.

 

[grahama]

Hi all,

thanks for all the replies and as requested here is a pic



The ceiling is basically the underside of the rafters.

another pic



I have not taken any walls out but the existing openingl between the lounge and the new extension is being opened up from 2.1m to 3.5m.


any other thoughts following this pic ??

Graham

 

[tony1851]

Ask the person who specified 40mm thick plywood to give you a lift with getting the 8x4 sheets up; one or both of you will finish up in casualty.

Can't see why any timber boarding is needed for that roof.
Ridiculous if you ask me.

 

[RogerBoyle]

I am not a building inspector or ouwt like that LOL but I have fitted several roofs and I do not see where or why it would need any ply on it at all from that photo

 

[grahama]

A small apology for the build plan designer, it was 12.5mm ply

OK, no ply but should I be putting a vapour barrier to the underside of the rafters ??

Graham

 

[RogerBoyle]

Ive never fitted one unless it has been specced for it in the rafters