Roof design & construction

[unclebobby]

I am building a 2 storey shed with office over (but possible future accomodation potential). The roof will be covered in artificial slate onto raised collar trusses. The raised collars will become the office ceiling and are 1m above wall plate level.
My plan at this stage was to keep it all simple with a membrane over the rafters, battens, slate and eaves ventilation. Insulation installed after roofing would be batts stuffed between rafters.
I am now wondering if I should infact board the rafter before slating, and wondering if it should be a "warm roof".
Any opinions and ideas how to go about it ?

 

[freddiemercurystwin]

Its not normal to do a pitched roof this way although you could if you really wanted. Anyhoo Use a breathable membrane and keep a 25mm gap between the insulation and underside of the membrane (allow for a bit of sag in the membrane) and no need to vent the roof at the eaves. Fit the insulation between the collars (ie at ceiling level) rather than all the way up to the ridge between the rafters.

You realise if you ever convert it to accommodation it will need Building regs!

 

[unclebobby]

thanks for that. i've got a breathable membrane although the slate suppliers documentation states that a cold roof should still be ventilated as these are close fitting slates.
what is the normal way to construct a pitched roof these days ?
i suppose some of my dilema is should i go for a warm or cold roof, especially as some of the roof (up to the raised collar) is part of the room, and can I still install insulation at a later date from the inside.

 

[theoldun]

. The roof will be covered in artificial slate onto raised collar trusses. The raised collars will become the office ceiling and are 1m above wall plate level.

Are you sure you have got that right?
oldun

 

[unclebobby]

yes. the trusses are 8"x2" at 400 centres.

 

[theoldun]

My sincere apologies, I read your post wrong.
I am well past my sell by date.
Wish you well. oldun

 

[freddiemercurystwin]

thanks for that. i've got a breathable membrane although the slate suppliers documentation states that a cold roof should still be ventilated as these are close fitting slates.

If that's what's recommended then best to follow it.

what is the normal way to construct a pitched roof these days ? i suppose some of my dilema is should i go for a warm or cold roof

I don't understand your dilemma. You could go the warm roof route although it is unusual and of no benefit to you. Several slants on a new pitched roof but all follow the same principle http://www.insulation.kingspan.com/uk/pdf/k7.pdf

 

[unclebobby]

thanks again.
i'll go cold roof, no boarding and usual ventilation. and later between rafter insulation.
one final question. slate spec states 25x38mm battens for <450mm rafter spacing. mine are 400mm except for 3x 550mm spaces for roof lights (and 200mm each side of roof lights) - will i still be okay with 25x38mm batten ?

 

[r896neo]

Yes but if your using a breather membrane you need to counter-batten it. i.e 1 batten up the rafters holding the membrane and then a second horizontal for fixing the slates to.

 

[freddiemercurystwin]

Yes but if your using a breather membrane you need to counter-batten it. i.e 1 batten up the rafters holding the membrane and then a second horizontal for fixing the slates to.

Eh!

 

[geraint]

Yes but if your using a breather membrane you need to counter-batten it. i.e 1 batten up the rafters holding the membrane and then a second horizontal for fixing the slates to.

Eh!

but he is foreign...

 

[unclebobby]

plenty of space between the 8" deep rafters for insulation and an air gap under the membrane so not planning on counter battens

 

[noseall]

you only need counter-batten where the insulation sits above the rafters or where there is solid board sarking.

 

[r896neo]

Obviously not the same for all products then. The klober breather membrane i have used stated it must be counter battened.

Apologiesi just assumed it was not the same for all breather felts