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Final Roof Buildup Decisions


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#1 Crofter

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Posted 10 April 2016 - 08:43 AM

Before I start sawing up a large pile of expensive wood, I need to confirm my roof buildup plans. Notionally this is (from outside to in)
- box profile steel sheeting
- 50x50 counterbattens
- 50x25 battens
- breather membrane
- 11mm osb sarking
- 150mm rafters fully filled with mineral wool
- 50mm PIR boards
- 25mm battened service void
- 9mm plasterboard

I would really like to improve the performance of this by increasing the PIR depth, ideally to 100mm. However this is not straightforward. To add it on the inside means a loss of ceiling height. I think in the vaulted ceiling rooms, I would get away with this. I could certainly consider vaulted or at least coombed ceilings throughout but I'm not sure how that would look.
If I shift the entire roof upwards, then that growth will end up being taken up by taller fascias etc and the whole roof could become very top heavy looking. On such a small build I think that this could look out of proportion.
Or I could put PIR between the rafters- is this as much of a faff as I think it is?

#2 declan52

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Posted 10 April 2016 - 09:00 AM

Should it not be counter battens then membrane then the 25mm Batten.

#3 jsharris

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Posted 10 April 2016 - 09:02 AM

View Postdeclan52, on 10 April 2016 - 09:00 AM, said:

Should it not be counter battens then membrane then the 25mm Batten.

I'm guessing here that the 50 x 50s are to allow fixing at the peaks of the box profile, so they will be on the box profile pitch.

#4 ProDave

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Posted 10 April 2016 - 09:36 AM

For box profile normally tthe top battens go horizontally. The counter battens go vertically following the rafters. the only confusion is which are the battens and which are the counter battens. Does it matter, it's just a name?

Don't forget if you increase your roof thickness to check id doesn't exceed the ridge height limit for a portable building.

#5 Crofter

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Posted 10 April 2016 - 10:26 AM

View PostProDave, on 10 April 2016 - 09:36 AM, said:

For box profile normally tthe top battens go horizontally. The counter battens go vertically following the rafters. the only confusion is which are the battens and which are the counter battens. Does it matter, it's just a name?

Don't forget if you increase your roof thickness to check id doesn't exceed the ridge height limit for a portable building.

Yes I may have mixed up my terms, the first set of battens onto the sarking would be vertical (letting them drain) and the next layer horizontal. The box profile is fixed in the troughs, not the peaks, so needs horizontal battens. I want to raise these off the sarking to provide a drainage space.

There is no ridge height limit for a portable building. The height limit of 3.048m is measured internally, to the ceiling.

#6 Crofter

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Posted 10 April 2016 - 10:51 AM

Battens aside, any chance of a comment on the actual query? Namely:
- fitting PIR between rafters. How well does this actually work.
- stealing a bit of ceiling height. How much do you think I can get away with. I only need it right at the eaves so could steal it via a partial coomb.

This affects how tall my wall panels are, so I need to make my mind up as I am starting framing this week.

#7 Crofter

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Posted 12 April 2016 - 10:13 AM

OK, decisions made...
I have upped the roof insulation so it now comprises 150mm between rafters plus 100mm below. This ties in with the 50mm layer in the walls, see below:
Attached File  Cross section details.jpg   15.79K   10 downloads

I have stolen a bit of ceiling height, making it 2350mm above joist level, i.e. minus the chipboard and floor covering. I think I can get away with this especially in the vaulted ceiling room.

Externally, I think the use of steel cladding barge boards helps break up things and it works aesthetically:
Attached File  Gable View external.jpg   18.68K   10 downloads

What do you guys think?

#8 jsharris

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Posted 12 April 2016 - 02:40 PM

Sounds good to me, and I like the idea of the contrasting barge boards, it helps reduce the "visual thickness" of the roof.

Another thing you could do to reduce the apparent thickness of roof at the gable ends is to use thinner ladder frames for the small overhang. MBC do this, to hide the 400+mm roof thickness, by having the last rafter on the gable wall, with a thinner ladder (I think ours was studwork, so 89 x 38 timber) projecting over the verge. This meant that the thickness of the barge board subframe is only 89mm, which reduces the thickness of the barge boards themselves to this, plus the soffit thickness, plus the batten thickness plus the roof covering thickness.

Edited by jsharris, 12 April 2016 - 02:41 PM.


#9 Nickfromwales

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Posted 12 April 2016 - 09:40 PM

View PostCrofter, on 12 April 2016 - 10:13 AM, said:

OK, decisions made...
I have upped the roof insulation so it now comprises 150mm between rafters plus 100mm below. This ties in with the 50mm layer in the walls, see below:
Attachment Cross section details.jpg

I have stolen a bit of ceiling height, making it 2350mm above joist level, i.e. minus the chipboard and floor covering. I think I can get away with this especially in the vaulted ceiling room.

Externally, I think the use of steel cladding barge boards helps break up things and it works aesthetically:
Attachment Gable View external.jpg

What do you guys think?
All sounds VERY VERY risky to me.
It would probably be a good idea to maybe build a prototype of this to be sure your heading in the right direction tbh. As luck would have it, I have a ring beam already set out around the side of mine, and your more than welcome to come and do a practice build if you'd find that easier :D
Just a suggestion B)
:ph34r:

#10 Crofter

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Posted 12 April 2016 - 10:02 PM

Are you by any chance implying that I might be overthinking this! :D
Just wait till I demand answers as to which roofing felt to buy, currently paralysed by indecision :o

#11 Nickfromwales

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Posted 12 April 2016 - 10:29 PM

Nope. Just want a free man shed :D
No harm in trying ;)
Regards, nick.