Standing Seam Roof Build Up

[CJO]

After some advice and experiences from those on this forum website regarding our potential Non Ventilated Warm Roof.

 

I have spent an evening/afternoon reading previous threads about this subject matter and that has raised a couple of questions.

 

Our internal roof pitches will all be open plan (vaulted) and our house will be an upside down layout.

 

Our architect and SE have specified the following build up from inside to out.

 

12.5mm plasterboard

Service void

Vapour control layer

9.5mm sheathing board

125mm Rafter

75 PIR insulation between

100mm on top

18mm sarking board

Standing seam metal work roof.

 

My partner and I have increased our expectation of our self build and want to make it achieve passive elements.

 

I’m thinking…

 

12.5mm plasterboard

Service void

Vapour control layer

9.5mm sheathing board

125mm Rafter

Cellulose infill for sound and insulation

100mm PIR on top

22mm OSB

Tyvek Metal Underlay

Standing seam metal work roof.

 

Potentially have an extra 50mm deep rafters to play with as the SE downsized the Architects spec…

 

Reading the previous threads regarding noise and condensation risks has got the grey matter questioning methods.

 

I have read a lot of Du Ponts website regarding roof build ups, watched Roofer Steve on YouTube explaining vapour control layers and also looked at VM Zincs information regarding build ups. Our doesn’t sound impossible.

 

Is there a concern infilling our rafters would result in needing deeper rafters?

 

Look forward to reading any responses.👍🏻

 

[Redbeard]

What you describe is a hybrid Warm roof - some on top of the rafters and some between. In some circumstances there is a risk of the dew-point occurring at the interface. A condensation risk analysis (CRA) can guide you, but a colleague had a good rule of thumb: If you have two-thirds of the R value above the rafters and one-third below, the interface is unlikely to get cold enough to reach the dew-point. If I remember correctly the lambda of cellulose is (or was when I used it) 0.036W/mK, and PIR is 0.022W/mK. So for 125 cellulose it's 3.47m2K/W (actually less, as that does not take into account the 'intrusion' of the rafters)  and for 100 PIR it's 4.54m2K/W, so not too far off but probably not two-thirds/one-third. A CRA (particularly if done in the WUFI software) will be more accurate.

 

Edit: A thought: If noise is an issue why not consider rigid wood-fibre on top? 140-180kg/m3 as against about 32kg/m3 (IIRC) for PIR)?

Edited Sunday at 15:31 by Redbeard
Noise and WF

[Nickfromwales]

5” rafters are quite small? At what frequency and what length? 

[SimonD]

1 hour ago, CJO said:

Vapour control layer

9.5mm sheathing board

125mm Rafter

Cellulose infill for sound and insulation

100mm PIR on top

22mm OSB

Tyvek Metal Underlay

Standing seam metal work roof.

 

If you do osb sheathing @ 11mm you can use this as your VCL, just tape and seal all the joints.

With respect to your buildup sub the metal roof, you need to be a little careful. There are only a small number of standing seam metal roofing manufacturers that explicitly have a buildup that doesn't require a ventilation gap so double check with them that the Tyvek metal underlay system on warm roof is accepted by them (I know it's been designed to allow condensation to drain from below the metal roof and there are approved systems) - and double check with building control. You are also much better of using sarking boards, which are the preferred substrate, second is plywood and 3rd is osb. Pull out resistance of your standing seam clips is greatly reduced in osb where you really need to use screws not nails, which most contractors will use because they won't want to spend the time screwing the clips into the board. So you have to look at this in the round and how the metal roofing contractor designs the clipping for the wind exposure of your house.

[Iceverge]

Any drawings of your roof and wall interface? 

 

If it's complex then I would recommend moving your airtight layer outboard of the rafters. 

[CJO]

Apologies for the delay and thank you for very quick responses to my questions.

 

@Redbeard thank you for explaining the rule of thumb, I had a small read about it was initially thought it was about the amount of rafter covered not relative to thevR values of insulation used (I'm not fibbing when I say I had a small read on it). I am now looking at woodfibre sarking boards following @SimonD suggestion, they seem to also provided a very good vapour permeable substrate.

 

@Iceverge I was on the understanding I need a nice strong vapour control layer internally and all materials leading up through the roof ideally need to be more permeable than that... until we reach the standing seem roof.

 

@Nickfromwales a quick bit of maths has the rafters working out to be around 3500mm. They will be butted up to an I beam ridge that will be packed out with timber. Roof pitch is 40 degrees.

 

Please have a good nose through the drawings, our architect is good but hasn't provided us with true detailing. Our SE drawings a more accurate and informative.

 

I look forward to any more insights you can provide.

24066-R01 - Structural Calculation Report 240925 (dragged) 5.pdf 01923-HBA-DR-1008_BR02_Proposed Section AA copy.pdf 01923-HBA-DR-1012_BR02_Window Junction Details copy.pdf

[Russell griffiths]

I wouldn’t consider osb under the sheets, pull out strength of an 18mm screw is not great. 
25mm sarking boards for me. 

[Tony L]

1 hour ago, CJO said:

I am now looking at woodfibre sarking boards following @SimonD suggestion, they seem to also provided a very good vapour permeable substrate.

 

@SimonD didn't suggest woodfibre.  My understanding is, woodfibre sarking boards are used when extra insulation is required, & they're likely held down onto rafters with softwood battens, so their sub-optimal pull out qualities aren't a concern.  If you already have a ton of Kooltherm on your roof, as your drawings show, you don't need to worry about another few mm of insulation.  I think @Russell griffiths is suggesting 150mm x 25mm treated softwood planks.  These don't fully cover the roof - I've seen them installed with 10mm or more gaps between each plank, so they support the standing seam & the gaps assist in drying, should the wood become damp.  I'm no expert, so listen to others' views, but I'd want some ventilation under the standing seam.  I'll try to find a picture...

 

Of course, the problem with the treated softwood 150 x 25mm is, the materials cost is going to be more than OSB (or even 22mm ply) & they're going to take a lot longer to install, as well.  I haven't done the maths yet - I need to, because this is exactly what I'm planning to do for my roof.

[Russell griffiths]

9 minutes ago, Tony L said:

 

@SimonD didn't suggest woodfibre.  My understanding is, woodfibre sarking boards are used when extra insulation is required, & they're likely held down onto rafters with softwood battens, so their sub-optimal pull out qualities aren't a concern.  If you already have a ton of Kooltherm on your roof, as your drawings show, you don't need to worry about another few mm of insulation.  I think @Russell griffiths is suggesting 150mm x 25mm treated softwood planks.  These don't fully cover the roof - I've seen them installed with 10mm or more gaps between each plank, so they support the standing seam & the gaps assist in drying, should the wood become damp.  I'm no expert, so listen to others' views, but I'd want some ventilation under the standing seam.  I'll try to find a picture...

 

Of course, the problem with the treated softwood 150 x 25mm is, the materials cost is going to be more than OSB (or even 22mm ply) & they're going to take a lot longer to install, as well.  I haven't done the maths yet - I need to, because this is exactly what I'm planning to do for my roof.

I priced up ply versus sorft wood 100x25 the ply was a lot dearer. 
but this was in 2020. 

[Tony L]

3 minutes ago, Russell griffiths said:

I priced up ply versus sorft wood 100x25 the ply was a lot dearer. 
 

That's good to know.

 

This is something like what I have in mind for my build (left side image).  What this drawing doesn't show clearly is, there's an air gap above the red membrane (4).  The air gap starts at the gutter & goes all the way up to the vented ridge.  Also, I'll leave gaps in between my 150 x 25s - the gaps will be as big as I think I can get away with & still be able to walk on the roof without any risk of damage; the bigger the gap between each board, the cheaper the roof.

 

 

[Tony L]

Russell's 100 x 25 is probably a better suggestion than my 150 x 25 - assuming there's negligible increase in flex.  The 100s likely save more in materials than they cost in additional labour.  Furthermore, I can see that fitting these is a job that I'd enjoy doing myself, so it doesn't matter too much if they're going to take an extra day.

[Russell griffiths]

I remember why the softwood option was cheaper 

I left large gaps between each board so used a load less boards

not saying how big that’s up to you to decide. 
all fitted with 90mm galv ringshanks nails

my labour was free, it took a bloody age. 
 

I researched this forever and found that a lot of EU countries used big gaps, and they have been installing this stuff longer than us. 

Edited 6 hours ago by Russell griffiths

[Tony L]

4 minutes ago, Russell griffiths said:

I left large gaps between each board so used a load less boards

This has got me thinking: due to planning constraints I've ended up with a messy roof design - I can likely go for wide gaps between the sarking boards on the sections that will likely never be walked on, & this will give a worthwhile saving, but I'm guessing my standing seam will be going on before I get onto cladding the dormer faces & cheeks, so I'll need the adjacent roof sections to be more robust.