Advice and recommendations

[nickw]

Guys any advice welcome , simple roof with rafters, can I full fill between rafters if I using a vcl and also as belt and braces and extra insulation celotex foil backed pir.? 

I figured it'll be from in out

Plasterboard

Celotex  25mm or 50mm

VCL 

150 MM PIR rafter infill

18mm  t&g osb 3

Membrane

Standing seam metal roofing

 

So questions are

Is a full rafter infill ok

Is there a better way, as I really don't know about these things

What membrane for the osb , so many options and variations. Had looked at the Klober but which one is best, Permo air, Permo sec, or another make like Cromar vent 3.

 

Any suggestion or advice very welcome

 

 

 

 

 

Edited May 5, 2019 by nickw

[Oz07]

Don't think so. Usually 50mm vent gap for condensation. Boffins will know more than me 

[nod]

As above 

whatever you decide to put in You must allow for 50 mil airflow 

[Onoff]

How deep are your rafters? If 150mm then 100mm pir max...I think.

[Russell griffiths]

Have a look at your roof manufacturers recommended build up, any metal roofing needs to either go on a warm roof or needs to be ventilated. 

I have been trying to work out the details for months, and have decided to do my own thing. 

Ventilated gap, and then gaps between sarking boards instead of osb. 

[Thedreamer]

I've used roof shield on ours.

 

https://www.proctorgroup.com/products/roofshield#key-features

 

I think the 50mm gap was standard practise for your old fashion non breathable felt and some manufacturer claim that no gap is required, but most people still incorporate a gap for good measure.

 

Not an issue this time of year, but you have to be careful if it's frosty, as this can ruin the membrane.

 

Lots of stuff on the forum about membranes, but invest in a good one. No point spending thousands on your final roof covering if your membrane ain't going to last.

 

If I was starting again I would consider going for warm roof with insulation on top of the rafters. Some on here have used a wood fibre board to provide this insulation and provide strength to the roof.

 

 

[ToughButterCup]

11 hours ago, nickw said:

[...]

can I full fill between rafters if I using a vcl and also as belt and braces and extra insulation celotex foil backed pir.? 

[...]

 

Yes. That is our roof build-up. We have 80mm of celotex underboarding, and 200mm between the rafters. 

[AliG]

@nickw asked me about this as we have full fill insulation in our roof.

 

It depends on whether you have a warm(insulation above the rafters) or cold (insulation between or below the rafters) roof.

 

If you have a warm roof VM Zinc can be installed without any ventilation. This requires a VCL below the insulation and a breathable membrane on top. This is detailed here on the VM Zinc website. My flat roof area is of this construction.

 

https://www.vmzinc.co.uk/our-solutions/vmzinc-roofing/vmz-standing-seam-warm-roof-on-rigid-insulation-structural-roof.html

 

My pitched roof area is tiled. It is a cold roof with insulation between and below the rafters. It is still, however, full fill with PIR as the ventilation is below the tiles which are raised off the sarking on battens. Thus I do not have any ventilation into the roof and any unsightly vents in the eaves. Again there is a VCL below the insulation and breathable membrane on top of the sarking. The  sarking also has small gaps between the boards to allow it to breath, it is strips rather than OSB boards. You should not use T&G in this application.

 

The problem with a VM Zinc roof in a cold roof application is that it is fixed directly to the OSB, as well as wrapping tightly around the edges usually. As it cannot be vented between the OSB and the zinc it needs ventilation below the OSB. This is detailed in these methods on the VM Zinc website

 

https://www.vmzinc.co.uk/our-solutions/vmzinc-roofing/vmz-standing-seam-ventilated-roof-on-open-gap-timber-boarding.html

 

https://www.vmzinc.co.uk/our-solutions/vmzinc-roofing/vmz-standing-seam-cold-roof-on-vented-plywood.html

 

Suffice to say if you want a Zinc roof it is better if it is a warm roof.

 

Edited May 6, 2019 by AliG

[Onoff]

1 hour ago, AnonymousBosch said:

 

Yes. That is our roof build-up. We have 80mm of celotex underboarding, and 200mm between the rafters. 

 

But what have you then got on top in terms of ventilation gap if any and roof membrane?

[PeterW]

4 minutes ago, Onoff said:

 

But what have you then got on top in terms of ventilation gap if any and roof membrane?

You don’t need a breather space with a lot of the new membranes. You can also get away with sarking and counter battens for air space 

[Onoff]

14 minutes ago, PeterW said:

You don’t need a breather space with a lot of the new membranes. You can also get away with sarking and counter battens for air space 

 

So it would be helpful for the OP to know the full build-up.

[Russell griffiths]

It’s really up to @nickw to go back to his roof manufacturers and find out what they want under their sheets, 

if he doesn’t install to their spec he probably won’t get a warranty. 

 

I will I’ll be using a membrane called klober permo sec metal, which is a breather membrane with an extra layer of spun polypropylene on top to allow any condensation to escape, wicked expensive but I have not been happy with the roof makeup that the manufacturers want. 

[AliG]

That is useful info @Russell griffiths

 

So Klober perm sec metal is 8.5mm thick and has a mesh that allows water to run down through it.

 

This it slightly raises the metal above the sarking, similar to what a batten does with a tile and allows an unventilated metal pitched roof.

 

See page 2

 

http://klober.co.uk/media/uploads/56c711838b31b.pdf

 

The only thing not clear is, does the bottom edge of the metal have to be left open somehow allowing moisture to exit and presumably drip into the guttering? I guess you could still wrap the zinc around the edge of the roof as long as you left a small gap behind it.

Edited May 6, 2019 by AliG

[Mr Punter]

Our contractors used the Klober permo type product in the past under a zinc roof.  It has a layer of what looks like course trimmed pubic hair to create a gap.

[Russell griffiths]

@AliG

i Am hoping to use double sided tape to stick the eaves end of the membrane onto the eaves flashing, leaving it slightly short from the sheet end to allow an open end to the mesh stuff for moisture to drain out from and into the gutter. 

[nickw]

Looks like I'll amend it slightly to from roof down

steel roof

permo sec metal membrane

18mm osb

50mm pir on rafters

150mm full fill on rafters 

tyvek airguard vcl

50mm battons

25mm celotex GA4000 

12.5MM Plasterboard

 

should suffice I hope.

[AliG]

The recommendation from both Kingspan and Celotex is that in this scenario the insulation above the rafters should be equal or greater than the insulation below the rafters or there may be condensation issues.

 

The easiest is usually to use 100mm above the rafters and 100mm between. This is also usually the cheapest board to buy. You would just push the 100mm right up and leave a space underneath.

 

200mm of PIR like this will give you a u-value of 0.11, I am not sure that the extra 25mm underneath would make a big difference relative to the cost. Also you should really do this as it would make the insulation below the rafters thicker again.

[nickw]

thanks Ali for the info, I had seen the recommendation re the above and between being the same but was hoping that 50mm would be ok. If I use the permo sec membrane would I have to worry about the condensation though?, with the extra 25mm below I take your point re the cost and bugger all difference in u-value.

Overall I was also trying to keep the roof thickness down, I will have one neighbour who will be measuring with a laser to make sure I don't go over anything that combined with trying to get a usable loft attic space just meant keeping the thickness down.

At the moment I have planning for slate roof, we are putting in for metal roof now the mortgage company has agreed but it almost seems more trouble than it's worth.