Flat warm roof- VCL needed with celotex?

[Bagheera]

Hi all,

I'm building a timber framed garden room with a flat warm roof.

my build up for the roof will be:

joists
18mm OSB3 sub-deck
vapour control layer
100mm foil backed celotex GA4100 or similar
11mm OSB3 top deck
EPDM

My questions are:
1. Do I need a VCL considering the foil on the insulation acts as one?
2. Would trying to use foil tape somehow to connect the undersides help?
3. If not, what is a good product that conforms with b regs EN... (I've seen that screwfix polythene sheets do not conform)
4. What tape and how much overlap do I need to connect together the polythene sheet (if indeed best option)?
5. When fixing the top deck aren't I going to pierce the VCL as I need to screw through the insulation and sub deck into joists below, is that an issue and is there a self sealing product that can withstand being pierced without allowing vapour through?

Any clarification would be fab. Cost is somewhat of a consideration but mainly want to make sure it's not going to have condensation in the roof. Attached is a photo of current progress. May make it clearer.

Cheers!

 

[Notch7]

Fill any big gaps with foam, tape all the joints with foil tape on top.

You could put in a vcl, but I dont think its necessary. If the garden room is not attached to the house, then it may spend some time unheated. In which case the inside could be colder than out sometimes.

Id be a bit nervous of using 11mm top deck.

Are you doing a sedum roof?

If you are putting in firrings they need to go under the insulation

 

[Bagheera]

Thanks for the reply Notch7. It's detached from the house and I'd not thought about how it may be cooler inside than out, though I doubt cool enough for warm water vapour to condense? That said I'm sure the EPDm is vapour proof so that wouldn't be an issue though I take your point about taping from above. Hopefully there won't be any gaps to fill but if there are any indentations from delivery I'll fill with foam.

No firings as joists are sloped (should have gone with firings in retrospect)
No sedum roof, (at least not at first)
I went 11mm as this is what I used to sheath the walls and when I built them on the floor I noticed that they were very solid to stand on unsupported between 16 inch centre studs. On the top deck, they'll be supported by PIR insulation from below so I'm sure they'll be fine. I've seen an EPDm go directly over insulation and also insulation go straight on roof joists. It will be for access and maintenance only. I'm working on my own so I don't know how id get 18mm up to the top, it's so darn heavy!

I suppose if there were any condensation issues in the future I could add a VCL below the rafters and above the plasterboard.

 

[Notch7]

18mm up to the roof -lean 4 x 2 against building side at angle and maybe temp fixed in place, slide the board up till it reaches balancing point and push a bit and let go.

Mark out your joist positions accurately and transfer to top of decking.

You will need 160 or 180 screws

You will struggle getting the epdm up on the roof, that is heavy!

 

[tomfe]

You'll be fine with 11mm ontop, for the cost you may as well add a vlc below your joists.
I would get a hand to put them on your roof. You can slide them up a ladder and walk up behind it etc but it's so much easier AND SAFER with 2 people. If you've not had the 18mm delivered yet as ask the delivery man to give you a hand and offer them some cash.

 

[Notch7]

A vcl is only thin plastic really.

The disadvantage of vcl below joists is if you have downlighters. It still can help limit vapour transfer.

Foil backed plasterboard could be an option.

 

[tomfe]

A vcl is only thin plastic really.
The disadvantage of vcl below joists is if you have downlighters. It still can help limit vapour transfer.
Foil backed plasterboard could be an option.

You'll have the same problem.

Sorry vlc should go ontop of the deck like op said. You can use the fancy self adhesive vlc or just a plastic sheet.

 

[Bagheera]

Thanks everyone. Super fast!
I'd forgotten about my downlights which would go through a VCL below joists so making it a bit useless.

Great shout to mark out my top 11mm with joist positions and number them and their position before I put insulation on. Not thought of that either.

Already got the 11mm and 18mm delivered already. Couldn't ask the fella to give a hand as he also delivered my joists which needed to go up first. Some nice tips for getting it up there though!
I have to say that the rope method on YouTube has been very effective for moving the boards around.

If I do go for an extra sheet can someone point me to a decent product that b regs would approve of? Also any suggestions for how to do the VCL on the walls while we're about it