Vapour barrier for ceiling - weight and fixing

Just putting together the list for bits for the roof.
The plan is to have a vapour barrier underneath the joists which will initially support the insulation and eventually be covered by plasterboard.
I was planning to use the DPC left over from the concreting but this is pretty heavyweight stuff and visualising trying to fix this to a ceiling is not giving me good vibes.
So:
(1) What weight of plastic sheeting should I use for this vapour barrier? Web sites don't seem to be specific about vapour barrier the way they are about DPC.
(2) Is stapling O.K. as a method of fixing?
I saw http://www.screwfix.com/prods/48677/Power-Tools/Nailers-Staplers/Tacwise-140EL-Pro-15mm-Nailer-Stapler and wondered if this would make the fixing easier - fixing anything over your head and holding it up at the same time is likely to be entertaining for spectators but not participants. 25 does not seem a lot for a labour saving device. I have a manual stapler but this is possibly a bit cumbersome. Then again I might just want a shiny toy to make me feel better.
(3) Most vapour barrier seems to come in very large sheets. Is it better to cut the sheets down and lose some coverage through overlap but gain on ease of handling?
The basic plan is: Put up the joists at 400mm spacings. Vapour barrier under 4 joists. Fill 3 gaps with insulation. Fit 1m (coverage) wide metal roofing sheet. Proceed to the next 3 or 4 joists and repeat.
The reason for doing it one sheet at a time is to allow working up through the adjacent joists when fixing the metal roofing. The metal roofing is very light and so it doesn't seem advisable to plan to work from the top of the roof. To do this would require the construction of special crawling boards.
I have ordered Plastisol Hi30 from http://www.clarkesofwalsham.co.uk /. Has anyone else worked with this kind of sheeting?
Cheers
Dave R
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You don't. You use foil-backed plasterboard, if you're replacing the plasterboard anyway.
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Andrew Gabriel
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writes:

The one downside is that I'm trying to fit the insulation as part of the roof installation. If I get rained on then fibreglass and vapour barrier should not suffer unduly. However plasterboard may well not like this. I would prefer to install the insulation from the top as this seems easier than installing it from below then trying to keep it in place until I can get the plasterboard up. As far as I can see, to do this I would have to push up the roll of fibreglass then install something to keep it in place such as netting or a polythene sheet. If I'm going to have to do this then I might as well install the support first, drop the fibreglass in from the top, then put the roofing sheet on. The plasterboard is not a replacement - this is a new build.
Other posters have suggested that foil backed plasterboard is unreasonably expensive compard to normal plasterboard.
I will check.
Thanks for the suggestion :-)
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writes:

<snip>
From Travis Perkins with my Trademate account, vapour check plasterboard is approximately 5 + VAT more than the standard item. I calculate that I need 10 sheets to do my ceiling. So that is 50 + VAT extra.
Cheapest vapour barrier I have found so far is http://www.diy.com/diy/jsp/bq/nav.jsp?action tail&fh_secondid07871&ecamp=trf-005&CAWELAID&6891125 at 16.98 although there is not information about the weight of the plastic (which prompted my original question).
I already have enough heavyweight black plastic left over from the floor but it would need cutting into manageable pieces.
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writes:

Just found this: http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionIDE36
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A few posters near the end of that thread know what they're talking about, but most of them don't. The fact that the builder didn't know how to use it without wrecking it is no surprise. I've used it myself several times with no problems at all, and that's why I DIY rather than use a clueless builder. Using an expert tradesman is another matter - I just used one who's done some very nice leadwork for me.
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On 06/05/2010 15:19, David WE Roberts wrote:
snip

I have used that or something very similar but as an additional layer to a leaking lean-to roof.
The sheets were supported by transverse bearers rather than longitudinal rafters. I don't remember any problem walking on the sheets when supported that way but I certainly wouldn't want to risk it when the rafters run in the same direction as the profile rather than across.
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Undeclared part of the roof design is transverse bearers.
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