Loft joist spacing

6 Mar 2011
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United Kingdom
Sorry if this is a stupid question, but I am trying to get to grips with this DIY stuff.

I read that standard loft joist spacing is 60 cm or 40 cm. I also read that the vapour barrier is supposed to be continuous. Does that mean that the vapour barrier should continue under the joists?

In my house, there seems to be a 2 cm gap between the underside of the joist and the ceiling plaster board. I think the ceiling is spaced from the joists by some metallic strips that the ceiling bopards are mounted on.

Now, if the vapour barrier is supposed to be continuous, I guess you are supposed to space the joists at 60 cm between centres. Say you are using 4 cm wide joists, then you would move 2 cm of wool aside on each side of the insulation strip so that the paper can meet in the middle under the joist.

But .... my joists are at intervals of 63.5 cm. Which I guess is no conincidence because my roof trusses are 3.5 cm wide. That leaves an exact gap of 60 cm between each joist for the insulation, not 60 cm between centres. But is also means that the vapour barrier is discontinuous, by the width of the joist. Is this normal practice?
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There is an easier way ! Just buy either a roll of plastic sheeting or three or four of the thin plastic sheets used as protection whilst decorating ( often sold as individual 5 m x 4 m sheet in a packet for E 1 or E 2 ) and simply drape the across the joists, taping all joins and then lay fibre insulation on top
Kevin652, you should count yourself lucky that MOINtainwalker has answered your post.

As he is part of a Elite build force (sniggers) :LOL: he ususally finds it a burdend to answer such trivial posts. ;)

@MOINtainwalker, hows FMT doing? Has he toned himself down yet? Let him know theres a new man in town ;) ;) ;)
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Joist spacing typically depends on the load they are supporting. Normally the idea is to build economically with as few materials [ie joists] as possible, but greater loads would mean more joists closer together

Another factor is modular sizes of sheets, so for a 2.4m plaster or ply board say, you would want joists spaced at 400 or 600mm centres to avoid cutting sheets

Ceiling do not have to be fitted to the underside of the ceiling joists, but again it cuts down on work and materials if they are. But say an acoustic ceiling, may need to be separate from the structural joists.

Vapour barriers are intended to prevent passage of moisture from one side (the room) to the other (the loft), and they must be continuous to function properly- otherwise moisture can breach the barrier and it becomes ineffective

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