decking on flat roof

I want to lay decking and a half-height trellis surround on the flat roof of my kitchen/bathroom extension to make a roof terrace.
Basically I'm laying 3x2 direct onto the felt roof in the direction of drainage flow, and screwing the decking directly to them. I plan to make it in four seperate sections, so the whole thing can be more easily lifted for felt cleaning/replacing when necessary.
My question - decking is heavy stuff. I can work out the joist spacing in the extension (450mm centres), but have no way of knowing how big the joists are without destroying the ceiling. Does the fact that the joists are spaced at 450 indicate enough strength to support the weight of my decking?
Thanks for any advice..
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Are you ready for all the leaks caused by laying the 3 x 2 on the felt? BT,DT!
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How old is the extension ?
we had the same idea but planning permission made it clear that it would not be allowed.
Phian
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On 11/05/2011 22:50, Phian wrote:

I don't know the exact age of the extension - at least 40 years, though the felt was renewed a few years back. As to leaks - I'm replacing a ramshackle old decking thats been up there for more than 20 years without causing any hassle. It's made of lightweight palings rather than decking - hence my original question about weight.
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