Velux windows

I have some scaffolding going up soon to repair a leak on the valley fo my roof. At the same time I propose to put two Velux roof windows in, ready for a loft conversion which I shall undertake next year.
I am intending on renting the property in the future and was wondering if anyone can tell me whether I need to make one of the VELUX windows a VELUX fire escape window. Also as it will be about 50foot up do I also need to supply a ladder to get someone from the roof to the ground?. Any knowledge/wisdom would be greatly appreciated.
Thanks Mark Barlow
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Hugo Nebula's 'Loft Conversion FAQ' will probably be of use. (Google Groups for that subject - it may be at www.diyfaq.org.uk)
Hugo mentions that you can have a roof with rafters but no joists, with a (steel) ridge beam to prevent spread. I can't visualise how that works - can anyone explain?
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