RobertL explained on 03/03/2010 :
Thanks, it was cracking I was worried about. I hadn't even though
about falling through!

The corrugations are about 6" apart and about 2" deep.

I fell through one of my roofs last year - only about 6' down onto
piles of soft materials, but it could have been much worse. It cracks
without any warning at all. I was using a plank BTW.

If you are going to use planks, the plank needs to be strong enough to
support all of your weight without transferring any onto the roof
mid-span, it also needs to have the weight only supported over the
beams via spacers and the weight under the spacers needs to be put onto
the roof via some material with some give in it.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk