accessing roof

So on the holiday we just got back from the vent over rear kitchen fell apart, it was all in pieces, so I guess even with the cover over it the sun still gets to them, it is a 2004 model, anyway have bought what should be the correct elixer cover and want to know how smart would it be to extend slide out then put a board over it and step from a ladder onto it then onto the roof?
I do have a 8 foot step ladder, the thing must be all of 12 feet tall, so it appears it would be a safe way to get up on top, I do not like heights and this would appear to be one way to get on top, just curious what you all think?