Half Life Help-please

Just wondering if anybody could help me through a bit of half life. I'm in the chapter named Blast pit and i cant seem to reach the roof of the silo. Apparently your meant to start the fan, quickly climb the ladder, asn then jump onto the fan and let the stream of air take you up. Now i've done all that but each time i jump on the fan i just keep falling through and dying. I know this might be a newby request for aid but i really am dumfounded, i have no idea what i've done wrong. So if anyboby could please offer me some helpful advice it would be much appreciated :}

^^ yes, you need to climb up to a higher position, wait for the fan to be at full speed, and then jump.

Well if what they said doesn't help open up the console, type in "sv_cheats 1" (or something like that) then enter "noclip" and simply fly to the roof. Then enter "noclip"again and all is well.