My hair has been bleached - Is it ok to dread it? - Dreadlocks FAQ's
Bleaching dries and weakens the hair - anywhere from a lot, to so little you'd never notice it. Modern bleaching kits are a little easier on hair, especially when the instructions are followed. Dreads tend to lock well in slightly damaged hair so you'll have no problem there - but over time, since hair in the dread stays around forever (rather than shedding away) there is a good chance that in 5 years or so you'll see breakage in the bleached areas. Not to worry though, they will be at the ends/tips so it's not going to be a big deal.
If you continue to re-bleach the roots take extra special care not to re-bleach the previously bleached hair. It will weaken and damage it in a serious way and you'll have breakage along the body of the dread.