Bleach can be damaging to roof shingles if it is not used properly. It is important to dilute bleach properly and to only use it on a small, isolated area of the roof at a time. If bleach is applied undiluted or in large quantities, it can strip the protective coating from the shingles and cause them to become brittle and prone to cracking. Additionally, bleach can fade the colour of the shingles, which may not be desirable if you are trying to maintain the appearance of your roof.