Hi @faigey,
Welcome to the Bunnings Workshop community and many thanks for your question about replacing roofing screws that have started leaking.
It definitely sounds like the rubber washers have degraded over time due to the exposure to changing weather conditions. Replacing them will certainly help to stop the issue.
Unfortunately, the only way to replace these screws is from above, so if you are not comfortable with this, it's probably best left to the professionals.
Depending on the height, pitch and circumstances around the job, roofers will often use edge protection scaffolding or wear fall protection such as this Bailey Fall Protection Pro Roof Workers Kit attached to a Bailey Fall Protection Edge Type Roof Anchor to ensure their safety while carrying out this kind of job.
When walking on roof structures, the general rule is to follow the screw lines as they indicate roof battens are underneath which will be able to support a load whereas the sheets will not. As the sheets are polycarbonate, there is a certain amount of flex in them that will allow walking on top without damage.
Please be aware of the risks involved and make an educated decision on whether this is a job you're comfortable doing and take all reasonable precautions to ensure your safety. If you're not comfortable with the risks, I'd suggest contacting a handyman to assist.
Let me know if you have any other questions.
Jacob
