You need to read up some threads from that area, as pretty much every aspect will have been covered over the years.
Have you understood the area lading of the roof (trusses) & taken appropriate measures?
Have you read water / plumbing regs with regards to back flow (etc).
Have you seen the thread with the commercial "in roof" diverter as a pre-engineered option on harvesting in a low profile manner, its all there as resources, based on others professional / bodgineered set up.
Your problem is likely grey water / potable inlet (badly put, as I'm no plumber) & potential for contamination via back flow which is why keeping the two separate is / was the route of many. ..since some of those earlier posts there are other options.
You calculated the viable run off as a first step?
Thanks @MRGUS:
- I've had a look through the previous threads, but as I'm not sure what terms I should be searching for, I've struggled to find the relevant previous discussions. Is there a wiki or something where previous information is collated / summarised?
- Yes, the structural engineer has confirmed that the joists (part of a steel frame) will happily take 2 tons of water.
- I've had a look at the water regulations and am aware of the necessity for an air gap or similar to prevent backflow into the mains.
- A filter in the gutter is an option, but access to the roof to clean it will be problematic. I'd rather have the filter in the attic as it would be much easier to access / clean / unblock.
- I calculated the runoff previously and the recommendation was around 1.5 to 2 m3. However, I'm not sure whether I'll have enough space for 2m3, so I thought I would start with a single IBC and consider connecting it to another in the future if there is enough space and I can maintain the temperature of the water below 20ºC.