I’ve heard of flexible membrane tanks being used in the basement of houses for rain water storage but does anyone know of using one’s roof cavity?
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I briefly considered that, a small amount (to gravity-feed to toilets) using these flat tanks you fit on 4x4 roofs. In the end it was easier to use a normal pressure pump direct from the outside tank.
Other than that… not much. And besides, water weighs a lot, I’m sure there are limits involved that kick in very quickly 
You mean like a geyser? 
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Standard practice in older British and Irish houses.
I remember these header tanks that fed geysers.
I’m thinking more about using this space rather than the tanks installed outside. I don’t think these tanks are a pretty sight and they take up a lot of space…
I would be VERY careful. A 5000l tank weighs about 5 tons full… and even putting in small 500l tanks wouldn’t be a good idea. Chat to the architect first. The roof/ceiling isn’t normally designed to hold that sort of weight.
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In most cases I think roof trusses are pretty much on their design limits.
I think it can work if a house gets designed with that in mind right from the start, for starters a 5000l tank must be installed on a solid level surface which in this case probably means a concrete slab ceiling.
If you go with smaller tanks which can fit in a standard roof space, then the price goes up.
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Having built up a few years ago, custom roof trusses and all that, I learned that roof trusses are designed for a specific purpose.
Existing roof trusses, to go now and add a bladder afterward on top of those trusses, trusses that would have been a custom-made design with Engineering signoff on building plans originally, is not a good idea no.
If you insist on doing it, rather speak to roof truss experts.
Yip.
I did a sort of a diy extension a few years back and I had to provide the truss company the plans, tiles I was going to use and specify if any geysers were going to be installed.
I specified I wanted to add solar and this was added into the design where they added an additional 20kg/sqm.
You can’t just go add tanks on the trusses.
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Can be done however you need to keep the following in mind: water weighs 1kg per litre therefore keep the volume to the weight your ceiling can support, if you do have a truss roof then they are not designed to take a load on that bottom cord.
With that in mind keep it within reasonable volume and I would not bother with a truss roof unless you take professional advice.