Raised deck with rainwater capture underneath

[roger191141]

I'm hoping to build a deck about 50cm above ground level. It will abutt a new conservatory. I propose to construct a water reservoir underneath. Approximate deck dimensions are 6m x 5m. The deck will be supported by three concrete block dwarf walls in parallel and an end wall creating 2 partions. My thoughts are to line the partitions with butyl pond liner, connect them with a pipe, fit an overflow to drain to a garden pond and fit a submersible pump to access the water. Approximately half the house roof area and the conservatory roof (about 6m x 6m) will drain into the reservoir.The reservoir would be capped with black corrugated impermeable sheeting.
Am I crazy or does this sound a goods idea? Any advice on construction will be very welcome.

[welsh brickie]

by all means capture the water from the consevatory roof,But having any water no matter how small, stagnant under the decking will encourage bugs/flies to breed and cause snails&slugs to crawl across the decking.

[roger191141]

That's a good point. Slugs & snails I can cope with but keeping midges out will be a problem. Even a well-capped water butt still harbours them. I'm off to research economic ways of debugging stored water.