DIY Doctor UK DIY Forums

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.

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.

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.