Apologies if the title makes this sound rather more than grand than it actually is – it's just a very simple piping system to capture the rainwater falling on the roof and then re-direct it inside the greenhouse.
I started with three 3 metre lengths of 20 mm plastic conduit that I'd bought a year ago for the solar panel installation, before I discovered that I couldn't fit two pairs of cables inside each pipe and therefore ending up buying 25 mm instead for that application.
Two lengths of the conduit I had were white and one black. I had a few fittings already to hand, but topped up the shortfall with a few more from eBay. The inspection covers on the PVC elbows and tee were sealed up using 'Evostik' Pipe Weld adhesive, and then pressure tested by holding my hand over the open end(s) and blowing into the other.
The long lengths of irrigation pipe were drilled every 150 mm or so – I started with 3 mm holes near the feed end and increased to 4 mm and then 5 mm as I progressed further along the pipes, the idea being to encourage water flow along the whole length of each pipe. The open ends are temporarily sealed with blu-tack until I get around to making some proper plugs.