The slope from the tanks down to where the drain box is is an issue as the level is too steep for a stable base with the potential issue of subsidence and undermining of the closest tank.Consequently I had to build a (fairly simple) box. The first one I’d used 20mm treated pine had started to warp, so take 2 with 38mm treated pine and several structural reinforcements. I’d also liberally applied several layers of bituminous paint.
Hopefully the extension will be long lasting…