H3 or H4 treated pine? water tank base

Hi guys,

On their advice I just bought a load of H3 treated pine from hardware store. The dude said no problem, should get at least twenty years out of it using it this way.

I am a little sceptical.

I have dug out a level pad for a 15000litre water tank and instead of laying a slab or building a raised timber pad, i came up with the idea to lay a nice and level sand base then a good dose of gravel over this and then simply lay out my treated pine onto the gravel as bearers/joists for some old hardwood decking i scavenged from the tip.
I wanted H4 but they only had h4 sleepers, and too expensive. So i just bought some H3 70 x 35.

So, h3 isnt meant to be in contact with the ground. Do you guys think that my sand and gravel pad will be enough drainage?

Or is this all going to go to ???? real quick?

G'day.

15,000 litres is about 15 tonne in weight. That is a lot for 70x35 to bear.

You may find that the timber will embed into the sand/gravel during prolonged wet weather.

H4 really should be used for ground contact.
But then again.....if you have to replace it in 20 years, will you still be there or sold up and moved on...

Yeah, its a colorbond tank so it will be pretty easy to simply empty it and tip it over to replace the base when it needs it.
Im just hoping to get more than ten or fifteen years out of the h3 on a sand/gravel base.

As for the weight - i bought enough to space them at about 200mm centres, and ill lay them flat, not up on edge so they bed on top of the gravel moreso than cutting into it.

I would simply use a sand base, but the tank manufacturer says not a good idea because of rust and they wont warranty it. They also dont like a cement slab as the bottom traps moisture.

Would anyone reccomend laying plastic over the top of the h3 before nailing down the decking? Or will the trouble come from beneath anyway if it comes. Beneath where it is sitting in the gravel?