I am planning on building a 3.5 by 4m garden shed/workshop, and I am thinking that I'll construct it on pressure treated wood post foundations.
I have some 100x100mm pressure treated pine posts. My plan is to paint on some end grain protection, and dig them into a hole lined and backfilled with crushed stone. My thinking being that the crushed stone will prevent moisture from pooling next to the wood. What are people's thoughts on that? Is the gravel idea even necessary with pressure treated wood? Would I be better with concrete in the hole? Is there anything else I might not have thought of? (Well, the answer to that is yes!)
Some background: I'm on the east coast of Scotland so it's not the driest, I've never seen water pooling in my intended location but my reason for building is that an existing shed has rotted away to nothing. It had been built on untreated timber placed straight on the ground, so far as I can tell.
Best Answer
I live in the Pacific Northwest of the US where it is also fairly damp. In 2000 my neighbor and I built a 140 ft long stretch of fence between our properties. For materials we using 6"x6" 8ft pressure treated Douglas fir posts every 10 ft with a pair of 2"x6" 10ft p.t. cross members between each. The posts were rated for ground contact against insects and rot and came with a lifetime warranty.
After doing a fair amount of research, we ended up digging 3 ft deep holes and put about 4 inches of crushed gravel and a few hand-size rocks in the bottom of each. This is to raise the post above any standing water that might accumulate there. Since it's also very windy here certain times of the year and it tends to blow perpendicular to direction of the fence, we filled the holes with concrete.
Probably a bit of overkill, but it was so much work—especially digging the post holes in the rocky ground—we just wanted make sure it was done right. That's going on 17 years ago now and it's still standing and remains very sturdy.