I am building a new house and it sits on piers. The sub-floor I am using is yellow tongue particle board.
I have just bought some bamboo floors and wondering what's the best type of underlay for it and ideally how thick should it be, as well as what should I be looking for when choosing the underlay.
Best Answer
I hate to be the bearer of bad news, but particle board is not a good choice of subfloor, unless it has gotten better over the years. (It has, at least in Australia, see edit below)I asked the question of floor suppliers and here on SE about fastening flooring to particle board with an answer that said nobody will guarantee the floor will stay in place.
Your picture of the floor was very dark, but it did look like it was 3/4" T&G nail down. If your subfloor was plywood, you can go directly over the subfloor, no underlayment needed. It looks like you may need to screw and glue down a plywood underlayment, 1/2" would be my choice, but others may have had luck with thinner, over the particle board floor to keep the fasteners in place that hold the bamboo floor.
Edit
You need to look over the install guide for the yellow tongue, it appears it can have flooring nailed directly to it. It uses some terminology in the guide that I am not familiar with. I can guess what it means, but the idea I get is that you do not need to place an underlayment over it like I originally posted. But you need to read the install guide to determine it too.