How do I level 18mm plywood on top of joists?

The joist should be level to begin with or reasonably level anyway. If there not you could try getting some thin ply like 6mm or thinner to use as packers to level out the joists were there a bit low. Regards Frankie BT Roofing Services

Hi, I would suggest using packers to level the joists.

If the floor is way out or loose then joists or showing signs of damp or dry rot then replace the entire lot. If not then you should pack out the joists as necessary using plastic packers which do not rot or expand or move over time & a pack comes with lots of different sizes so you can add together to get correct height. You can but these from b&q or screwefix for about £13 a bag. Hope this helps.

NEVER use plastic packers for joists as they are non structural and designed for spacing or packing windows. . Use natural slate which is non compressive. I would also recommend using a tongue and groove floating chipboard floor which you glue down to avoid squeaks.