Covering a wooden floor with 3mm hardboard.

Should be ok. Mine is held down with 1/2" staples fired by a (manual) staple gun, and is ok.

Use 8x4 sheets to minimise the number of joints - with the long edge perpendicular to the floorboards. Try to avoid having any joints coinciding with floorboard edges. Give some thought to any small areas which you may to get at to access wires and pipes - and make provision for these now.

use 4mm WBP ply instead

RT

In article , Set Square writes

aaand, shiny side down to give the carpet something to grip. You can go right up the skirting, grippers for the carpet (if used) can go on top. I taped the joints with gaffa tape on top and sealed the joint at the skirting with a sealant gun as I had a draughty floor.

Lay the hardboard at 45 degrees to the floorboards to avoid all undulations being parallel to the walls - seems to 'feel' better.

Forgive my ignorance, but what is WBP ply? What's the advantage of this vs hardboard?

Water and Boil Proof ply, says it all, really.

hardboard is stiff cardboard and thus not moisture resistant whatsoever.

RT

Depends on the grade. IIRC oil tempered hardboard is what you need.

Carpet tape sticks better to the shiny side though! (more contact area / less dust).

I was going to crawl under the floor, cut access hatches and tape from the bottom but decided against it ;-)

Rough side up def best to stop the carpet skating. I initially planned to lay one sheet of board then lift it a tad to place a piece of tape (face up) on the floor, then lower board, then adjacent board onto it - an obvious faff and totally hopeless. At that point decided to tape on top & to my amazement it stuck pretty soundly, but then I use nuclear strength gaffa and not that post-it strength stuff they sell in the sheds.