Hard Board of Plywood?

[a1topdog]

My floor boards have gaps and slight uneveness from being lifted and relayed.

Before the carpet goes down I was thing about laying a hardboard or plywood.

All rooms to be done will be bedrooms.

Which shall I go for and what thickness?

Also will it have to be secured to the floor boards?

Many thanks.

 

[DAZB]

Lay 3mm Hardboard and nail it at about every 6-8 inch using Annular or Ring nails but don't use nails longer than about 15mm because of the pipes and cables below your floors.

 

[a1topdog]

Thanks.