Installing carpet directly over particle board sub-floor used to be industry standard, but these days it's frowned upon. Particle board is often used as a sub-floor because it is cheap and generally flatter than plywood, but it has problems of its own. However, if you have particle board sub-floors in your home, fear not. There's an easy fix before you install your carpet.
Types of Particle Board
There are three main types: MDF (Medium Density Fiberboard), OSB (Oriented Strand Board) and basic particle board.
MDF is heavy and uniformly brown, with no visible fibers. It isn't often used as sub floor, and is more often used to make furniture, speaker boxes and faux-paneling. OSB and fiberboard are distinguished from one another by the size of the wood fibers. OSB has larger wood strips, arranged roughly in a directional pattern (or oriented). Basic particle board has smaller, but visible, fibers, and is the type most often used as sub-floor.
The Problems with Particle Board
Particle board is used because it's cheap and flat. The reasons for these benefits are also the causes of its problems. Particle board is made when waste wood is glued together and pressed into a sheet. Wood warps because it tries to regain it's natural shape, and trees don't grow in straight lines. By making boards from small pieces of wood, it has no natural shape to warp back to.
However, these small bits of wood do not hold fasteners well. Particle board tends to break around staples instead of holding, and screws often pull out, taking large chunks of the board with it.
Also, if particle board gets wet, it tends to swell, and the adhesive holding it together fails. If adhesive is used to install your carpet, it could deteriorate your particle board.
Carpet can be installed over particle board. It just needs something in between. Most contractors will use 1/4" to 3/8" plywood as an an extra layer, or underlayment, over particle board before installing carpet. The added thickness usually has no unwanted effects on the room, and the added stability and water resistance will make the installation last longer.
Use no glue and plenty of staples when installing the plywood underlayment, then install your carpet as the carpet manufacturer recommends.