The degree of difficulty of removing subflooring depends on how the sheet was installed in the first place. It's a good idea to screw, not nail, sheets of subflooring. Some carpenters prefer to nail and glue the pieces. However, nails and glue are huge obstacles. Using screws, and no glue, allows you to remove a piece in short order. The screws also allow future trades easy access to the underside of the attic floor.
To remove a piece of tongue and groove oriented strand board subflooring, the first step is to determine the thickness of the material. The tongue edge of the subflooring can be found on the long edge of the oriented strand board. One edge has a tongue and the other a groove. The tongue of one piece of oriented strand board slides into the tongue as the material is installed so the long edges don't sag in between the floor joists. You need to make a cut line down the long edge between sheets of subflooring to disengage the pieces of from one another.
2. Cut the tongue
Some cut the tongue with a circular saw and adjust the cut depth to one-eighth inch less than the thickness of the subflooring. You never want to cut into the top of an attic floor joist or attic truss bottom chord. This will weaken the wood-framing member.
Once the tongue has been cut on the two long edges, the next step is to remove fasteners. If a nail gun was used, it tends to countersink the nail heads below the surface of the oriented strand board. This makes it difficult to pull the nails.
3. Clear the surface
Use a wet-dry vacuum to remove all sawdust from the surface of the oriented strand board after cutting away the tongues. Sweep the entire piece with care. This will help you see the fastener heads used to attach the subflooring to the floor joists.
4. Remove fasteners
There are two demolition tools that might help removing nails from wood. The first tool is called a cat's paw. The tip of this tool has a V groove you drive with a hammer under the head of the nail. Once the nailhead is engaged, you can apply sideways pressure on the tool in order to extract the nail from the subflooring.
The tool pulls the nail out far enough so you can finish the job with a traditional claw on a hammer.
5. Pry up the panel
When all the fasteners have been removed, it's time to pry up the subfloor panel from the floor joists. If you're lucky and no glue was used between the subflooring and the floor joists, the panel should pop right up.
A flat demolition pry bar might help start this lifting job. Drive one end of the tool into the open seam between two floor joists and use the tool as a lever to lift up on the piece of oriented strand board you want to remove. If you meet resistance, it means you missed a fastener or two -- or glue could be your next obstacle. If it's glue and it has a great bond, you may have to cut out each rectangle of oriented strand board in between the floor joists and do a creative job of patching the floor.
6. Reinstallation tips
Assuming there's no glue, the piece of oriented strand board should lift out easily. When it comes time to reinstall it, you'll have to cut and nail pieces of wood blocking under the long edge of all the pieces of subflooring. This blocking takes the place of the tongue you cut away. You need to slide it under the other pieces of subflooring you didn't remove but that are adjacent to the piece you removed.