Building or replacing a sub floor is not particularly difficult, but careful thought should be given to the type of product used. Standard MDF (Medium Density Fiberboard) is a wood composite held together using urea formaldehyde resin. All types of formaldehyde resins release a harmful gas long after installation. Gases are emitted for extended periods when using urea formaldehyde resin products. When formaldehyde-free products are not available, seal the sub floor using layers of paint primer and finish.
Measure the piece(s) of MDF needing for repairing or installing the sub floor using a measuring tape. Pull a chalk cord over the lengths needing cut and snap a straight edge to use as a guide. Do this by pulling up on the cord and releasing.
Cut the MDF board using a circular saw. Wear a dust mask to avoid breathing the dust.
Sand all surfaces and edges using a disk sander with medium grit sandpaper. Switch to a fine grit sandpaper for a smoother finish.
Paint the MDF board with latex primer using a low-nap roller. Use a brush on the edges. Let the primer dry for two hours.
Lightly sand the surface using the fine grit sandpaper. Wipe the surface with a clean rag to remove dust.
Add a second layer of primer to the surface. Let the MDF dry for another 2 hours.
Sand the surface one last time using the fine grit sandpaper. Wipe the primed surface with the rag.
Paint the MDF with a finish layer of latex paint. Allow the piece(s) two hours to dry and install MDF sub floor using galvanised screws and a power screwdriver.
Keep MDF material dry to avoid damage. Shawn Sadler of Painting Ideas & Techniques states, "Even with MDF being one of the best types of wood products to paint, problems can still arise when coating it's surface. Moisture in the wood causes painted areas to crack, bubble or lift."
Lumber yards and home improvement centres might tell you that MDF containing phenol formaldehyde is safer, but it is not a formaldehyde-free product. Healthy Building states, "Phenol formaldehyde results in 90 per cent less formaldehyde, but does not completely eliminate formaldehyde emissions altogether."