Start by evaluating the existing floor assembly. The new hardwood flooring must be installed on a wood subfloor of approved thickness that is at or above grade, flat, and well-fastened to appropriately spaced joists. You can check the joist spacing from the basement or crawlspace, but to confirm the subfloor material you’ll have to remove a small section of vinyl. Make sure the vinyl is well-adhered, and plan to remove any existing trim or cove base.
Don’t install nail-down flooring over particleboard underlayment — particleboard doesn’t hold nails adequately and will literally dissolve if it ever gets wet, wrecking the whole installation. Use a pin-type moisture meter to check the moisture content of the subfloor before you start. Vinyl is a moisture retarder but not a moisture barrier; if there are high levels of moisture in the subfloor, it will eventually affect the finish floor as well.
Finally, follow the manufacturer’s installation instructions or the NWFA (National Wood Flooring Association) guidelines and plan to use a fastener length that provides a minimum penetration of 5/8 inch into the wood subfloor.
If it’s not possible to meet the above conditions but a nail-down floor is the only option, you’ll have to remove the existing vinyl and upgrade the subflooring and joist system as needed. Another alternative is to go with a glue-down installation, which some manufacturers approve for their solid flooring products. If you go this route, you must closely follow their specific instructions for installation over vinyl.
Posted with permission from Hanley Wood, LLC. Originally posted on JLConline.com by Daniel Boone on July 13, 2012 at http://www.jlconline.com/flooring/installing-hardwood-flooring-over-vinyl.aspx.