This article is divided into three sections. Be sure to also read What You Need To Understand Before Deciding To Use Hardwood Flooring Over A Concrete Slab. Then read Installing An Engineered Hardwood Floor Over A Concrete Slab.
First thing to understand is that all 3/4" solid hardwood floors have to be nailed-down. This means you need some sort of wood substrate to nail the solid wood planks too. The National Oak Flooring Manufacturers Association recommends you either install a sleeper system or install plywood on top of the concrete slab first before proceeding with the solid hardwood flooring.
Note: Either method with raise your entire floor 2 inches or more, which can cause problems with entry doors. Also, all the additional sub-floor construction will add considerable amount of costs for both the material and labor to install the sub-flooring.
Be sure to acclimate the solid wood flooring and plywood to the room before installing. This requires all the wood to be in the room for 2-3 days to adjust the moisture content in the wood to be approximately the same as the room. If in cartons, the cartons of flooring need to be opened and spread out to allow all planks to acclimate.
The sleeper system is where a coating of petroleum-based mastic is troweled onto the concrete slab and then polyurethane film on top of that. 2-1/4" wood studs are laid flat on top of the film and then another layer of polyurethane film is laid on top of that studs. The hardwood flooring is then nailed into the studs.
A more popular way is to lay polyurethane film over the concrete slab and then nail or screw down 5/8" CDX plywood over the film. The solid wood planks are than nailed into the plywood sub-flooring. Yet another method is after the polyurethane film is laid down and properly taped is to lay 1/2" CDX plywood of the top of the film and then lay another layer of 1/2" CDX plywood over the top if the other ply wood in the opposite direction and then nail or screw the two layer of plywood together. This prevents any fasteners from puncturing the polyurethane moisture barrier. Both the plywood sub-flooring and the solid hardwood planks need the proper gapping to allow for expansion.
The above gives you a general overview of the various methods used today. It is best to consult with your local flooring specialty retailer to determine what is best in your situation. Most solid wood flooring installation failures over a concrete slab will probably be from excessive moisture present in the room and this will not be covered by the manufacturer's warranty.
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