Gluing solid and engineered wood flooring to concrete

Gluing Wood Flooring to Concrete
The three most important factors in fitting wooden flooring to concrete, whether it is engineered flooring or solid hardwood, are moisture, level, and preparation.
Moisture. If there is the slightest suspicion that the concrete has damp issues, then do not even put wooden flooring in the same room, let alone fit it. If the house is relatively new, or the concrete floor has been installed by a competent builder, then there will be an adequate damp-proof course, and you should have no problem. However do bear in mind that in a new build it can take months for a concrete screed to dry out completely. Similarly any structural work or plaster needs plenty of time to dry. If your floor is to be installed by a professional fitter or a builder, then they will have the equipment to test the concrete for moisture content. However if you are doing it yourself, there is a simple test. Get some squares of polythene (about 18 inches) and tape them individually round each side to the concrete at various locations on the floor. Leave them for one or two days. Provided there is no condensation underneath, then the concrete should be sufficiently dry. If the house is an old property, the chances are that you are replacing old carpet with wood. In this case, was there any evidence of damp when you took the carpet up? If there was, then you need to consider applying a water-proofing barrier onto the concrete before you fit any hardwood flooring. There are a number of products available from builders’ merchants.
Level. Check your concrete floor to see that it is level. It should be flat to within 5 mm over a 3 metre run. If it is not, then either grind down any high spots, or alternatively apply a self-levelling compound, making sure that it is fully dry before attempting installation of the wood flooring.
Preparation. The concrete must be clean, free of grease, and particularly free of dust, otherwise the glue might not stick to the concrete, leading to failure of the wooden floor.
Gluing Down. There are two types of wood flooring glue available - one which is usually applied from a cartridge using a special gun, which effectively replaces joists in that you squeeze out strips at about 350 mm centres - one which you spread over the whole floor using a notched spreader. With both of these systems it is important to follow the manufacturer’s instructions, and to avoid spreading too much before you start to fix the floor. As with a nail down system of fixing hardwood flooring, it is essential to plan out your work before you start, leaving a 15 mm gap between the wall and the first board, and ensure that the first row of boards is perfectly straight. See fitting floors. As well as using pegs or spacers, an alternative is to miss out the first row and instead, nail or screw a batten to the concrete just where the first row would end. You can then use this as something to push against as you work your way across the room with subsequent rows, removing it only when the floor is finished, and replacing it with a row of floor boarding. Finally, take your time. Fitting a wooden floor is task which should not be rushed. Prepare enough flooring for two to three rows and only apply enough glue to fix these, otherwise you will find yourself kneeling in the glue. The next thing you know will be glue on the surface of the new floor. So keep everything clean. Once you have fitted a few rows, you should be able to work from the wooden floor rather than the concrete. Also make sure that you do not get any glue between the boards, i.e. in the tongue and groove.
Gluing Solid Hardwood Flooring. The natural tendency of hardwood boards to bend and twist is a factor which needs to be considered. If boards are not reasonably straight you have to find ways of holding them in place while the glue sets sufficiently to hold them. With a nail-down system this is relatively easy because once the boards are cramped or pushed into position, they are held immediately with the fixings. With solid boards you may have to use weights or a method of bracing, e.g. battens against the opposite wall and wedges. For this reason it is easier to glue down narrow boards since they are easier to straighten. With wider boards, and particularly long ones which are not straight, consider cutting them into shorter lengths and use these to start and end rows.