I hope you will be able to give me some advice. I have a basement bathroom in which we had glued down flooring some years ago. I have noticed that the flooring is lifting in areas. It seems that there is something under the places that have lifted. When you press on them you can feel some resistance and hear some crackling. I am suspecting that there may be some of that white mould, or is it salt, that is coming up through the concrete? Could this be causing the problem and what can be done about it? I want to replace the flooring and I am wondering if ceramic floor tiles may be a better choice than the vinyl flooring.
Thank you for your time
There are often problems with adhering any type of floor covering to a concrete basement floor slab due to several reasons. The concrete floor may be uneven and have cracks that may expand and contract, slightly, with seasonal changes. The concrete is normally cooler than the surrounding air and may be damp due to condensation or moisture in the soil under the slab. I will try to offer some suggestions, but you may have problems with any style of flooring you use.
The first issue to address is the crackling and white powder under the flooring. The crackling is a sign that the surface of the concrete is deteriorated and is spalling. The white powder is probably not mould and you may be correct that it is salt. These salts are know as efflourescence and will leach out of the concrete, or from the cracks, due to moisture in the concrete. As previously stated, the moisture may be due to condensation, especially in a bathroom, or from below the slab. It may be difficult to determine where the source of the moisture is, but that may be irrelevant.
The first thing to do is pull up the old flooring and inspect the surface of the concrete. If it is crumbling or spalling, it will have to be patched before any further work is done. If there are substantial cracks, greater than 3mm wide, these will also have to be dealt with. If the cracks are moderate to large in size and efflourescence is seen, then adhering new flooring directly to the concrete is not a good idea. If there are small cracks and minimal deterioration, new flooring may be possible. The surface of the concrete should be cleaned and dried before patching small areas and cracks with a high quality concrete patch. This patching should be applied with an additive to help it adhere to the old concrete.
Once the patching is done, the floor may be smoothed out by applying floor-levelling compound. Caution must be taken to use compound designed for use on concrete, not wooden subfloors. The crackling material you hear may be old floor leveller that is deteriorated. If new vinyl flooring is to be installed, the higher the quality, the less likely it will be to lift. Most vinyl flooring has a composite backing that is not very moisture resistant on the underside. This will deteriorate, over time, and may be partly responsible for the current problems you are facing. There are 100 % solid vinyl floorings available that are moisture resistant and ideal for use on concrete floors. Check with the flooring supply store for the correct adhesive to use with this flooring in your application. Caulking the exposed edges along bathroom fixtures and baseboards with mildew resistant silicone caulking will also reduce the chance of lifting.
Ceramic tiles are an ideal floor covering for bathrooms, but may not be practical in your situation. These tiles are very hard and durable, but are also brittle. Because of this property, the substrate or subfloor that they are installed on must be very stable. If the concrete basement floor slab is cracked or has even slight seasonal movement, the tiles may crack. If there are no cracks and only the surface is deteriorated, tiles may be possible if installed over a metal mesh that is secured to the concrete and covered with a thin-set mortar. If cracks or evidence of movement in the floor is seen, there may be a better option.
Installation of a wooden subfloor over top of the concrete will allow installation of any style of flooring desired. This may have limited applications due to low ceilings or other obstructions in the basement bathroom area. If enough headroom is present, treated wooden strapping may be installed and secured to the concrete to allow for installation of a plywood subfloor. This will also allow for shimming and levelling of the strapping to minimize any slope existing in the current concrete floor. The subfloor and any material in contact with the concrete should be treated to prevent rot and mould due to dampness or moisture under the new flooring. If plywood 19mm thick or over is used for the subfloor it may be sufficient for installation of ceramic tile or may easily accommodate a mesh and mortar bed. If vinyl flooring or other material is to be installed, a smooth thin layer of subflooring over the plywood may be required to allow a smooth surface for to the new flooring.