As a general rule, we would say use a primer of some description to prepare your floor prior to tiling. However, it’s not always necessary to use a primer, as there are many high quality adhesive products available which are specifically formulated for installing large format natural stone tiles and porcelain floor tiles onto many different substrates without the use of a primer.
For instance, an old concrete floor that was tiled previously will probably not need a primer, as long as the surface is clean and free from old adhesive and anything else that was stuck to the floor.
Priming Wooden Floors
Before you even consider the priming of a wooden floor, you need to be satisfied that the floor construction is suitable to take the tiles. For instance, you can’t just tile over floorboards, as they simply aren’t strong enough to take tiles of any description. The tiles will probably crack and peel up over a period of time. You must “overboard” floorboards with a suitable thickness of marine grade plywood (not the cheap “sheathing” ply you can get) - basically, the thicker the better (12-18mm), as this will give the floor more rigidity. The plywood surface will then be the surface to be tiled.
Most adhesive manufacturers say their flexible products are suitable for use on “wood” and generally they are, but we would recommend the use of a primer on the wood surface anyway to ensure you have the best possible surface for the adhesive. We hear stories all the time about tiles peeling up after a while due to poor floor preparation, or a combination of bad practice and poor floor prep! Even after your floor has been overboarded, it will probably be trampled on by other trades like plasterers, or even the tiler themselves. Dirt, plaster, adhesive and all manners of stuff will be ground into it before the tiler gets round to tiling it. So your nice new clean ply floor isn’t so clean anymore and if it’s not prepared properly prior to tiling, the adhesive will just go onto the dirt on the floor and might not stick! This is why it is better to prime it than not to.
Priming New Concrete Floors
New concrete floors, or screeded floors are a different matter. Once the concrete or screed has been down for a while and you are satisfied that it is fully dry, the surface will be dusty from the effluence left on it after the concrete cured, or there will be general building dust from builders and plasterers walking all over the floors as they carry out other building work. While adhesive manufacturers say you can tile straight onto new concrete or screed, we would strongly recommend the use of a primer to ensure there is a good “key” for the adhesive to take to. Imagine if the floor was not fully clean and there was still dust covering it, the adhesive will sit on top of the dust and not the concrete itself. Eventually, this can lead to delamination where the tiles simply peel up! Not good…..
Priming Liquid Screed Floors (Anhydrite)
Primer is absolutely necessary if you have an anhydrite screed (aka calcium sulphate). This is a liquid screed pumped onto the sub-floor that self-levels. It’s quite common in modern construction and has many benefits over sand and cement screeds, but you can’t tile straight onto it with standard adhesives. If you do, there will be a chemical reaction between the screed and the adhesive, and the adhesive will eventually separate from the screed.
There are specific preparation requirements for anhydrite screed – assuming these have been done, you will then need to use a good quality acrylic primer to seal the floor. Two coats will be required at 90 degrees to each other i.e. one coat one way across the room and another coat the other way. You will need to let the first coat dry before attempting the second one. Typically, the primer is watered down by two parts water to 1 part primer, to allow it to soak into the screed. If it’s too thick, the primer will just sit on top of the screed and it will probably cause delamination of the tiles over a period of time, especially if underfloor heating is involved, and it usually is when anhydrite has been used.
There are a number of good quality primers you can use. Some people choose to use standard building product like SBR that are widely available in builder’s merchants as it’s a more economical way of going about this. We recommend the use of premium brand products like the Kerakoll Primer A which is specifically formulated for this kind of work, and any other floor priming.
What you absolutely shouldn’t use is PVA. This isn’t meant to be used as a floor primer – it’s ok for school glue and sticking wood together, but not much else. It might give you a reasonable key for the adhesive to stick to, but it will not stick to the floor properly and your tiles will probably peel up. It’s just not worth the risk for the sake of £20 or so.
If you require any further technical advice, feel free to speak with one of our specialist team who will be happy to discuss your technical requirements.