Before you start your project please consider...

• The floor will need to be over-layed with WBP or Marine Ply, minimum 15mm thick, primed on the under-side, and screwed into the joists with counter-sunk screws at 300mm intervals (that is to British Standards). 
• It may be necessary to lift floorboards / chipboard sheets to reinforce the joists with noggins if the floor has a lot of deflection.
• Note, it is acceptable to replace floorboards / chipboard with 18mm WBP or Marine ply and tile onto that.  It is also possible to tile directly onto tongue and groove floor boards (using appropriate flexible adhesive), but the boards must be very securely fixed with no deflection (otherwise the tiles will crack / come loose).  It is not appropriate to tile directly onto chip / particle boarded floors, these must be overlaid with ply, or removed and replaced with ply.
• If you are planning a new kitchen or bathroom, it is much easier to board the floor once all the old furniture / units have been removed and BEFORE the new bathroom suite / kitchen units are installed.  Please consider this when planning your project - it will save you time, money and ensure a professional, long-lasting installation.  
• If you have new kitchen units installed before boarding the floor then please ensure the kitchen Fitters leave sufficient gap between the worktops and the washing machine / dishwasher etc to accomodate the ply, the tiles, and the adhesive, usually at least 30mm.  Otherwise it will not be possible to tile under these units (unless you raise your worktops) and this will be a less professional finish. 
• You need FLEXIBLE adhesive for tiling onto any wooden substrate (whether ply or tongue and groove boards).  Please be aware that flexible adhesive can only accomodate LATERAL movement, but NOT vertical (caused by deflection, or a "springy" floor).  A simple test is to fill a cup with water and place it on your floor.  Jump up and down near the cup and see if any water spills.  If it does, then the floor will need to be strengthened and boarded prior to attempting tiling.

If tiling onto a concrete floor:

• Most concrete screeded floors are not flat and level (due to the drying out process).  Even on new build properties, or newly built conservatories, it is industry practice to level screeds using latex self-leveling compound BEFORE the final floor finish is applied (whether ceramic / porcelain / stone tiles, carpet tiles, vinyl tiles, lino etc).
• Some customers are put off by the thought of having to have their concrete screed "leveled" prior to tiling, and instead ask "can't it just be tiled over as it is, and any out of level areas built up with tile adhesive?"
• The answer is yes it can, however, this is NOT the correct way to solve an out of level floor:
• Firstly floor tile adhesive (especially flexible adhesive, required for porcelain and natural stone tiles) is much more expensive than leveling compound (so it is a false economy to use more adhesive than is necessary just to save a few pounds on leveling compound);
• Secondly, adhesive manufacturers specify a MAXIMUM bed thickness for their adhesives.  If you exceed this, for example trying to compensate for an out of level area on the floor, then there is a risk that shrinkage may occur in the adhesive. This means that hollow points beneath the tile may form, which can then cause the tiles to crack if a chair leg, or a high heel happens to be placed above it.  Also, shrinkage effectively pulls the adhesive away from the tile, so tiles may become unstuck and loose;
• Compensating for out of level floors with tile adhesive may take much longer in order for the Tiler to achieve a professional finish with no lipping between tiles (so it will cost you more overall).
• If there are high points on the screed, the Tiler may recommend chiselling them off prior to applying the leveling compound.  I have encountered customers who have been put off by that suggestion, stating "I don't know, that sounds like a lot of work".  Let the Tiler do the work (or get another Tradesman in) - high spots can be cut off of a screed in a couple of hours.  As long as the cost for the preparation work is clearly shown on the quotation, and is a fair and reasonable cost for the time it will take, then don't worry about how much work it is for the Tiler - that is why you are hiring a Tradesman, so you don't have to do the physical work.  Good preparation is the key to a successful tiling project. 

We hope these planning tips have been useful, and please feel free to contact us for a friendly discussion and free advice about any aspect of your project.  Good luck!