The most common reason for leakages behind baths and shower trays is movement in the fitting itself (along with obvious broken tiles etc) and the first thing to check is that your fitting is secure. There should be an absolute minimum of movement in either fitting and with a shower tray it is advisable to fit a resin, rather than a plastic one.
Baths can be secured, while in place by removing the bath panel and, (if it is a steel bath) securing through the under-hanging bath lip to the wall after making sure the feet are firmly screwed to the base and all connections on the frame are firm.
Please see our project on replacing a bathroom suite at the replacing a bathroom suite project. If the bath is plastic, you will need to make up some timber “bearers” ( 2inch x 1inch treated timber). Cut the timber to lengths of 400mm and plug and screw them firmly under the edge of the bath, between the main body of the bath and the end walls. There is just room to get in at either end but it is a struggle!! Its is also sensible to apply a liberal amount of very strong construction adhesive to the back of each bearer. A bearer can be placed under the edge of the bath at both ends as well. You will not be able to drill, plug and screw these as there is no room, but if you use the adhesive above and find a way of wedging the bearers in place until the adhesive is dry, you will get no further movement.
When the bath is secure it will no doubt need re-sealing. It rarely works for long to keep placing new sealant over the old. Strip off the old adhesive using a sealant remover. This comes in a tube, as does the sealant, and is applied by mastic gun. When you have removed the sealant, use a rag dipped in solvent thinners or white spirit to wipe all surfaces clean. (wear gloves for this operation and open the windows to allow the vapour to disperse) This will de-grease the top of the bath or shower tray and also the tiles. Leave to dry for a few hours and then apply a high modulas sealant. You can now be sure of good adhesion and hopefully a trouble free joint.
You can also get specific sealant removing products that will also do the job. Your local DIY store should have these in stock. An example can be seen below
It is also worth checking the grout to your tiles at this point. The smallest pinhole in the grout will suck water in behind the tiles, where it will filter through the adhesive and onto the floor below the bath. This kind of leak is not even visible until it has done a fair bit of damage. If you find any grout that is suspect, use a grout rake to rake off the top few mm. You can then mix and apply a new filling and be sure the joints are full. See our replacing grout project for grouting instructions.
Take a look at our video section on tracing and finding leaks and watch the bathroom repair film on how to fix a bathtub or shower spout leak.