Your question is a bit like saying to a doctor, I have a headache whats wrong?
Without a close inspection you wont know what to use, you cant just assume sand and cement.
Use whatever is right for the age of the property. If it is an old and listed building you may have handmade clay or peg tiles, then it will not be cement to use but traditional lime mortar.
If they are concrete tiles I would use sand and cement.
Clay tiles expand and contract with heat and moisture more than concrete tiles and lime is more forgiving and flexible than a pug of sand and OPC (Ordinary Portland Cement). If your working on a property with lime, when the old bonnets are lifted you also have the option to save as much of the old mortar as possible, break up the lumps back into a crumb and add water again, this way you make it into a reusable lime mortar again, as the lime reacts again with the water. But you will probably still have to buy more lime. When it dries it will set as before. Lime is not available (i dont think) from your usual builders merchant, dont be confused with powdered Hydrated or hydralime, thats not traditional lime mortar. I buy it mail order, it comes in a poly bag inside a bucket or box ready mixed. You can add soot to darken it, but it will soon age on a roof anyway. Look for Chalk Down Lime ltd in Sussex on the net. Keep it air tight in the bag in the bucket and you can use it for weeks on end, unlike cement. I do window restoration on period buildings and carry lime to point in around window to brickwork sometimes, and I have had a 20kg bucket of lime for a year in my van, it cost about £7. It dries a bit with time, but add water and away we go again.
You can send a sample to Chalk Down Lime who will match the mix for colour and texture.
If the tiles you are working on are concrete and set with Portland cement, go with what the other guys say earlier, you will soon know it, the cement will not fall away from the tiles like lime, and in fact it will be almost be impossible to get off clay tiles, they will break for sure.
With clay tiles and lime, the lime come off quit well and makes everything reusable with less damage. When restoration work is done on old lime built buildings never use OPC. Once OPC is used you damage the building, and any further work causes more damage to adjoining tiles, so if someone has used OPC in the past allow for this with time and money to replace it with lime.
And I doubt you will have the option to choose from re-pointing to re-bedding, when you get up there and start the job, it will become pretty obvious if they are loose and need total re-bedding. If on the other hand they are sound and just have some pointing gaps, you can simply point them in.
