The leaks are getting out of hand so rather than paint I´ve decided to tile is there an addative that will make the cemento cola and grout doubly waterproof?

The best thing you can do is to fibreglass it using the red resin and paint.

You can then tile it if you want. There are specific 'SIKA' products for adding to grout/mortar to make it waterproof - a good ironmongers will have these products as will Leroy Merlin in Gandia

Leaks out of hand ... I know the feeling!

Might be something useful here?
Cómo impermeabilizar azoteas y terrazas
If you are tiling over existing tiles it might be sensible to put down a new impermeable membrane between the layers but you will need everything to be dry first.

What sort of tiles will you be using? Sika do grouts and waterproofing additives for cement and are very high quality products. Have a look at their website. Some of their products are sold in Leroy Merlin.

What is the surface of the roof at the moment? Flat roofs and roof terraces around here are covered in caucho rubber paint, and then fibreglass matting and then caucho again. Then more matting laid the other way and then more caucho. This is then tiled.

Can you not use ..Swimming Pool water proof paint..

Hola

Swimming pool grout is waterproof - but more expensive

Davexf

Can you not buy one of those big sheets of rubber they use these days when building? Then slap a layer of cement over it and ceramic tiles (if you want to walk on it) or paint.

Thanks for all the replies I'm using tiles simply because I inherited some boxes of floor tiles with the house and they've been taking space up in the shed for at least 10 years.The cement additive seems the way to go and because the roof has no access my dodgy tiling will stay a secret

Wellington time

Having done the maths,painting is the cheapest option.All the paints I've looked at are in a similar price range.Has anybody any recommendations as to the brand with the best results?

the best brand i know is Mapei, don´t know the price for it though - excellent for all wet roomd , leaks etc.

I bought TKROM based only on the fact that all the local contractors use it and that I could get trade price on it.The quality will only be evident in a year or two
(it's guaranteed for three) bought the fibre from Leroy Merlin two times ten metres for a nineteen metre roof.......and it was two and a half metres short. Leroy Merlin were there usual dis-interested selves

I had excellent service at the Le Roy's in Huelva but the rest are a waste of fresh air

Le Roys in Marbella is absolutely useless..you might aswell ask someone in the car park about what you need. In fact I did once and he said he would carry my wallet! Which was nice!

Yes I agree the Marbella one is a waste of time.