Fork lift truck trainers and manufacturers rightly advise against driving across a slope for obvious reasons.  I find it hard to imagine that every surface a fork lift truck is driven on is perfectly flat.  I know that trucks should be driven directly up/down the slope but also appreciate in the real world that there are /must be occasions where the advice is ignored.  Rather than ignore advice I want to know what angle constitues a slope (or where to find the information).  If I can find that information then I can place markings of where/where not to drive.  Any assistance is appreciated.

There is no definitive answer in this case. Different types and sizes of FLTs will have different tipping points. The load will also affect the tipping point of whatever FLT is carrying it.

The reason you do not drive across a slope is that you don’t really know at what angle you are attacking the slope, as slopes as you have mentioned are very rarely regular and it is very easy to just go over the tipping point without realising it.