You will need to take into consideration the existence of any ramps or sloped surfaces that must be traversed and determine what type of equipment is required in order to maintain safe operation when ascending or descending the sloped surface.
One way of describing a ramp or sloped surface is to describe it in terms of percent of grade.
One way of describing a ramp or sloped surface is to describe it in terms of degrees of slope angle.
You can use one of these conversion charts to translate Percent of Grade into Degrees of Slope Angle and vice versa.
The manufacturers' gradeability specifications which are published in specifications sheets and sales literature are engineering specifications which are a measure of the mechanical efficiency of the power train components under ideal conditions and often have nothing to do with safe forklift truck operation or how a user can expect the vehicle to perform under normal operating conditons.
The recommended maximum grade for sit-down rider forklift trucks (ITA Class I (Electric), Class 4 (I C Cushion Tire) and Class 5 (I C Pneumatic Tire) Powered Industrial Trucks) is 15% Grade which is about 8-degrees of slope angle, which is approximately one foot of rise for every 7 feet of run.
The recommended maximum grade for stand-up rider forklift trucks (ITA Class 2 Powered Industrial Trucks) is 5% Grade which is about 3-degrees of slope angle, which is approximately one foot of rise for every 20 feet of run.