When installing horizontal glass (rooflights) the glazing performance changes; this must be taken into account in the U-value. For example, glazing with a stated U-value of 0.60 W/m2K should be entered as 0.90 W/m2K in PHPP when it is installed horizontally to take account of this. EN673 has a calculator to determine this accurately.
Note that rooflights are very difficult to shade from overheating in summer.
It is very hard to achieve good installation details with a rooflight, although manufacturers have developed installation kits that improve performance.