Windows are typically installed using one of two installation system options – a barrier or drained:
Barriers do not provide for forgiveness to water infiltration through or around the window opening and should be limited to areas of low rainfall potential or where the area where the window is located is protected by overhangs or porches.

Drained installations are designed to manage small amounts of water infiltration through or around window openings, by collecting and draining the water back out to the exterior. Central to the performance of a drained opening is a pan flashing.
A pan flashing is installed (or should be installed) below every window opening and is designed to direct any water which may leak through or around the opening back out to the exterior. When a window is sealed into a rough opening the side jambs and header should be sealed, however the sill should be left free to drain to the exterior.
In my days as a post frame building contractor, we found at least 10% of all our window installations leaked, when using standard flanged vinyl windows. When we went to ordering vinyl windows with J channels attached, we found our window leak callbacks were all but totally eliminated.
In an extensive study done by ABTG (Applied Building Technology Group, LLC) 113 laboratory and 127 field tests were conducted on installed windows. Remarkably, the laboratory installations had far fewer instances of leaks than the field window installations (actually no real surprise)!
About 20% of field-tested window installations leaked through the window-wall interface (in layperson’s terms – between the window unit and the rough opening). Extraordinarily – over 50% of all these same windows experienced water leakage through the window unit itself!!
Given this information, it is amazing window sill pans are not required by code!
The conclusion of the testing was providing a subsill drainage path was considered more effective than attempting to achieve better quality in window manufacturing and installation.
If you are like me – you are scratching your head wondering something similar to…why manufacture a product which is inadequate in over 50% of all cases?
Now maybe the windows you will install in your new post frame (pole) building will never leak. Odds are there is a potential they will. There are commercially available solutions such as products manufactured by Sure Sill, which are available at your local The Home Depot™ or available online at www.homedepot.com.

Knowing what I do now would I invest in sill pans?
Most probably.