How to achieve a crispy skin when roasting a chicken?
The issue in achieving crispy skin is managing the moisture content. At least one day (but as many as 3) before you plan to roast the chicken, salt the exterior and interior with kosher salt.  Place in the refrigerator uncovered.  This will draw out moisture.

I also prefer roasting at much higher temperatures, in the 425F to 450F range. This will help as well.

Finally, try to use chickens that were not frozen first...or, if they were, plan ahead so they thaw very slowly in the refrigerator.  Quick thawing pf chicken causes the release of an excessive amount of liquid when cooking.