Sagging ceiling drywall is usually caused by using 1/2” thick drywall that is too thin for the 24” spacing typical in most roof truss installations. Our Mosby Building Arts standard for roof trusses that are framed 24” on-center is to use 5/8” thick drywall or 5/8” thick fire-rated drywall which has fibers to reinforce and maintain the flatness of the ceiling. This also increases the added fire retardant quality of the material we sometimes desire in some applications.
Another common cause for this problem is too little attic ventilation that can allow excessive moisture to build up in the attic. The summer heat tends to “steam” the moisture and drive it into the drywall ceiling material and promote sagging when it gets moist & soft. I suspect your sagging ceiling may be a combination of these two factors.