This study aimed to evaluate the possibility of root fenestration or oroantral communication by evaluating the distance from root apex to the sinus floor and buccal cortex in maxillary posterior teeth using cone-beam computed tomography (CBCT) images. The study included 2182 roots of the maxillary posterior teeth from 219 patients after reviewing CBCT images of 462 patients according to the location of roots by two endodontists. The distances from each root apex to the maxillary sinus floor and buccal and palatal cortices were evaluated according to sex and age, and the mean values were compared by one-way analysis of variance and Mann-Whitney U-test. The distance between root apex and maxillary sinus floor was the greatest in maxillary first premolars and shortest in the mesio-buccal roots of maxillary second molars. The distances from root apex to the buccal and palatal cortical bones were significantly greater in male patients than those in female patients (P < 0·05). The palatal roots of maxillary first molars exhibited the highest incidence as well as the greatest mean length (1·96 mm) of protrusion into the maxillary sinus. The distance from root apex to the sinus floor was found to increase with age, except in case of maxillary second premolars. Understanding the relationship of maxillary posterior teeth with the sinus floor and buccal cortex could provide clinicians valuable information to help reduce iatrogenic damage.