The height limitations of this chapter, as shown in Table 2, shall not apply to the following structures: church spires; belfries; silos; cupolas; domes not used for human occupancy; chimneys; ventilators; skylights; water tanks; bulkheads and other necessary mechanical appurtenances usually carried above the roof level, parapet wall or cornice for ornament extending above such height limit not more than five feet; radio or television receiving antenna; or a public utility transmission tower or cable. No such uses shall in their aggregate coverage occupy more than 20% of the roof area on which located.