Height exceptions. The height limitations of this chapter shall not apply to telecommunications structures or antennas, church spires, belfries, cupolas, sirens and loud speakers for emergency purposes, chimneys, water tanks, silos, bulkheads and similar features, and necessary mechanical appurtenances usually carried above the roof level. Such features, however, shall be erected only to such height as is necessary to accomplish the purpose they are to serve and may be required to be screened by the Planning Board.