The height provisions of this chapter shall not apply to the erection of church spires, belfries, towers designed exclusively for ornamental purposes, flagstaffs, chimneys, flues, gas holders, water towers, standpipes, stage towers or scenery lofts. The height provisions of this chapter shall not apply to bulkheads, elevator enclosures or water tanks occupying in the aggregate less than 10% of the area of the roof or lot on which they are located.