Every five years, the actuary of the retirement board shall make an actuarial investigation into the mortality and service experience of the contributors and beneficiaries as defined in this chapter, and he or she shall make a valuation, as of June thirtieth of each year, of the various funds provided for by this chapter. Upon the basis of such investigation such board shall:
1. Adopt for the retirement system such mortality, service and other tables as shall be deemed necessary;
2. Certify the rates of deduction from salary necessary to pay the annuities authorized under the provisions of this chapter.