There shall be such distribution of the profits of such society among the workmen, purchasers and members as shall be prescribed in the certificate of association, at such times as therein prescribed, as often at least as once in twelve months, but no such distribution shall be made until a sum equal to five per cent of the net profits shall have been appropriated for a contingent or sinking fund and such appropriation shall continue to be made until there shall be accumulated a sum equal to thirty per cent of the capital stock of such society.