Mo. Rev. Stat. § 292.500
All contractors and owners when constructing buildings in cities, where the plans and specifications require the floors to be arched between the beams thereof or where the floors or filling in between the floors are fireproof material or brick work, shall complete the flooring or filling in as the building progresses, to not less than within three tiers or beams below that on which the iron work is being erected. If the plans and specifications of such buildings do not require filling in between the beams of floors with brick or fireproof material, all contractors for carpenter work in course of construction shall lay the under flooring thereof or a safe temporary floor on each story as the building progresses to not less than within two stories, or floors below the one to which such building has been erected. Where double floors are not to be used, such owner or contractor shall keep planks over the floor, two stories or floors below where the work is being performed. If the floor beams are of iron or steel the contractor for the iron or steel work of buildings in the course of construction or the owners of such buildings, shall thoroughly plank over the entire tier of iron or steel beams on which the structural iron or steel work is being erected, except such spaces as may be reasonably required for the proper construction of such iron or steel work and the raising and lowering of materials, to be used in the construction of such building, or such spaces as may be designated by the plans and specifications for stairways and elevator shafts.
§ 292.500, RSMo