The object of using two sets of joists is to prevent the passage of sound, and sometimes, as in dance halls, to prevent the vibration of the floor being communicated to the ceiling. While this construction is undoubtedly the most perfect (when wood must be used) for accomplishing these objects, it is very objectionable from the standpoint of fire protection, as it affords free passage for flames in both directions, and the material is so disposed as to be rapidly consumed. It should, therefore, never be used in public buildings unless protected underneath by metal lath and plaster, and above by some fireproof material, such as "Salamander," laid between the floor boards. It is probably hardly necessary to say that this is not an economical construction, as it requires much more lumber to obtain the same degree of stiffness for both floor and ceiling, when divided into two beams, than it does when all the wood is in one beam.
It has also been quite a common custom in the construction of school house floors, in some sections of the country, to lay 2x4-inch scantlings on top of the floor beams, and at right angles to them, for the purpose of ventilating the rooms through the floors into the vent shafts.
Such construction should never, under any circumstances, be adopted, as it forms a veritable fire trap in case a fire should be started in the room above.