Deflection is the degree to which a building material bends when it bears a load. A trampoline floor would have high deflection; a stone slab would have fairly low deflection. Building codes set the maximum deflection allowed for the framing of a home under the kinds of loads that can be expected in a residence.Typically, codes set deflection limits for roofs, walls and floors.he L/X Formula. The maximum allowable deflection of a ceiling is expressed as a fraction in the format L/X. The L is the length, in inches, of the clear span of the joists or rafters that make up the ceiling framing.