sentences:  Above the targeted bottom height (coplanar with the compacted sand and gravel topping) a separate grid of rebar or welded wire mesh is usually added to reinforce the concrete, and will be tied to the under slab "girder" rebar at intervals.The under slab cast girders are used especially if it the slab be used structurally, i.e., to support part of the building.Floors in wood-frame homes are usually constructed with joists centered no more than apart, according to most building codes.