Abstract: The deflection of decking planks under load is tested using a portable deflection measurement instrument, after installation of the planks over spaced joists. The instrument has a bridge on support legs spaced to rest on the deck at points apart from a span between joists of one or more planks to be tested. The legs can rest on planks laterally adjacent to the plank(s) under test, so the bridge straddles over the plank, or the bridge can be cantilevered or can rest on the tested plank but at points beyond the span to be tested. A contact foot or tup is movably mounted on the bridge and coupled to a platform or similar receptacle by which weight is applied to the plank. The resulting displacement between rest and loaded states is measured using a distance scale, and applied as a function of the span length and loading criteria to assess the deck structure.