A fiddley deck is a raised, exposed deck open to the engine room or boiler space below. The space between the machinery space and the fiddley deck is known as the fiddley. The fiddley deck serves as a raised coaming for ventilators or intake vents, increasing protection from downflooding by green water on deck. The enclosed air space acts as an air reservoir, as well as providing a removal or installation route for large pieces of machinery. The vertical height acts similar to a chimney, drawing heated air out of the machinery spaces. The smoke stacks are typically run through the fiddley when one is present.
For additional ventilation, the fiddley deck may open, consisting of a grating of bars on a coaming and capable of being fit with a hatch cover. In this case, fiddley can refer to the grating, the coaming, or both dependent upon context. As the fiddley directly accesses hot working spaces, on a steamship operating under power the fiddley and fiddley deck would get quite warm - areas around the fiddley often formed ad-hoc areas of respite or refuge for sailors during cold weather.