Why so many terraces have an open roof void
It is not a defect in the sense of something that went wrong; it is a building practice that was normal when these houses were built and that later standards superseded. A Victorian or Edwardian speculative terrace was constructed rapidly, in runs, with a continuous roof structure spanning the whole row and the separating walls built up to carry the ceiling joists and no further. Slating over a continuous structure was faster than closing each wall to the underside of the covering, and the fire risk that concerns us now was not the design consideration it later became. The consequence is a void that can run uninterrupted for several houses, sometimes an entire terrace, above the ceilings. What that means in practice is worth stating plainly, because it is easy to treat as a technicality. A fire starting in a neighbouring house that reaches the roof space has an unobstructed path into your own roof, above your ceilings, and can be established across several properties before anything is visible from inside any of them. Fire services encounter this regularly in terraced streets, and it is the reason a fire in one house so often damages the roofs of two or three. The same void carries smoke, and it carries sound — voices, music and footfall from the neighbouring loft pass over the wall as readily as anything else. Modern requirements close it: the wall between adjoining dwellings must provide fire resistance for its full height, taken up to the underside of the roof covering, so that each house is a complete compartment. Retrofitting that in an unconverted roof means crawling in a confined space and working awkwardly around rafters and purlins, which is why it so rarely gets done on its own. During a loft conversion the roof is already open, the structure is already being altered, and the work is a small line in a much larger contract. That timing is the whole argument for doing it properly rather than minimally.
