The existing ceiling joists are unlikely to be adequate to take a conversion floor, so additional new joists will be required to comply with the Building Regulations. The size and grade would have been specified by the structural engineer, who would have taken into account the span and separation distance for a given loading. The new joists span between load bearing walls, and are normally raised slightly above the existing ceiling plasterwork by using spacers below the joist ends. This spacing must be sufficient to prevent any new floor joist deflection from touching the ceiling plaster below. The new joists run alongside the existing joists. Above window and door openings, thicker timbers are used to bridge the opening, so that pressure is not put on the existing opening lintel. RSJs are also specified to distribute the load, and in some installations are used to carry the ends of the new joists. If head height is limited then thicker joists, more closely spaced can be specified.
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