An RSJ or 'Rolled Steel Joist' is s a steel beam, normally used for support in building structures.
RSJ's are sometimes also known as UB's or 'Universal Beams'.
An RSJ is an H shape in cross section and comes in a range of sizes depending on the load it needs to carry.
They are in common use in the building trade today and are typically used to support the load when a supporting wall is removed in a house undergoing internal modifications.
RSJ's are used when knocking through from one room to another.
As they are often concealed between the ceiling and the floorboards on the floor above, Surveyors will advise further structural investigation if the RSJ is not apparent.