Over the years, old building (and above all their timbered ceilings) will have become crooked and distorted to a greater or lesser extent. The timbers will have suffered from creep and shrinkage, and not infrequently, the beams will sag in the middle.
The structural timber elevations plan forms the basis for the decisions about the method to be used for re-levelling the structure . If the level differences are minor (between 1 and 3 cm), the level can be restored simply by means of the in-situ poured concrete. However, if the differences are greater, then a separate re-levelling procedure needs to be established before the concrete fill, or the concrete must be installed as a skew layer, with a compensatory layer being used to establish a completely level structure. This would consist of lightweight or aerated concrete.
One of the most common problems, a sagging ceiling, is solved by reversing the sag first (by spragging from below or by pulling-up with previously built auxiliary supporting structures) and then installing the concrete under load.
Level check with a laser rule