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Last edited 11 Aug 2023
Floor level (FL)
Finished floor level (FFL) refers to the uppermost surface of a floor once construction has been completed but before any finishes have been applied. So, in concrete construction it may be the uppermost surface of a screeded finish, or in timber construction, FFL will denote the top level of floorboards, chipboard or ply decking.
Finished floor level does not take into account any applied finishes as these may be added by owners, occupiers or tenants, will vary in thickness, e.g a thick shag-pile carpet has different thickness to a laminated floor, and may be replaced.
Structural Floor Level (SFL) or sometimes Structural Slab Level (SSL) in the case of concrete frame buildings, describes the floor level prior to the addition of finishing items such as screed, underlay and flooring. The numerical measure will normally be written next to the abbreviation on a drawing often with a + to indicate vertical increase or - in the case of basements. The figure will be a certain amount above or below whatever the agreed setting out position or datum is. For example the setting out position might be 0.00, the SSL +3.0 and the FFL +3.3.
Building Ground Level (BGL) might be used to describe the setting out position for the building ie the 0.00 from which all other levels will be measured. The height of this in relationship to the site itself will depend on the approach taken and agreed by the project team, though it will normally be slightly higher than the Ordinace Survey (OS) Level or Natural Ground Level (NGL) to allow for structural foundations.
Ordinance Survey (OS) level, is based on the accepted levels or measures of the lay of land across the United Kingdom, these are calculated through the use of Ordnance Survey Bench marks (BMs) which are survey marks that are made by Ordnance Survey to record the height of the landscape above Ordnance Datum. Based on these and the process of surveying, the more localised OS or what might be call Natural Ground Levels are calculated and recorded.
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