SSL stands for Structural Slab Level: the level of the top of the structural slab as cast, before screed, topping or floor finishes are added.
What SSL means
SSL is Structural Slab Level: the top of the structural concrete slab as it is cast, before any screed, topping, adhesive or floor covering goes on. It is the number the formworker and the concreter actually set to, and the number the structural engineer nominates on the general arrangement plans.
SSL exists as a separate abbreviation because the structural floor and the floor you walk on are two different surfaces at two different heights. The architect nominates the finished floor level; the structural engineer works back from it by the specified build-up and nominates the slab level. Get the two mixed up and you either lose your finishes thickness or end up with a step where the drawing shows none.
On steel or timber framed floors the same idea is usually written SFL – structural floor level – because there is no slab. SSL is the concrete-frame version of the same term.
How SSL appears on a drawing
SSL appears as a spot level on structural plans, and as a labelled line on sections and slab details. In Australia and the UK it is written in metres to three decimal places against the project datum:
SSL RL 24.290
The relationship to the finished floor is the part worth reading carefully. If the architect has FFL RL 24.350 and the floor build-up is 40 mm screed plus 20 mm tile and adhesive, the slab is set 60 mm lower:
SSL RL 24.290 = FFL 24.350 LESS 60 FLOOR BUILD-UP
Wet areas and recessed thresholds get their own callout, usually as a set-down off the general slab level:
SSL RL 24.240 - SLAB SET DOWN 50 TO WET AREA
On a multi-storey concrete frame with an assumed ground floor datum, the same information reads SSL +3.240 (FFL +3.300). Either way, the useful check is that the difference between SSL and FFL equals the build-up called up in the section and the finishes schedule.
Terms it gets confused with
- FFL – finished floor level, the top of the finishes on Australian sets, and higher than SSL unless the slab itself is the finish. Some UK sources take FFL as the top of the screed and exclude applied coverings, which narrows the gap to SSL; check which reading your set uses.
- SFL – structural floor level. Same intent as SSL, used on framed floors and by offices that prefer it generally.
- TOC – top of concrete, and TOS – which is genuinely ambiguous: on some sets it means top of slab, on others top of steel. Read it in context and query it if a level depends on the answer.
- Soffit level – the underside of the slab, normally written SL or U/S SLAB. I have occasionally seen an abbreviation legend use SSL for something other than the slab surface, so I read the legend on every new set rather than assuming, and the rest of the level abbreviations are in my A-Z list of engineering drawing abbreviations.
- FGL – finished ground level, outside the building, which is a completely different surface again. On a slab-on-ground it is the top of the slab – the SSL – that the NCC’s minimum height above external finished surfaces applies to, not the finished floor level.
AU and UK versus US
SSL and SFL are standard in Australia, New Zealand and the UK, written as levels in metres to a datum – AHD in Australia, Ordnance Datum in the UK, or an assumed site datum. US structural sheets normally express the same thing as an elevation in feet and inches and label it T/SLAB, T.O.S. or TOC, often against an assumed ground floor of EL 100'-0". SSL is not a common abbreviation on US drawings, so if you are producing shop drawings from a US structural set, expect top-of-slab callouts rather than SSL.
Where to check on your own set
- Structural general arrangement plans – the primary home of SSL. Check whether the notes say levels are to top of slab or top of screed.
- Architectural plans and sections – the matching FFL and the build-up. Subtract and see whether the two sets agree.
- Wet area and threshold details – set-downs, hobs and rebates, which are where the slab level departs from the general level.
- Abbreviation legend and general notes – confirm SSL means the slab surface on this set.
- Datum note in the title block – so SSL values from two consultants can be compared at all.
References
- Designing Buildings Wiki – Floor level (FL), SFL and SSL (supports the SSL definition; note it uses the narrower UK reading of FFL that excludes applied finishes)
- Levels on drawings (AS 1100.301 conventions summary)
- Geoscience Australia – Australian Height Datum
Other meanings of SSL
Outside construction, SSL overwhelmingly means Secure Sockets Layer, the legacy web encryption protocol superseded by TLS – which is why searching the bare abbreviation returns web hosting and certificate pages. In audio it also means Solid State Logic. On a building drawing it means neither.