TOS means top of steel: the level of the top of a steel member, normally the top of a beam’s top flange, used to set out the frame.
What TOS means
TOS means top of steel – the level of the top of a steel member. For a beam that is the top of the top flange. For a column it is normally the top of the cap plate, though some offices quote the top of the shaft instead, so it is worth confirming on each job.
Steel frames are set out from TOS, not from finished floor level. The fabricator cuts, drills and detail-checks against it. The erector shims base plates to achieve it. The surveyor checks it before the deck goes down. Every downstream dimension – deck bearing, cleat position, service zone, ceiling height – hangs off that one number, which is why it gets its own callout instead of being derived from the floor.
How TOS appears on a drawing
On a framing plan, TOS is either annotated member by member or given once for the whole level with local exceptions noted.
ALL TOS RL 8.400 U.N.O.
SLAB: 150 TOTAL ON 54 DECK - TOS RL 8.400, TOC RL 8.550
B12: 410UB53.7 - TOS RL 8.400
B14: 360UB44.7 - TOS RL 8.250 (DROPPED 150 - SLAB STEP, SEE 6/S22)
COL C3: 250x250x9 SHS, TOS RL 12.900, U/S BASE PLATE RL 4.200
The slab line is doing the work there, and it is worth spelling out. The deck bears on the top flange, and the 150 is the total slab thickness measured from that bearing – not the topping over the ribs, which on a 54 mm deck is about 96. So the concrete surface lands 150 above TOS, at RL 8.550. B14 is dropped a clean 150 because the floor itself steps at that line, so its own slab finishes at RL 8.400, a 150 step down from the main floor. If instead a beam is dropped to carry a thicker slab, say so and give both levels rather than writing a build-up in the brackets.
The column line carries two set-out levels for the same reason. U/S base plate RL 4.200 is where the steel starts; the concrete underneath is lower again by the grout bed, commonly 25 to 50 mm, so the pier TOC is a different number.
The US form, on a project datum:
TOS EL. 112'-6"
The section designation travels with the TOS callout, so it helps to know which family names it. Australian and New Zealand hot-rolled sections follow AS/NZS 3679.1 – 410UB53.7 is a universal beam of 410 mm nominal depth weighing 53.7 kg/m. Structural hollow sections follow AS/NZS 1163 and are sized by face and wall thickness, as in 250x250x9 SHS. The American equivalents are AISC shapes: a W-shape such as W16x36 is a wide flange of nominal 16 in depth at 36 lb/ft, and hollow sections are HSS, as in HSS 10x10x3/8. Design sits with AS 4100 in Australia and AISC 360 in the US, with AISC 303 setting out who provides what on American shop drawings.
Terms TOS gets confused with
- Top of slab. The dangerous one, because it is the same three letters. On a composite floor, top of steel and top of slab differ by the full slab thickness measured off the top flange – deck depth plus topping, commonly 120 to 200 mm. A set that uses TOS for the slab should use something explicit such as T.O. STL for the framing. Check the legend before you detail anything.
- BOS, bottom of steel: the beam soffit. That is the number the services coordinator wants, not TOS.
- TOC, top of concrete, which on a composite deck sits above TOS by that same full slab thickness.
- Top of deck. TOS is the flange, not the sheeting bearing on it. The deck profile adds its own depth on top.
- Base plate level. Not the top of the concrete under it. The grout bed sits between them, so a base plate RL and the pier or footing TOC are two different numbers.
- TOR, top of rail, on rail and crane drawings. A crane runway beam has both a TOS and a TOR, separated by the rail and its pad.
- Top of support. On plant and piping drawings TOS is sometimes read this way. It usually still resolves to the top of the steel the support is welded to, but not always.
AU and UK versus US
The abbreviation is used everywhere. What changes is what follows it: metres to three decimal places against AHD in Australia, metres against Ordnance Datum in Great Britain, feet and inches with EL against NAVD 88 or a project datum in the US.
American structural sets very often set the ground floor at EL 100′-0″, so a TOS of 112′-6″ means 12 ft 6 in above the ground floor, not 112 ft above the sea. Reading that as a real-world height is a mistake that only shows up when you try to reconcile the structural set against the civil survey.
Where to check on your own drawing set
- The structural abbreviations legend and general notes. This is the definitive statement on TOS versus top of slab for your project.
- Framing plan notes: the blanket “ALL TOS RL x U.N.O.” and every local exception around stairs, plant areas and drop slabs.
- Typical sections through the floor, which show whether TOS is the flange or the deck bearing, and whether the slab figure quoted is total thickness or topping over the ribs.
- Column base details, for the grout bed thickness between the base plate and the concrete.
- Approved shop drawings. If fabrication has started, the shop drawing TOS is what is actually being built – reconcile against that, not only against the design set.
More steel and level abbreviations are collected in the engineering drawing abbreviations list.
Other meanings of TOS
Outside drafting, TOS most commonly means Terms of Service, and Top of Stack in computing. Within drafting the collision that matters is internal: Top of Steel versus Top of Slab on the same sheet, and occasionally Top of Support on plant and piping drawings.