Last updated July 27, 2026
Soffit is the underside of a structural element — a slab, a beam, an arch, a flight of stairs. A soffit level on a structural drawing is the level of that underside. On a drainage drawing the same word means something almost opposite: the highest point inside a pipe. And to a builder pricing a house, a soffit is the board under the eaves.
Three trades, one word. This page covers the drafting senses first, then the drainage one, then the eaves one — skip ahead if you came for the last of those.
The thing that trips people up
Nearly every level a drafter annotates is measured to a top surface. FFL is the top of the finished floor. SSL is the top of the structural slab. TOC is the top of the concrete. TOS is the top of steel.
Soffit is the odd one out. It is the one common level measured to an underside — and then drainage flips it again by using the same word for the top of a pipe bore. That single asymmetry causes most of the confusion around the term.
Soffit versus ceiling
They are not the same thing, and the cleanest statement of the difference comes from the Architerms dictionary:
A soffit is the underside of an architectural element. A ceiling is the overhead surface of a room. A soffit can be part of a ceiling, but not all ceilings are soffits.
A slab soffit left exposed is the ceiling. Put a suspended grid under it and the ceiling is now the grid, while the soffit is still the underside of the slab, sitting in the plenum with the ducts. Two different levels, often a few hundred millimetres apart, and the drawing needs to be clear about which one it is calling out.
There is a third usage worth knowing, because it turns up in architectural documentation: a dropped bulkhead in an interior ceiling, used to conceal beams and services, is also called a soffit. As the UK’s Designing Buildings Wiki puts it, “Inside buildings, the term soffit may refer to any portion of a ceiling that is lower than the rest of the ceiling.”
Working out a soffit level
Once the definitions are straight the arithmetic is simple, and it is the reason soffit level is worth annotating at all.
For a flat slab, the soffit is the structural slab level less the structural thickness:
soffit level = SSL − slab thickness
For a downturned beam, it is the slab level less the overall beam depth — not the slab thickness:
beam soffit level = SSL − overall beam depth
And because SSL is FFL less the floor finishes, you can get there from the architectural level too:
soffit level = FFL − floor finishes − slab thickness
Worth stating explicitly because it is where the mistakes happen: a soffit level is never derived from FFL directly. The finishes have to come off first. On a job where the architect is working to FFL and the structural drawings are working to SSL, that step is the one that gets skipped.
Why the soffit decides your clear height
Headroom is measured from the finished floor up to the lowest thing above it — and in a structure with downturned beams, that is the beam soffit, not the slab soffit. It is also the number that decides how much room is left for ducts, trays, sprinkler mains and pipework.
This is why services coordination arguments are almost always soffit arguments. The available zone is the gap between the beam soffit and the ceiling, and every discipline wants it.
A vocabulary flip worth knowing. In concrete the underside of the beam is the soffit. In structural steel the same level is normally called BOS — bottom of steel — and you will see that on the drawing rather than “soffit”. Same idea, different trade.
Soffit formwork
The soffit form is the one carrying the load. Designing Buildings Wiki: “The formwork base or soffit must be capable of resisting the initial dead load of the wet concrete and the dead load of the dry set concrete.”
That is why side forms can come off early while the soffit form and its props stay until the member can carry itself. When a formwork drawing distinguishes side forms from soffit forms, that difference in stripping sequence is what it is talking about. The actual stripping times come from the project specification and the engineer — they vary with member type, span, temperature and mix, and we are not going to quote a number here.
Soffit in drainage: the top of the pipe
Here is the one that catches people out, and it is worth reading even if you never touch a stormwater drawing.
In drainage and hydraulics, the soffit is the highest internal point of the pipe bore — the exact counterpart of the invert, which is the lowest. A UK drainage glossary puts it as “the uppermost internal surface of a pipe or conduit… the highest point on the internal circumference of a circular pipe”. You will also see it called the obvert; the two words mean the same thing.
So on a structural drawing soffit means an underside, and on a drainage drawing it means an inside-top. Same word, inverted sense. If you work across both — and plenty of us draw a slab and a stormwater line in the same week — that is worth having straight.
It is not an obscure usage, either. Current NSW Government guidance for plumbers and drainers uses it directly: “The invert level of a trap or gully weir must be a minimum of 10mm higher than the soffit of the pipe to which it connects.” Read “soffit” there as an underside and you would set the level wrong.
Arches, bridges and box girders
The underside of an arch has its own name: the intrados. Soffit and intrados are used interchangeably for arches, with intrados the more formal term.
In bridge work, soffit stops being a surface and becomes a member. Caltrans, in its structure technical policy, defines it flatly as the “bottom slab of box girders”. On a box girder drawing the soffit is a structural element with its own thickness, reinforcement and details — not a face of something else.
And the eaves soffit
If you arrived here about the board under a roof overhang: that is an eaves soffit, the lining that closes off the underside of the overhang between the fascia and the wall. It is the same idea — the underside of a construction element — applied to the eaves.
In Australia the trade usually calls the product eaves lining; manufacturers here list it under “eaves and soffits” to catch both names. North American usage leans much harder on “soffit” alone, which is why a search for the bare word returns a wall of vinyl and aluminium panel suppliers.
We do not cover eaves products — this is a drafting site. But that is the distinction, and it is why the word behaves so oddly in a search engine.
The short version
- Structures: the underside of a slab, beam, arch or stair. Soffit level = SSL − structural depth.
- Steel: the same level is usually annotated BOS, bottom of steel.
- Formwork: the load-bearing face, which is why it strips last.
- Drainage: the top of the pipe bore — the opposite of the invert. Also called the obvert.
- Bridges: the bottom slab of a box girder, a member in its own right.
- Housing: the eaves lining. Different trade, same underlying idea.
Related on this site: SSL, FFL, TOC and TOS, or the full A–Z of drawing abbreviations.
References
- Soffit — Architerms architecture dictionary. The soffit-versus-ceiling distinction.
- Soffit — Wikipedia. “The underside (but not base) of any construction element”; the intrados; the Italian and Latin etymology, first recorded in English in the 1610s.
- Soffit — Designing Buildings Wiki (UK). The dropped-ceiling/bulkhead sense.
- Formwork — Designing Buildings Wiki (UK). The soffit form as the load-bearing face.
- Soffit — Drainboss drainage glossary (UK). The uppermost internal surface of a pipe.
- Soffit Level — SWMM5 reference. Soffit and obvert as the same level.
- Floor waste and fixture requirements — NSW Government, guidance for plumbers and drainers. The 10 mm invert-above-soffit requirement.
- Invert level — Wikipedia. Invert and obvert as the lowest and highest interior levels.
- Structure Technical Policy 9.4: Deck, Overhang, and Soffit Design — Caltrans. “Soffit – Bottom slab of box girders”.
- Eaves and Soffits — James Hardie Australia. Australian product naming.