Explainer · what is a section drawing
What is a section drawing?
By Saumyajit Maity · Published 25 Feb 2024 · Updated 7 Aug 2024
A section drawing is the one that lets you see inside the building. Where a plan slices horizontally and an elevation shows the outside face, a section slices vertically straight through the building and shows you the cut — wall thicknesses, floor build-ups, how storeys stack, headroom under a stair, the way a roof meets a wall. It is the view that reveals construction, which is why a section is often the drawing that tells you whether a building actually works in three dimensions.
This explanation covers what a section is, the vertical cut that defines it, how cut material is drawn, the section line that tells you where the cut was taken, and how to read one. A handful of CAD blocks are drawn specifically in section — a bath cut through, a detail at a window jamb — and understanding the view explains exactly when those section blocks earn their place.
A section, defined
A section is the view you get by passing an imaginary vertical plane through the building, removing everything on one side of it, and looking at the cut face. If a plan is a horizontal cut seen from above, a section is a vertical cut seen from the side. It exposes the inside of the construction in a way no other view can.
The value of a section is that it shows relationships you cannot see from outside: how thick the floor slab is, how high the ceilings are, how a staircase climbs from one level to the next, how a foundation sits below ground. It turns the building from a set of surfaces into a thing with depth and internal structure, which is why structural and detailed design relies on it so heavily.
What a section reveals
A section shows two kinds of information at once. First, the cut itself: every wall, floor, roof and slab the vertical plane passes through is shown sliced, so you read its thickness and how it is built up. Second, everything beyond the cut: the far wall of a room, a window in elevation, a staircase, furniture — drawn as it appears looking into the cut space.
This is where vertical dimensions live in their most useful form. Floor-to-floor heights, ceiling heights, the rise of a stair, sill and head heights, the depth of a roof void — all are measurable directly. A section is how you prove there is enough headroom under a beam, how you show a floor build-up to a builder, and how you communicate the way levels relate across a sloping site.
Poché and hatching: reading the cut
When a section cuts through solid material, that material is filled in so it reads as 'cut, not seen beyond.' The old hand-drawing term for this solid fill is poché; in CAD it is usually a hatch pattern. Different patterns indicate different materials — a brick hatch, a concrete hatch, an insulation symbol — so a reader can tell at a glance what each cut element is made of.
Learning to separate cut from beyond is the key skill in reading a section. Hatched, bold outlines are the things the plane sliced through and are nearest you. Lighter linework behind them is what lies beyond the cut, shown in elevation. Once that distinction clicks, a section stops looking like a tangle of lines and starts reading as a clear slice through the building.
The section line — where the cut was taken
A section is always linked back to a plan by a section line (or cut line): a heavy line drawn across the plan showing exactly where the slice was taken, with arrows indicating the direction you are looking, and a label such as A–A or 1–1. The matching section drawing carries the same label, so you can pair them up.
This matters because where you take the cut changes what the section shows. A section through a stairwell tells a different story from one through a solid wall. Designers choose section locations deliberately to reveal the most important relationships — usually through the most complex or telling part of the building. When you read a section, always glance back at the plan to see where its cut line runs, so you know what part of the building you are looking through.
Section vs elevation — the common mix-up
Sections and elevations are the two views most often confused, because both are vertical and both can show a wall with windows in it. The difference is the cut. An elevation has no cut — you are looking at an uncut outside face. A section cuts through the building, so it shows sliced walls and floors with hatching, and reveals the interior behind the cut.
A simple test: if you can see wall thicknesses and floor build-ups drawn as solid, hatched slices, you are looking at a section. If you only see a clean outer face with no cut material, it is an elevation. The two work together — an elevation sells the appearance, a section proves the construction — and a coordinated set lines them up so the same features appear consistently across both.
How to read a section
Start by finding the section's label and tracing it back to the cut line on the plan, so you know where the slice runs and which way you are looking. Then identify the cut elements — the bold, hatched walls, floors and roof — and read their thicknesses and build-ups. These are the nearest things to you.
Next read what lies beyond the cut: the far wall in elevation, the stair, any windows seen through the room. Check the level marks for floor-to-floor and ceiling heights, and look at how the storeys stack and how the roof resolves at the top. Detail sections zoom right in to show, say, how a window sits in its opening — and this is where section CAD blocks help, supplying a ready-drawn cut through a bath or a fixture so you do not redraw the slice from scratch.
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Questions
Frequently asked
What is the difference between a section and an elevation?+
A section cuts vertically through the building and shows the sliced interior — wall thicknesses, floor build-ups, what is inside the construction. An elevation has no cut; it shows an uncut outside face. If you can see hatched, sliced walls, it is a section.
What does the hatching in a section mean?+
Hatching (or poché) marks material the cut passes through, and the pattern signals what it is — brick, concrete, insulation and so on. It lets you tell at a glance which elements are cut and nearest you, versus the lighter linework that lies beyond the cut.
What is a section cut line on a plan?+
It is a heavy line drawn across the plan showing exactly where the section was taken, with arrows for the viewing direction and a label like A–A. The matching section drawing shares that label, so you can pair the plan and the section.
Why do I need a section if I have plans and elevations?+
Because only a section shows vertical construction and how floors stack — headroom, floor build-ups, the rise of a stair, the depth of a roof. Plans give layout and elevations give the face; the section proves the building works in the third dimension.
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