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What is a section on a construction drawing?

A section is the house cut straight down and seen from the side. It shows ceiling heights, floor build-ups, roof pitch and the guts of the construction — everything a floor plan and a facade elevation cannot show.

The section explained in three lines

Imagine the house being sawn straight through, vertically, from roof to foundation, and one half being driven away. Then you stand directly in front of the cut face and draw what you see — no perspective, fixed scale. That is a section.

Compare it with the two other basic drawings: a floor plan is the same exercise but with a horizontal cut at around 1.2 metres' height, seen from below looking up. A facade elevation is the house seen from outside without cutting anything at all. The section is the only one of the three that shows what is inside the floors, walls and roof, and how the storeys sit in relation to one another.

How to find the section line on the floor plan

A section always comes as a pair: the line on the floor plan and the section drawing itself. On the floor plan, the section is marked by:

• A heavy line across the plan, showing where the cut is taken. It can have a bend in it if the cut is routed to pass through several points of interest — that is called a stepped section.
• Two arrows, one at each end, pointing the way you are looking. If the arrows point downward on the plan, you are seeing the house from south to north in the section.
• A letter at each end, typically A-A, B-B, C-C. The section drawing carries the same label so you can find it again.

If you flip the section the wrong way round in your head — mirror it — everything else is correct, but on the wrong side. That is why the arrow direction is not decoration. Check it, every time.

What you can read on a section

The section is the house's document of heights. The most important things it states:

• Ceiling height — the clear height from finished floor to finished ceiling, room by room. Older flats often have 2.8–3.2 m; production houses from 1960–80 often around 2.3–2.5 m.
• The thickness of the floor slab — what sits between the ground floor's ceiling and the first floor's floor, typically 25–40 cm including joists, insulation and floor build-up. That is why two storeys at 2.5 m clear height each do not add up to a 5 m tall house.
• The floor build-up — from the bottom: a capillary break layer, insulation, a concrete slab, possibly underfloor heating, floor covering. In a renovation with a new floor, it is the section that shows whether there is enough height to work with.
• The roof pitch — given in degrees or as a ratio (e.g. 45° or 15°). Together with the ridge height, it determines how much usable space there is on a first floor.
• The knee-wall height — how tall the eaves storage wall is on a first floor. At 90 cm, a bed can stand against it and cupboards can be built along it; at 40 cm, almost nothing fits.
• The staircase — where it starts, where it lands, and whether there is enough headroom over the steps. Sections through stairs are almost mandatory, because stairs are the most height-sensitive part of a house.
• Levels — height figures at ground, floors, eaves and ridge, measured relative to a shared zero point.

How to read the lines in a section

There are two kinds of lines in a section, and the difference is the whole key to reading one:

• Heavy lines (often with hatching): what the saw cut has passed through. Walls, floor slabs, foundation, roof structure. The hatching indicates a material — diagonal lines typically for masonry or concrete, dots or waves for insulation, small squares for timber seen across the grain.
• Thin lines: what you see behind the cut face but which has not been cut through. A window in the background wall, a door at the far end of the room, a staircase further inside the house.

That distinction is why a section is not a mess of lines: everything heavy and black is construction, everything thin is the room's background.

Where is a section taken?

A section is taken where the house is most complicated in height — not straight through the middle for the sake of symmetry. The typical places:

1. Through the staircase, because headroom over the steps and the stair's run between storeys can only be shown here.
2. Through the tallest room, or through a double-height living room or a collar-tie loft.
3. Through the bathroom on a renovation, because floor falls, drains and dropped ceilings are all matters of height.
4. Across the house through the ridge, so the whole roof geometry is included.
5. Through a dormer or an extension, when that is what the project is about.

An ordinary single-family house typically has two sections in its drawing set: one lengthwise and one crosswise.

A calculation the section answers: can the loft become a room?

The section is what decides whether a loft storey can be put to use. Say the house is 8.0 m wide with a 45-degree roof pitch, and there is a knee wall of 1.20 m out at the sides.

At 45 degrees, the roof rises by exactly as much as it moves inward. From the outer wall to the centre is 4.0 m, so the ridge sits 4.0 m above the eaves. With a 1.20 m knee wall, the clear height at the centre is roughly 1.20 + 4.0 = 5.2 m — minus the thickness of the roof structure and the ceiling build-up, typically 30–45 cm. So there is plenty of height at the centre.

What is more interesting is how wide the band of usable height is. If you assume at least 2.1 m of headroom is needed, you can measure on the section how far in from the outer wall the ceiling reaches 2.1 m: from the knee wall's 1.20 m, the roof has to rise another 0.90 m, which at 45 degrees happens 0.90 m further in. That gives a usable band of 8.0 − 2 × 0.90 = 6.2 m in width, with lower eaves storage along the sides. At a shallower 25-degree pitch, the same calculation would give a markedly narrower band — and that is exactly what the section makes visible at a glance.

What may actually be fitted out and counted as habitable space is decided by the local authorities and varies from place to place. The section provides the geometry; it is not a permit.

Do I need a section for my home?

As a private homeowner, you need a section in three situations:

• When a building project needs approval — an extension, a dormer, a changed roof shape. A section is usually part of the material the authorities ask for.
• Loft or basement conversion — where height decides whether the project is possible at all.
• A major renovation with new floors, underfloor heating or a dropped ceiling — where every centimetre of height counts.

If you are just painting, furnishing, laying flooring or getting a quote for a new bathroom, a floor plan and an elevation of the relevant walls will get you through.

What HouseSense gives you — and what it doesn't

HouseSense produces a measured floor plan, elevations of individual walls, and a 3D model of the whole home with the storeys stacked. Wall heights are measured during the scan, so ceiling height is a measured figure, not a standard assumption.

A proper sectional drawing with construction build-up — insulation thicknesses, rafters, the layers inside a floor slab — is something HouseSense does not produce, and it would also be guessing: those layers cannot be seen by a scan that only sees the room's surfaces. If you need a section for a building project, it has to be drawn by someone who can look up the house's construction or open it up. The drawing may be dressed up; the figures must not be.