How to Read a Section Drawing

A plan tells you how a building is organised. A section tells you what it will be like to stand inside it — and of the two, the section is the one most people have never been taught to read.

This is a shame, because it is not difficult, and because the section is where almost everything a client actually feels every day gets decided: ceiling height, where the light comes from, whether hot air has anywhere to go, whether the stair is a pleasure or a chore.

What a section actually is

Imagine a vertical plane passed straight through the building, and everything on one side of it taken away. What is left is the section: a cut face, and beyond it, whatever the cut did not touch.

Two line weights carry almost all of the information:

• Heavy lines are the cut — walls, floors and roofs the plane passed straight through. These are usually filled solid; the convention is called poché.
• Light lines are everything beyond the cut: the far wall, a stair, a door you would see if you were standing there.

If you learn nothing else, learn to separate those two. Every section on this site is drawn to that convention, and the drawing viewer exists so you can zoom in far enough to tell them apart.

Four things worth checking

1. Floor-to-floor heights. Marked as levels — +0.000, +3.400. The gap between them, minus the structure and the services, is what you actually stand in. Anything under 2.7 metres clear will feel low once furniture is in.
2. Where the light comes from. Follow every opening. A room with one small window high on a north wall is a very different room from the one the plan suggested.
3. Whether there is a figure drawn in. A person calibrates everything at a glance. Their absence is one of the surest signs you are looking at a render dressed up as a drawing.
4. What happens at the top. Roof falls, parapets, gutters. Buildings leak at the top, and a section that goes vague up there is telling you something about how much of the building has actually been thought through.

The thing to be suspicious of

A section that has been drawn through the most generous part of the plan and nowhere else. Any building has one flattering cut. Ask where else the plane could have gone, and — if you are a client — ask to see that one too.

Try it

Open House of Three Walls and go to sheet A-301. Find the courtyard, roughly in the middle of the drawing. Notice that there is no roof line across it: the cut passes through open sky.

That single absence is the whole argument of the project. Three sides of the plot are party walls, so nothing could be lit from the perimeter alone, and cutting a void through the centre was the only way to get daylight and moving air into the middle of the plan. It takes about two seconds to see in the section. No photograph of that house could make the same point.

That is why we publish them.