Elevations and sections

The lesson

A plan answers where. An elevation and a section answer how high. They are both vertical views, and the difference between them is whether the drawing cuts through the building or looks at it from outside. An elevation is a flat view of a face with nothing cut: you see brick, windows, doors, the parapet, and the ground line. A section is a vertical slice: you see the thickness of the floors, the height of each room, the structure holding it up, and how the roof meets the wall.

Both views are located from the plan by a marker. A section marker is a circle split across the middle, the drawing number over the sheet number, with a tail and an arrowhead. The arrow is the important part: it says which direction you are looking after the cut is made. A cut line usually runs across the plan between two markers to show where the slice is taken. Turn the plan so the arrows point at you and you are standing where the section stands.

An exterior elevation is named by the face it looks at, and the naming follows the compass, not the paper: the north elevation is the face that looks north, drawn as you would see it standing north of the building looking back. That is why the north arrow on the plan is not optional. An interior elevation marker is a circle with four arms, one for each wall of a room, each arm carrying its own drawing number, and it sits in the middle of the room pointing outward.

Heights on a vertical drawing are given as level datums rather than as a chain of dimensions. A datum is a horizontal line across the drawing with a symbol and a label giving a name and an elevation, such as the finish floor of Level 1 at zero. Every other height is stated relative to that. Levels are chosen so that the number is useful on site: floors, roof bearing, and top of parapet are the usual ones, because those are the heights that get set out and checked.

• Level 1 finish floor is the origin, and it is set to zero so every other height reads as a distance above it.
• Floor to floor is the distance between two floor datums, and it includes the floor structure itself.
• Floor to ceiling is smaller than floor to floor by the depth of the structure and the ceiling void.
• Roof bearing is where the roof structure sits on the wall, and the parapet carries on above it.
• A datum on a site drawing may be a real survey elevation instead of zero, and the two must not be mixed.

That last distinction catches people out. On the building drawings, zero usually means the Level 1 finish floor. On the civil drawings, elevations are real heights above a survey datum, and the same floor might read as a number in the hundreds. A note somewhere in the set ties the two together, and finding that note before you compare a building height with a site contour is worth the thirty seconds it takes.

A section also shows you what a plan physically cannot: how the floor is built, where the beams sit, how deep the ceiling void is, and whether the duct you saw on the mechanical plan actually fits above the corridor ceiling. Those are the questions that make sections worth reading carefully rather than glancing at. When somebody says a drawing does not work, they usually mean it works in plan and fails in section.