How to read an architectural drawing

You have been handed a set of drawings and asked to approve them. You look at them, you can tell there is a house in there somewhere, and you have no idea what you are supposed to be checking.

It is an uncomfortable situation and a very common one. And it is worth resolving, because approving a drawing you do not understand is the most expensive way to trust somebody: whatever is not corrected there gets corrected on site, and there it costs a hundred times more.

Start with the title block

It is the box in the bottom right-hand corner, and hardly anybody reads it. It holds what tells you whether you are looking at what you think you are looking at:

• —Which drawing this is. Ground floor plan, section A-A, north elevation.
• —The scale. It determines how everything else is read.
• —The date and the revision. If two sets of drawings are on the table, this decides which one governs.
• —Who signs it, with their professional registration number. That is the person legally answerable for the drawing.

Scale: what 1:50 means

It means that one centimetre on the paper is fifty in reality. Half a metre, in other words.

Dimensions: where the sizes are

They are the fine lines running outside the drawing, with a number above them. That number is the real size, and it is the drawing’s only source of truth.

They come in strings, and each string measures something different: the outermost usually gives the overall plot or the overall building; the next one gives the grid; the innermost gives every opening and every wall.

Grid lines: the invisible skeleton

They are the chain-dotted lines crossing the whole drawing, with a letter or a number inside a circle at the end. Letters one way, numbers the other.

They are not decoration: they are the building’s coordinate system. When somebody on site says "the column on grid B at 3", they are pointing at an exact spot without measuring anything. And the grid is what lets the architectural plan and the structural plan line up.

The thick line crossing the plan

It is the section line, and it tends to be the most confusing thing on the sheet. It has an arrow at each end and a letter: A-A, B-B.

It marks where the house was sliced through in order to draw the section, and the arrow shows which way you are looking. Section A-A is that slice, seen in the direction of the arrows.

And there lies the relationship between the two drawings, which is the thing to grasp: the plan tells you how you move around the house; the section tells you how tall it is and how one thing sits on another. Stairs, double-height spaces, changes of level and roof pitches can only be understood in section.

The conventions worth recognising

• —A solid or hatched wall: it is cut by the plane of the drawing. It is solid material.
• —A dashed line: something above the cut. A beam, a cantilever, a mezzanine.
• —An arc at a door: the swing, that is, the way it opens. It is worth a look: a door that opens into a cupboard is a cheap mistake to fix on the drawing.
• —A triangle or an arrow with a number: a level. `+0.00` is the reference floor; `+2.50` is two and a half metres above it.
• —An arrow marked "N": north. It decides where the sun comes from, and in Colombia that decides which rooms overheat.
• —Walls in three tones on a refurbishment: what stays, what is demolished and what is built. There is always a key explaining it.

The architectural plan and the structural plan are not the same thing

You will be handed separate sets, drawn on the same grid but carrying different information:

• —Architectural: spaces, walls, doors, windows, finishes. What you see and how it is used.
• —Structural: columns, beams, slabs, foundations, reinforcement. How it stands up. A civil engineer signs it.
• —Plumbing and drainage, electrical, gas: where the services run.

Coordinating them is the architect’s job, and it is where the classic clashes turn up: a column in the middle of a bathroom, a waste stack running through a living room, a beam dropping down exactly where a window was meant to go.

Six things to check before approving

1. 1Walk the house with your finger. Come in through the door, go to the kitchen, to the bathroom, to your bedroom. Awkward routes are felt this way before they are felt on site.
2. 2Compare the sizes against something you know. Your current bedroom measures so much. Is the new one bigger or smaller? Numbers deceive; comparisons do not.
3. 3Check the doors. Which way they open, and into what.
4. 4Find north and ask which rooms get the afternoon sun.
5. 5Count the sockets and the lighting points on the electrical drawing. It is the cheapest thing to change now and the most irritating one later.
6. 6Add up the dimensions. If they do not tally, something is wrong.

And one that is not about checking but about method: ask about whatever you do not understand, however obvious it may seem. An architect who takes offence at explaining their own drawing is telling you something important about how the rest of the process is going to go.