Doors don't just open, they swing

Go and open a door. Any door, right now. This lesson can wait.

Watch the edge of it sweep across the floor.

Everything that edge passes over is floor the door has claimed. You can't put a chair there. You can't put a trash can there.

That's what the quarter-circle on a floor plan is. Not a decoration. A warning label.

On a plan, a door is drawn as three things: a gap in the wall, a straight line for the door itself, and a thin curved line showing where it sweeps.

That curve is called the swing arc, and it is doing something no other symbol on the page does: it's showing you a piece of floor that has to stay empty. Not because a rule says so, but because a solid object is going to pass through it several times a day.

There are four ways to hang a door, and three are usually wrong

This is the part almost nobody realizes. A door in a wall isn't one decision. It's two, and each has two answers:

1. Which side are the hinges on? Left or right.
2. Which way does it swing? Into this room, or into the next one.

Two choices, two answers each, so four possible doors in the same hole. They look nearly identical on a plan. They feel completely different to walk through.

So how does anyone pick the right one? Five rules, and every one of them is about what happens when somebody actually uses the door:

• A door should open against a wall, not into the middle of a room. Pushed flat against a wall, it's out of your way. Swung into open floor, it's a barrier you walk around forever.
• You should not have to walk around a door to get through the doorway. If the door opens toward you as you approach, you're stepping backward before you can go forward.
• Doors shouldn't hit each other. Two doors near a corner, both swinging in, will meet in the middle. Real houses do this. It's always a mistake and it's never fixed.
• A door should not show you the toilet. Bathroom doors get hung so that when they're open, the thing you see is the sink. This is a real rule that real architects genuinely think about.
• Doors swing into the smaller space, usually: into the bathroom, into the closet, into the bedroom, so the hallway stays clear. Hallways are for moving through, and a door swinging into one blocks the only route.

Exterior doors are the exception: front doors on houses almost always open inward, partly so wind and weather don't catch them, partly because a door that opens outward would sweep the porch.

And sometimes the answer is no door at all

If a doorway doesn't need to close, between a kitchen and a dining room, say, you can leave the hole and skip the door entirely. Architects call that a cased opening. No door, no arc, no claimed floor. Sometimes the best door is the one you didn't hang.

Follow along in the Designer, one step at a time

Open Tiny Architects Designer. This one is about ten clicks and then a lot of noticing.

1. Draw two roomsWalls › Outside › House OutlineWalls › Inside › Interior Wall👆 Show me

   Drag a wide rectangle, then run an interior wall across the middle to split it in two. Make one room clearly bigger than the other.

2. Put a door in the middle wallDoors › Single Door👆 Show me

   Click the wall. There's your gap, your door line, and your arc.

3. Click through all four doors

   Click the door. Little round buttons appear on the wall: two of them with arrows, and they are the two decisions: one flips the hinge to the other side of the opening, the other flips the swing to the other side of the wall.

   Now go round all four. Hinge left, swinging in. Hinge left, swinging out. Hinge right, swinging in. Hinge right, swinging out. Watch the arc jump around the doorway: same hole in the same wall, four completely different doors.

4. Now make one of them obviously wrongBathroom › Fixtures › Toilet

   Put a toilet in the small room, right behind where the door swings. Flip the swing so the door opens onto it.

   There it is: a door that hits a toilet. Somebody, somewhere, got paid to draw exactly this.

5. Try the − and + buttons too

   The same handles let you change the door's width, six inches at a time. Watch the arc grow with it: a wider door claims more floor. A 72-inch Double Door claims a lot.

6. Flip to 3D and walk up to itPlan → 3D👆 Show me

   The arc disappears, because in real life nobody draws a quarter-circle on their floor. But it's still there. It's just invisible until something is standing in it.

   ---

Now make your own, using what you just learned

Find the worst door in your own home.

There is one. There's always one.

1. Walk around and open every door fully. Not halfway, all the way, until it stops.
2. Look at what stops it. A wall is fine, that's the door doing its job. A piece of furniture, a radiator, another door, or your own body. That's a door hung wrong.
3. Draw that room in the app, with the door exactly as it really is.
4. Now fix it. Flip the hinge. Flip the swing. Try all four. One of the four is better than what you've got, and finding it takes about five seconds.

Then the interesting bit: work out why nobody fixed it in real life. Often the answer is that the light switch is on that side, and moving a door means moving the wiring inside the wall. Design is full of that: the wrong answer that stayed because the right one cost too much.