Why some walls are fat and some are skinny
On every floor plan ever drawn, some lines are thicker than others. That isn't decoration and it isn't an accident. It tells you what's inside the wall.
Line weight, what wall thickness tells you, and where it stops telling you
Draw your home in two line weights, then hunt for the walls that don't fit
Somewhere in your house there's a wall that's basically a screen: studs, drywall, air. Two steps away there's one packed with insulation, or pipes, or the wood holding your bedroom floor up.
Standing in the room, they look identical. Same paint, same light switch, same boring flat wall.
On a floor plan they don't. One is drawn with a noticeably fatter line, and that line is telling you something about what's inside the wall, before you have gone anywhere near it.
That difference is on practically every building drawing you will ever see. Almost nobody notices it. Once you do, plans get dramatically easier to read.
A fat line is a fat wall
Go back to the slice. We cut the house open about four feet up and looked straight down. Every wall shows up as the shape of its own cut edge, so a fat wall draws a fat line and a skinny wall draws a skinny one.
So the drawing isn't choosing to make some lines heavier for emphasis. The lines are different because the walls really are different sizes.
Thickness tells you what's inside
A wall is a sandwich, and the line weight tells you how much filling it has.
Outside walls are thick because they're doing several jobs at once. They keep the rain and cold out, which means insulation, and they usually hold up the roof and floors, which means structure. All of that needs room.
Most inside walls are thin because they do one job: making two rooms out of one. Studs, drywall, a gap for wires. Architects call that a partition, and it only has to hold up itself and a couple of picture frames.
Thick usually means outside, thin usually means inside. That single reading will get you a long way on almost any plan you pick up.
Everything is trying to fall down
Here's the thing nobody tells you about buildings: they are all, constantly, trying to collapse. The roof weighs a shocking amount. So does snow on it, and the floor above you, and the bathtub on that floor.
All that weight has to reach the ground. It travels down through the building along a path made of walls and beams (architects call it the load path) and a wall carrying some of that weight is load-bearing.
You cannot reliably tell load-bearing walls from a floor plan
A thick line doesn't mean the wall is holding the house up. Reading it that way goes wrong in three ways:
- A thin-looking interior wall can absolutely be load-bearing. An ordinary stud wall, drawn exactly like every partition around it, can be carrying the entire floor above. Nothing on the plan gives it away.
- A thick interior wall often isn't structural at all. It might be full of plumbing, and waste pipes need real room, which is why bathrooms and kitchens so often back onto one chunky wall. Or it's hiding a chimney or ducts, or it's been built thick to keep noise out.
- Even "outside walls are structural" has exceptions. Some buildings hang everything off a frame of columns, and the outside walls are just weatherproof skin holding up nothing but themselves.
This isn't the plan failing. It's the plan doing its job and stopping. A floor plan describes the shape of a building; which walls carry weight is a different question, answered on a different drawing (a structural or framing plan) and on a real project by a structural engineer.
The clue that actually works: stacking
There is one honest clue, and it isn't line weight: it's what's above and below.
Weight travels in straight lines, so a load-bearing wall almost always has something directly underneath it to hand the load on to: another wall, a beam, the foundation. Walls that line up floor to floor are doing structural work far more often than walls that don't.
A clue, not a verdict. But it's a real one: lining the floors of a building up is a large part of how anyone designs one that will actually stand, and it's the first thing a designer checks when a second floor goes on.
Take it into the Designer.
Next, follow along in the Designer, step by step.
Build it → 5 steps in the DesignerDo it in the app
Follow along in the Designer, one step at a time
Open Tiny Architects Designer. Blank page. Let's make the difference impossible to miss.
Draw the outsideWalls › Outside › House Outline👆 Show me
Drag a rectangle: make it wide, like a real house, not a tiny square.
Split it down the middleWalls › Inside › Interior Wall👆 Show me
Draw a line across the middle, cutting your rectangle into two rooms.
Now zoom in on a corner where they meet
Get close enough that the two walls are fat bands rather than lines. The outside wall is visibly chunkier, about half again as thick.
Measure them, so it's a number and not a vibeMeasure › Custom Measure
Drag across each wall.
- Exterior wall: 6 inches.
- Interior wall: 4 inches.
Those are typical numbers, not rules. Exterior walls vary a lot, and many are thicker than 6 inches, but they're almost always thicker than the interior walls. That's the pattern your plan now shows.
Flip to 3D and look at it from abovePlan → 3D👆 Show me
Same building, lid back on. Look down at the tops of the walls and the thickness becomes a physical thing: a wide ledge on the outside walls, a narrow one inside.
Flip back. Plan. 3D. Plan. The fat line and the fat wall are the same fact, drawn two ways.
Now make one of your own.
Draw your home in two line weights, then hunt for the walls that don't fit
Your turn →Your turn
Now make your own, using what you just learned
Draw the floor your bedroom is on, in two line weights.
What you should not do is decide which walls are holding the house up. You can't know that from a floor plan, and neither can anybody else looking at one. What you can do is notice that a wall is unusual and ask a good question about it, which, honestly, is most of the job.
