Wall thickness is where a sketch becomes a drawing. A diagram can treat walls as lines; a floor plan cannot, because every dimension that matters — room sizes, areas, whether the sofa fits — is measured between the internal faces of walls that are hundreds of millimetres thick. This page gives the numbers to draw with, shows layer by layer where they come from, and puts a figure on what drawing walls too thin actually costs.
The numbers to draw with
| Wall | Gridsnap default | Typical range | What decides it |
|---|---|---|---|
| External / envelope wall | 400 mm | 300–500 mm | Driven by insulation, not structure. 250 is a warm timber wall, 500 a passive-house envelope. |
| Load-bearing internal wall | 250 mm | 150–250 mm | The load path and the acoustics. What a structural engineer looks for first. |
| Partition (non-load-bearing) | 120 mm | 75–150 mm | A stud partition with plasterboard both sides. Below ~100 mm a door frame will not sit in it properly. |
Where the numbers come from, layer by layer
A wall's thickness is the sum of its layers, and the honest answer to "how thick, really" is a build-up. These four are worked examples with everything counted:
| Build-up | Layers (mm) | Total |
|---|---|---|
| Timber frame wall | 22 cladding · 30 cavity · 12 OSB · 200 insulated frame · 13 plasterboard | 277 mm |
| Brick cavity wall | 102 brick · 100 mineral wool · 100 aerated block · 13 plaster | 315 mm |
| Rendered block wall | 20 render · 150 EPS · 300 aerated block · 10 plaster | 480 mm |
| Stud partition | 13 plasterboard · 95 insulated stud · 13 plasterboard | 121 mm |
Two things worth noticing. The structural leaf is a minority shareholder — in the rendered block wall, 300 mm of block is structure but the 150 mm of EPS is what the energy standard bought. And the 400 mm external default sits between the brick cavity and the rendered block wall: a fair average, not a magic number. The same stacks decide a wall's U-value — you can rebuild any of them, layer by layer, in the U-value calculator and watch the thickness and the insulation trade off.
What thin walls cost: the area you do not have
Here is the arithmetic that makes wall thickness matter. Take a 10 × 10 m external outline — 100 m² gross. With 400 mm external walls, the internal space is 9.2 × 9.2 m = 84.6 m². Add a couple of internal walls and the net floor area — the number rooms are sold and furnished by — drops further. A plan drawn with thin-line walls reports the gross figure everywhere and is wrong about every room in the house, in the flattering direction.
This is also why traced plans go subtly wrong: an estate agent's plan drawn with thin walls cannot be corrected by scaling it, because the error is not the scale — it is the missing 300 mm at every wall. Trace with real thicknesses and the rooms come out at their true sizes. Gridsnap measures rooms between the wall faces automatically, and dimensions state internal sizes — the gross-vs-net trap is worth five minutes if you are checking someone else's plan.
Choosing a thickness, wall by wall
- External walls are driven by insulation, not structure. Decide the build-up (or copy a regional one) and the thickness falls out of it: ~250 mm warm timber, ~300–350 brick cavity, 400–500 insulated masonry, 500+ passive house.
- Load-bearing internal walls: set them where they really are and at their real thickness — 200–250 mm masonry, less in timber. They are the first thing a structural engineer reads off your plan.
- Partitions carry nothing; 100–125 mm is normal. Below about 100 mm a standard door frame no longer fits the wall properly.
- Wet-room and plumbing walls run thicker than dry ones because they swallow pipes: allow 150–200 mm behind a WC.
- US framing: a 2×4 stud is 3½″ (89 mm), a 2×6 is 5½″ (140 mm); with ½″ board both sides a partition lands near 4½″ (115 mm). In Gridsnap's imperial mode you type these as inches and the drawing stays exact.
Drawing it: one wall, one thickness
In Gridsnap a wall's thickness is a property of the whole wall — anything from 20 to 1000 mm — so a wall that genuinely changes section is drawn as two walls. The three presets (Outer 400, Bearing 250, Inner 120) are the values a new plan starts with, and they are deliberately settings: wall build-up is regional, so under ⚙ Edit wall types… you set your own three figures once and every later plan uses them. Assign a build-up from the library and the thickness follows the layer stack instead — which is also what gives the wall its U-value in the heat-loss sheet.
What the plan check deliberately does not do is second-guess you: there is no rule that flags a 60 mm external wall or a 400 mm partition. Thickness is a drafting decision the drawing records faithfully. Fire ratings, acoustic performance and required U-values are set nationally and checked by no drawing tool.
Frequently asked questions
How thick should walls be on a floor plan in mm?
Draw external walls at 300–500 mm (400 is a fair European average), load-bearing internal walls at 150–250 mm, and partitions at 75–150 mm (120 is a stud partition to the millimetre). The exact figure is regional — an Austrian insulated masonry envelope is 400–500 mm where a Nordic timber-frame one is closer to 250 — so treat these as defaults you adjust, not rules.
How thick is a standard internal wall?
A non-load-bearing stud partition is about 121 mm built: 13 mm plasterboard, a 95 mm stud, 13 mm plasterboard. A load-bearing internal wall runs 150–250 mm depending on material. In imperial framing the same partition is a 2×4 stud wall — 3½ inches of timber plus half-inch board each side, about 4½ inches (115 mm) total.
How thick is an external house wall?
It depends on the build-up, and insulation is most of the answer. A modern timber frame wall totals around 277 mm; a brick cavity wall about 315 mm; a rendered, externally insulated block wall about 480 mm. The structural part might be only 100–200 mm — everything else is what the climate and the energy standard demand.
Does wall thickness matter on a floor plan?
More than any other single drafting habit. Room dimensions and areas are measured between the internal wall faces, so walls drawn as thin lines overstate every room. On a 10 × 10 m external outline, 400 mm walls take the internal space to 9.2 × 9.2 m — the plan loses more than 15 m² to walls alone. A plan drawn with real thicknesses is the difference between a diagram and a drawing someone can measure from.
What wall thickness do I need for a pocket door?
A pocket door needs a wall thick enough to swallow the leaf plus its cassette — in practice 100 mm of stud is the floor, and the bay the leaf slides into must be clear of sockets, switches and pipes for at least the length of the leaf. Plumbing walls are a similar story in reverse: allow 150–200 mm behind a WC, because the soil pipe lives in the wall.
How do I know which walls are load-bearing on a plan?
You cannot tell from thickness alone, though thickness is the first clue — a 200+ mm internal wall usually carries something. On drawings, load-bearing walls are often the ones that stack from floor to floor and sit under beams or joist directions. If a renovation depends on the answer, it is a structural engineer's question, not a drawing convention's.