This is the window half of the floor plan symbols guide. Every symbol below is drawn twice — the convention on the left, the same window on a Gridsnap plan on the right — because the two differ in a way that surprises people the first time they see it, and it is worth knowing before you decide a drawing is wrong.
What a plan can tell you about a window — and what it cannot
Half the confusion around window symbols comes from expecting the plan to answer questions it structurally cannot. A plan is a horizontal slice; a window is a thing whose whole behaviour is vertical. So the information splits:
| The fact | Shown on | Why |
|---|---|---|
| Width and position along the wall | Plan | Measured off the plan, jamb to jamb. |
| Wall thickness the window sits in | Plan | The lines run between the two wall faces. |
| Sill and head height | Elevation or a note | A plan is one horizontal cut; it has no heights. |
| Whether the sash SLIDES | Plan | Two offset panels: parallel tracks are visible from above. |
| Which way any other sash opens | Elevation | Shown by the hinge marks; a plan cannot say. |
| Mullions and transoms | Elevation | The bars run up and across the face of the glass. |
| Whether it is glazed at all | Plan | Thin lines across the opening rather than a gap. |
This is also why a "window types" chart with a dozen distinct plan icons should be read with care: most of those icons are elevation symbols. On a real set of drawings, the plan locates the window and the elevation describes it.
Window symbols, drawn twice
Window
Also called: fixed, casement, sash — all the same in plan
Thin parallel lines running across the opening inside the wall thickness: the outer lines are the wall faces, the inner ones are the glass.
Count the lines: the textbook symbol draws THREE — two wall faces and one glazing line. Gridsnap draws FOUR, because it draws the glass as a pair, the way a real double-glazed unit sits in the frame. Both mean the same thing, and neither breaks the wall outline the way a door does.
Sliding window
Also called: slider, gliding window
Two sashes side by side on a track, drawn as offset panels — the same idea as a sliding door, at window scale.
This is the one window type a PLAN can tell apart, and Gridsnap draws it: two half-width sashes offset to opposite sides of the glazing plane. Compare it with the window above — a casement draws its pair of lines running the whole width, which says "one sealed unit"; these two say "these pass each other". Everything else about how a sash opens still belongs on the elevation.
Mullions & transoms
Also called: glazing bars, pane division
The bars that split the glass: a vertical one is a mullion, a horizontal one a transom. They are an ELEVATION mark — the plan cannot show them.
Gridsnap offers single, two or three panes across, split once across the middle, or a cross — and leaves it on "auto", which puts a centre mullion in once the glass passes 1500 mm wide. The bars appear on the elevations and in 3D; the plan symbol never changes.
Coupled windows
Also called: window pair, corner window, ribbon
Two windows butted jamb to jamb read as one unit: the joint between them is joinery, not a slice of wall.
Gridsnap detects the pair itself — same wall, same sill, same head, jamb on jamb — and draws the joint as a thin joinery line instead of the heavy wall-cut line, halves the butting frames into one mullion in 3D, and stops the cill board at the joint. A door with a sidelight beside it couples the same way.
Bay & bow windows
Also called: projecting window, oriel
The WALL steps out, not just the window: the plan shows the projecting shape with glazing across each of its faces.
There is no bay-window object, and there does not need to be one: a bay IS three short walls at an angle with a window in each. Draw the walls, put a window in each face, and the 3D model and the elevations follow — which a bay drawn as a single symbol never would.
Three lines or four?
Textbooks draw a window as three lines: the two wall faces and one line for the glass. Gridsnap draws four — the two faces, and the glass as a pair — because a modern window is a sealed unit with two panes, and drawing it as a pair is what lets the same geometry describe where the glazing plane actually sits inside the frame. Both readings are correct and both mean "there is glass here". If you are copying a plan by hand, three is fine.
The other thing Gridsnap does that a drawn symbol cannot: it knows which side of the wall is outside. It works that out from the plan itself — probing past each face of the window to see whether it lands in a room the walls enclose — which is what lets a window carry an inside board and an outside weathering cill independently, and why an interior window between two rooms correctly gets a board on both sides.
Window sizes worth knowing
- Sill height: 900 mm is the usual residential default — above a worktop, and high enough for furniture to stand under. Gridsnap uses 900 unless you change it.
- Head height: 2100 mm, lining up with the door heads so the tops of the openings read as one line around the room.
- Width: 1200 mm is a sensible default for a single window. Past about 1500 mm the glass wants dividing, which is why that is the width at which Gridsnap's automatic pane division puts a centre mullion in — and why a sliding window gets one at any width, since one pane has nothing to slide past.
- Daylight: aim for glazing of roughly an eighth of the room's floor area. A 16 m² bedroom therefore wants about 2 m² of window — one 1200 × 1500 opening is not quite enough on its own.
A bed head under a low window is the most-reported comfort mistake in a drawn bedroom, and it is a geometry problem rather than a taste one: the sill is below the head of the bed, so the draught and the glass are behind the sleeper. It is one of the things the plan check looks for.
Drawing window symbols yourself
By hand, the trick is that the lines must stop exactly at the wall faces and the jambs must be square to the wall — a window drawn a millimetre proud of the face reads as a bay. In Gridsnap you click a wall and the window lands inside it: the jambs cut the wall linework, the frame follows the wall thickness, and the same window appears on the elevation with its panes and on the 3D model with real glass. Move the wall and it comes along.
Next: door symbols and staircase symbols, or back to the full symbols reference. Reading a whole plan as a buyer? Start with how to read a floor plan.
Window symbol questions
What does a window symbol look like on a floor plan?
Thin parallel lines running across a gap in the wall, inside the wall thickness. The outer lines are the two faces of the wall and the line or lines between them are the glass. Unlike a door, a window does not break the wall outline — the thin lines are what tell you it is glazed rather than open.
How do you tell a window from a door on a plan?
A door is a gap with a swing arc or a sliding panel; a window is a set of thin lines that run the whole way across the opening with no arc and no panel. If the wall outline continues across the opening as thin lines, it is a window.
How are different window types shown on a floor plan?
Mostly they are not. A fixed pane, a casement, an awning and a double-hung sash are drawn identically in plan, because a plan is a horizontal cut and they differ in how the sash moves vertically or outward. There is exactly one exception: a slider, whose sashes sit on parallel tracks at different depths in the frame, and which is therefore drawn as two offset panels. Which way any other window opens is shown on the elevation, not the plan.
What is the symbol for a sliding window?
Two half-width panels offset to opposite sides of the glazing line, one per sash. There is no arrow: the offset is what says they pass each other, exactly as it does on a sliding door. A sliding window also always has a meeting stile, because a single pane has nothing to slide past.
What is the difference between a mullion and a transom?
A mullion is a vertical bar dividing the glass; a transom is a horizontal one. Both are elevation marks — they appear when you look at the window face-on, and a plan cannot show them because the cut passes through the glass at one height.
How is a bay window drawn on a floor plan?
As the projecting shape itself: the wall steps out into a bay or a bow, and glazing is drawn across each of the faces. A bay is not a special window symbol, it is a change in the wall line with ordinary windows in it.
What height is a window drawn at on a plan?
The plan does not show height, which is why sill and head heights are written as notes or scheduled separately. A typical residential window sits with its sill 900 mm above the floor and its head at 2100 mm, matching the door head, and that is what Gridsnap uses unless you change it.