Why does my whole floor plan sit crooked — and can I straighten it?
A scanned floor plan often follows whatever angle the house happened to be scanned at. It looks messy, but it is rarely a measuring error — and most apps can straighten the drawing without changing a single dimension.
Short answer: a scanned floor plan follows the angle the house was actually scanned at, which is rarely quite square to the edges of the page. Most apps can straighten the drawing so the house's main axis sits level on the page, without changing a single measurement — it is a rotation of the whole plan, not a change to the geometry.
Why a scanned floor plan sits crooked in the first place
When an app builds a floor plan from a scan, the drawing inherits the angle the phone was actually carried around the room at. If the house itself is not built quite square to the compass points — which is normal; most houses sit on a plot, not on a grid — or the scan simply started facing a random direction, the main walls end up sitting a few degrees off on the page. That is not a measuring error. The walls have the correct lengths and angles relative to each other; it is just the whole drawing that lies rotated on the sheet.
Is it the house that's crooked, or just the drawing?
Both can be true, and it is worth telling them apart. A house that genuinely sits at an angle on its plot — for instance because the road or the boundary line decided the direction — will always have an angle relative to north, however neat the drawing looks. A drawing that is simply rotated a few degrees because the scan started off-axis, on the other hand, can be straightened without changing anything about the house's actual shape.
The rule of thumb: if all the right angles in the house (corners that are genuinely 90 degrees) look skewed on the drawing, it's the drawing. If the corners look right but the whole house sits at a slant relative to the page's edges, it is likely just a viewing angle that can safely be corrected.
How to straighten a floor plan without changing the measurements
A "straighten" function rotates the entire drawing as one piece, around a fixed point, until the house's longest or most common wall line sits level or plumb on the page. It is a pure rotation — every wall keeps its length, every angle between walls stays unchanged, only the drawing's orientation on the page changes. It is the same as turning a piece of paper on a desk: the text on the page does not change, only which way it faces.
If the app has no automatic function, you can often achieve the same thing by manually rotating the drawing view until it looks right — but then you lose the fixed reference to where north actually is, because that rotation is only visual and is not saved into the drawing's data.
What can go wrong if you straighten it against the wrong wall
The most common mistake is straightening the drawing against a wall that is not representative — for instance a short cupboard wall or a small partition that happens to sit slightly askew for other reasons. Then the rest of the house ends up crooked instead. A good "straighten" function finds the house's dominant direction from the majority or the longest walls, not from a single random line.
When you should not straighten the drawing
If the house has several wings that are not parallel — an L-shaped house, an extension set at an angle to the original building — there is no single "correct" main direction to straighten against. In that case it often makes more sense to leave the drawing as it was scanned, or to straighten each wing separately if the app allows it.
Does it matter for the floor area that the drawing sits crooked?
No. The area is calculated from the actual measurements between the walls, not from how the drawing is oriented on screen. A drawing that looks crooked can easily have a completely correct area — and a drawing that has been neatly straightened can have an error somewhere else. Rotation is a display matter, area is a measurement matter, and the two should not be confused.
Can I just rotate the view myself instead of using "straighten"?
Yes, but it is not the same thing. If you only rotate the view itself — by turning the image on screen with two fingers — you are not changing the drawing's saved data, only how it is displayed to you right now. Close the app and reopen the drawing, and it returns to its original, unrotated state; export a PDF or a DXF file in the middle of a manual view rotation, and the file can come out in the original orientation regardless of what was on screen. A proper "straighten" function, by contrast, writes the new orientation into the drawing's actual data, so every view — screen, print, export — agrees on which way the house faces.
How does the app find "the right" direction to straighten against?
Most "straighten" algorithms look at the angles between every wall segment in the house and find the direction that the most, and the longest, walls share. It is the same as laying a ruler along the most common wall direction in the house and rotating the whole drawing until that ruler sits level. The method is reliable in ordinary rectangular houses, but becomes less clear-cut in houses with many different wall directions — a curved bay window, say, or several extensions at different angles — where "the right" direction really comes down to which walls carry the most weight in the calculation.
How crooked is "normal" for a scan?
A scan that sits 2–5 degrees off the page's direction is common and rarely something the eye notices without comparing it to a ruler. Once a drawing is 10 degrees off or more, it starts to be obvious even to an untrained eye — walls that should be level or plumb on the page look clearly slanted. This is not a measure of whether the dimensions are correct, only of how much the whole drawing is rotated relative to the edges of the page.
Can you see how much the drawing has been rotated?
In a good app you can always see how much a "straighten" function has actually rotated the drawing — either as a number of degrees in a menu, or indirectly, by the north arrow moving accordingly. It is worth checking if you later need to compare the drawing with the house's actual position on a plot or a map: a rotation of a few degrees is normal and insignificant, but a rotation of more than 15–20 degrees is worth double-checking, because it can be a sign that the "straighten" function has picked the wrong wall line as its reference.
What HouseSense does
HouseSense has a "level the plan" function that finds the house's main axis from the actual geometry and rotates the whole drawing so it sits neatly with the page — without changing a single measurement. The north arrow follows the rotation, so you can always see where north actually points, even after the drawing has been straightened.
How to check that a straightened drawing is actually correct
- Look at the house's corners. If most of them are close to 90 degrees on the drawing, the direction has probably been found correctly.
- Compare it with the north arrow. Does it point in a direction that makes sense for the house you know?
- Check a measurement you already know. A door's width or a room's length should be unchanged after a "straighten" — if it isn't, something has gone wrong elsewhere, not in the rotation itself.
- Export and view the drawing outside the app, for instance as a PDF, to confirm the rotation is part of the saved data and not just a temporary view.