The underside of a sloped face is rough, fuzzy and visibly sagging, while the upper side of the same face is clean. Steeper slopes are worse, and beyond some angle the surface degrades into loose strands.
Where the 45-degree rule comes from
Every layer on a sloped wall is offset sideways from the one below it. How far depends on the layer height and the slope: a steeper wall means a larger step per layer. The new bead is supported only where it overlaps the previous one; the rest of its width hangs over air.
Put numbers to it. On a 45-degree wall with 0.2 mm layers, each layer steps sideways by 0.2 mm. If the bead is 0.4 mm wide, exactly half of it is sitting on nothing.
That is the whole basis of the rule. It is not a material property or a machine limit — it is a geometric coincidence that 45 degrees puts the unsupported fraction at one half for the most common combination of layer height and line width. Change either number and the angle at which you are in trouble changes with it.
Which gives you the strongest lever available
Halve the layer height and the sideways step halves too. On the same 45-degree wall at 0.1 mm layers, only a quarter of each bead is unsupported instead of half — a dramatic improvement in droop, achieved without touching cooling, speed or temperature.
This is why fine layers produce good overhangs and why adaptive layer height, which uses thin layers on shallow slopes and thick ones on vertical walls, gets both quality and speed. If your slicer offers variable layer height, this fault is the best argument for turning it on.
Widening the extrusion has the same effect from the other direction: a wider bead means the same sideways step is a smaller fraction of it. That option is limited by nozzle size, but it costs no time at all.
The rest of the toolkit
- Increase cooling for overhang regions. Plastic that sets sooner sags less. Most slicers can raise fan speed for overhangs specifically.
- Slow overhang perimeters. Each bead gets more time to solidify before its neighbour lands. Note that this is the opposite of the advice for bridging, where speed provides the tension that holds the strand up — a bridge is anchored at both ends and an overhang is not.
- Drop nozzle temperature by a few degrees for the material. Cooler plastic has a shorter soft period.
- Reorient the model. Almost always more effective than any of the above, and free. A part rotated so its critical faces are vertical or upward-facing has no overhang problem at all.
Droop is not curl
Two failures happen on overhangs and they point in opposite directions.
Droop is what this page describes: the unsupported part of the bead sags downward under gravity, producing a rough underside that is otherwise dimensionally close to correct.
Curl is the free edge lifting upward as it contracts, which raises the surface above where the machine expects it and eventually results in the nozzle striking the part. Curl is the more dangerous of the two because it escalates.
A part can show both: droop across the sloped face and curl at the leading edge. Cooling improves both, layer height improves droop specifically, and only curl leads to a collision.
Judge it with a test print
An overhang test — a fan of surfaces at increasing angles — takes fifteen minutes and tells you where your machine actually degrades, in your material, with your cooling. Most people find their machine holds a steeper angle than the folklore suggests, and knowing the real number changes how you orient parts and whether you use supports at all.
Print it once for each material you use regularly. The difference between PLA and PETG on the same machine is larger than the difference between two machines on PLA.
Materials, and what they can hold
PLA holds the steepest angles of anything in common use, given decent cooling. PETG is a step behind and sags visibly earlier. The polyamides are worse again. ABS and ASA suffer because the cooling that would help them is the same cooling that makes them crack, so overhangs on those materials are usually handled by orientation and support rather than by settings.
Fibre-filled grades are quietly good here: the filler stiffens the strand as it cools and reduces the sag, which is one of several places where a composite outperforms its base polymer.