Top and Bottom Layers

What these layers do

Inside the walls, most of a part is sparse infill: a pattern with open space between the lines. Flat surfaces cannot be left open, so the slicer prints solid layers there: bottom layers where the part starts, top layers where a surface faces up. The first top layer has to span the gaps in the infill below it, like a short bridge, and each layer after that fills in the dips of the one before. That is why one top layer is never enough, and why sparse infill and top layers interact (see Infill Patterns and Density).

Where the settings are called what

What it controls	PrusaSlicer	OrcaSlicer	Bambu Studio	Cura
Number of solid layers	Solid layers, Top and Bottom	Shell Layers (top and bottom)	Top shell layers, Bottom shell layers	Top Layers, Bottom Layers
Thickness instead of count	Not confirmed	Shell Thickness (top and bottom)	Not confirmed	Top Thickness, Bottom Thickness, Top/Bottom Thickness
Pattern of the visible top layer	Top fill pattern	Surface Pattern	Top surface pattern	Top Surface Skin Pattern
Extra solid fill on slopes	Ensure vertical shell thickness	Ensure vertical shell thickness	Ensure vertical shell thickness	Not confirmed

Not confirmed means we did not find the name in that slicer's documentation; the feature may still exist.

Count versus thickness

Prusa uses exactly that example: 3 layers at 0.3 mm or 9 layers at 0.1 mm give a similar thickness. The lesson is that a layer count tuned for one layer height is too thin at a finer one. A thickness setting fixes this. OrcaSlicer's Shell Thickness raises the layer count whenever the count alone would give a thinner shell, which protects you at small layer heights, and Prusa notes its minimum thickness setting is useful with variable layer height.

How many top layers

Rule of thumb Prusa suggests at least 3 top layers to reduce sagging, and OrcaSlicer recommends at least 3 for most prints. More top layers add strength and give a flatter, better-closed surface, at the cost of time and material on every upward-facing area.

Add top layers, rather than infill, when:

• The top surface shows the infill pattern underneath, or has small holes (see Top Surface Gaps).
• You lowered infill density or switched to a sparse pattern, so the first top layer spans wider gaps.
• You moved to a finer layer height and kept the same layer count.

Bottom layers matter less for looks, because the bottom sits on the plate, but they still close the part. Watertight or load-bearing parts need enough of both.

Slopes: ensure vertical shell thickness

On a gently sloping surface, each layer's top skin is a thin sliver, and the shell can end up thinner than the layer count suggests. Ensure vertical shell thickness adds solid infill under sloping surfaces to keep the shell at the thickness set by your top and bottom layer counts. Bambu Lab's example: with 4 bottom shell layers, any part of a bevel with fewer than 4 layers of cover gets extra solid infill. Prusa lists Disabled, Partial and Enabled, warns that Disabled may cause print failure, and calls Enabled the standard setting. Leave it on unless you have a reason.

The top surface pattern

Only the topmost layer is visible, and its pattern can be chosen separately (in PrusaSlicer, Print settings, Infill, Top/Bottom fill pattern).

• Monotonic: Prusa describes it as rectilinear with every line laid in the same direction, which gives a uniform surface without ridges.
• Monotonic lines (OrcaSlicer calls it Monotonic line): Prusa says it avoids overlapping the perimeter for a smoother surface, but adds time and can make seams more visible. OrcaSlicer says monotonic line is usually recommended for top surfaces.
• Concentric, Hilbert curve, spirals: mostly a look. Prusa notes Hilbert curve and Octagram spiral lengthen print time.

Bambu Studio also lets you set the sub-top surface (the single solid layer just under the top) separately; by default it uses Monotonic to reduce marks from internal solid infill showing through the top.

Small top features and lettering

Raised text and small top details often have no room for both walls and top fill. PrusaSlicer's Only one perimeter option and OrcaSlicer's Only one wall on top surfaces print a single wall on flat tops, leaving more room for the top pattern. OrcaSlicer notes this only works when the matching shell layer count is above zero.

What to change first

1. Gaps or visible infill on top: add top layers, or set a shell thickness so the count follows your layer height.
2. Still rough: switch the top pattern to monotonic. If lines look starved, check flow before adding more layers.
3. Thin spots on slopes: confirm ensure vertical shell thickness is on.

Change one thing, reslice, and look at the preview layer by layer before printing.