# Top and Bottom Shells

Controls how the top and bottom solid layers (shells) are generated.

![top-bottom-shells](https://github.com/SoftFever/OrcaSlicer/blob/main/doc/images/top-bottom-shells/top-bottom-shells.png?raw=true)

## Shell Layers

This is the number of solid shell layers, including the surface layer.  
When the thickness calculated from this value is less than [shell thickness](#shell-thickness), the shell layers will be increased.

These layers are printed over the [sparse infill](strength_settings_infill), so increasing **shell layers** will increase overall part strength and top surface quality.
It's usually recommended to have at least 3 shell layers for most prints.

## Shell Thickness

The number of solid layers is increased during slicing if the thickness calculated from shell layers is thinner than this value. This avoids having too thin a shell when layer height is small.  
0 means this setting is disabled and shell thickness is determined entirely by [shell layers](#shell-layers).

## Surface Density

This setting controls the density of the top and bottom surfaces. A value of 100% means a solid surface, while lower values create a sparse surface.  
This can be used for aesthetic purposes, improving grip or creating interfaces.

## Infill/Wall Overlap

The top solid infill area is slightly enlarged to overlap with walls for better bonding and to minimize pinholes where the infill meets the walls.  
A value of 25-30% is a good starting point. The percentage value is relative to the line width of the sparse infill.

> [!TIP]
> Check [Monotonic Line](strength_settings_patterns#monotonic-line) to learn about its overlaying differences with [Monotonic](strength_settings_patterns#monotonic) and [Rectilinear](strength_settings_patterns#rectilinear).

## Surface Pattern

This setting controls the pattern of the surfaces.  
If [Shell Layers](#shell-layers) is greater than 1, the surface pattern will be applied to the outermost shell layer only and the rest will use [Internal Solid Infill Pattern](strength_settings_infill#internal-solid-infill).

> [!TIP]
> See [Infill Patterns Wiki List](strength_settings_patterns) with **detailed specifications**, including their strengths and weaknesses.

 The surface patterns are:

- **[Concentric](strength_settings_patterns#concentric)**
- **[Rectilinear](strength_settings_patterns#rectilinear)**
- **[Monotonic](strength_settings_patterns#monotonic)**
- **[Monotonic Line](strength_settings_patterns#monotonic-line)** Usually Recommended for Top.
- **[Aligned Rectilinear](strength_settings_patterns#aligned-rectilinear)**
- **[Hilbert Curve](strength_settings_patterns#hilbert-curve)**
- **[Archimedean Chords](strength_settings_patterns#archimedean-chords)**
- **[Octagram Spiral](strength_settings_patterns#octagram-spiral)**