# Sandbox.ModelBuilder.AddSurface

```csharp
ModelBuilder AddSurface(Surface surface)
```

Adds a surface that can be referenced by collision mesh material indices.

### Parameters

- `surface` (`Surface`): The surface to add. Null uses the default surface.

### Returns

`ModelBuilder`

Declared in [Sandbox.ModelBuilder](/api/Sandbox.ModelBuilder).
Assembly: `Sandbox.Engine`.