Rotate an object according to terrain in Unity (C#)

I currently have an item placement system for building. It works by instantiating a "ghost" of the object that shows where it can be placed, the ghost (semi-transparent) object is attached to the camera and instantiates the object in its place when the player clicks.

I get the position at which to keep the ghost object like so:

        var pos = transform.position + transform.forward * placeDistance;  // A position 'someDistance' in front of the player
        pos.y = Terrain.activeTerrain.SampleHeight(pos);  // Get the position at the surface of the terrain to place the object
        firePlaceable.transform.position = pos + Vector3.up * 0.001f;  // 'halfHeight' is used in case the pivot is not on the base

Now.. I need the object to rotate according to the terrain so that the fire place is placed more or less correctly rotated. Any ideas? What would the best plan be?

Solution

Use the terrain normal vector at the place' position.

For example you could do a raycast straight down from the fireplace. The resulting hit contains a normal that is your place' up vector.

By thinking of it... I assume you already doing a raycast to get the position to place the fireplace right?

Use the placement raycast to get the up vector instead of making a new one.

So basicly do

fireplace.transform.up = clickPlaceHit.normal;