Roof pitches that can be walked

Roofing material type
Many materials give better foot grip than others
Example:
Concrete tile comes in many textures and designs.
Slate tile is smooth and more slippery than shake tile that has a rough surface

Roofing material condition.
Worn roofing materials can affect on your footing (especially composition shingles with loose   granules) or wood shakes that can be worn and loose and may slide out.

In this example we will use the most widely used roofing material in good condition:
Composition Shingles
Roof Pitch 3/12 to 6/12
Can be walked easily

Roof Pitch 7/12
Can be walked easily, but with caution

Roof Pitch 8/12 to 10/12
Can be walked, but with extreme caution and not advisable.

Roof Pitch 12/12 and above
Cannot be walked on

See Visual Examples of roof pitch
How to determine your roof pitch