What roof pitch that can be walked on can be determined by many things. Including the following: 

 Footwear.
 Clean and dry soft soled sneakers are usually the best.

 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: Here
How to determine your roof pitch: Here

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