Should depreciation on a 40 year shingle be the same as a 20 year shingle in OKlahoma on a roof 6 years old?

Is this to calculate the value of the roof for a warranty replacement? Generally a roof warranty is only for the materials and is prorated based on the warranty period of the roof and the time elapsed. For a 20 year roof the depreciation would be 6/20 or 30% of the original material price of the shingles. For a 40 year roof it would be 6/40 or 15% of the replacement cost of the shingles. Read the warranty that came with your shingles to get the details of how it would be applied in your case. Local laws may also change how the warranty is applied.