Even so, it's a 20 year old roof. Unless they were expensive 35 year shingles they have run the course of their life expectancy. In any event shingle warranties are prorated. Let's say the original shingles were rated for 20 year life span, which is pretty well standard builder quality shingles. Perhaps a 25 year warranty. Let's assume 25 years. The shingles have been on the house for 18 years. Assume the actual cost of the shingles new was 3000 (not labor, underlayment , flashing, ice shield etc, just the shingles because that is all the warranty covers) . The prorate is 18/25 (.72) . The amount the shingle company would credit toward a new roof is 3000 - (3000 x .72) or 840.00. You would need to pay the additional material and labor costs. If they were 20 year shingles the prorated amount would be 300 payable to you.