Calculating the replacement cost of a uPVC after 20 years?

uming that a building has a life expectancy of 50 years and a discount rate of 5%. If a uPVC gutter have a life expectancy of 20 years. Can I calculate the NPV (net present value) like this it's initial cost is $ 2,782. to replace it, is it correct to multiply it by the year purchase rate of 12.46221 giving a amount of $ 34, 669.87 to replace it in 20 years. Is this method correct. Those who know about life cylce costing and the use of discount tables in Construction Economics.