Comparative Analysis will give u the local market's indication. Retro search similar properties 2 years........ ex. 20 similar sales, list the roof types then the number of sales for each (if they vary)............and -----after adjusting for all the other salient characteristics (location, view, lot size/utility/topography, view etc, dwelling actual age, effective age and condition, style, floorplan, basement (if any), IG Pool, tennis court, spa, garage, etc. etc.) - u will have the answer you seek. OR you can secure the services of a local Experienced Appraiser who can do it for you .........negotiate a fair fee based on property type and "comp" difficulty, and analysis time. Let us know how it goes.