Should roof shingles be replaced?

[Jack K]

I have been getting estimates to replace the roof on our home.

Some don't want to remove the shingles while others say they will not place new shingles over the old ones.

The prices are several hundred dollars different.

What's the difference?

[home improvement guy]

There are several reasons for removing the existing shingles, the major one being the pitch of the roof. A low-sloped roof, one that has 4 inches of rise for every 12 inches of run should have only one layer of shingles. This is due to the added weight of a second layer of shingles. A low-cost shingle weighs approximately 22 pounds for every square foot of area covered. The rafters may not be designed to support this amount of additional weight plus any additional weight from a heavy snow or high winds. When the pitch of the roof is increased, it may be possible to add a second layer of shingles.

You should look at the present condition of the existing shingles. If they are cupped or curled, the formation may be visible through the second layer of shingles. This is known as ghosting and can reduce the looks of the home. Also a second layer of shingles will have a reduced life expectancy. For example, a lightweight shingle with a manufacturer's recommended life expectancy of 20 years may only have a 12- to 15-year life expectancy. This is due to the ventilation of the attic and the heat and moisture accumulation between the old and new shingles. The decision to strip and replace the existing shingles also depends on how long you intend to remain in the home. A minor savings now can be costly in the near future. I strongly recommend the existing shingles be stripped, damaged decking boards replaced and new shingles installed. I also recommend the use of "Ice and Water Shield" on the first 4 feet of roofing.