Are you considering a new roof? Following are some things I have gleaned from other professionals which you may not have considered. They are worth your time to review.
5 Reasons Not To Have A New Roof Installed Over An Old Roof
1.) Areas that have or had leaks can't always be addressed properly
There is a good chance that your old roof had some problem areas including possible leak spots, whether you noticed them or not. Without tearing off the old roof and properly identifying these types of trouble spots and determining where the leak was coming from and traveling to it is impossible to tell what areas of your roof may need some special attention.
2.) Any rotted wood under the existing roofing will only get worse
There could be areas that have rotted wood hiding under the old roofing. These rotted areas need to be identified and replaced before a new roof is installed. Obviously if your roofing contractor is only doing a lay-over roof installation then these rotted areas will remain covered up and only get worse as the years go on. Also the nails holding down the shingles in areas with rotted wood cannot properly do their job and you have a much higher risk of shingles blowing off in those areas.
3.) Eaves, rakes and valleys need special treatment
This is a big one. The eaves, rakes and valleys of your house need special attention when your home's roof is being installed. This is especially important in colder climates like Massachusetts, where we are located. In the winter time the eaves of your house are under attack by Mother Nature, whether it is through ice dams, snow build up, or just the constant freezing and thawing that occurs throughout the winter season. When a new roof is properly installed the roofing contractor needs to put new aluminum drip-edge around the entire perimeter of your roof.
Next they need to apply a 3 foot wide section of ice & water barrier around the perimeter as well as in any valleys on your roof. Then they can begin to install the new roofing. Without tearing off the original roofing there is no way to properly install the new drip-edge or ice & water barrier. On a lay-over type of roofing install, the roofing contractor is counting on the existing products on the home's roof to still be up to par and be able to handle the winter conditions. All too often the old products fall short whether it was because they have outlived their lifetime, were sub-par to begin with, or maybe they were never there to begin with (all to often the latter is the case with ice & water barrier).
4.) Extra roofing weight is no good for old rafters
One of the more obvious problems with a lay-over re-roof is the added weight of the extra layer of shingles. On most newer homes this is not an issue, however many older homes have rafters that are considered undersized by today's framing standards. It is not uncommon to see 26 rafter systems on many of these houses. Now in most situations a 26 rafter is undersized to begin with and you certainly don't want to be adding the weight of a new roofing layer on top of an old roofing layer to these already undersized rafter systems. With newer roofs using 210, 212, engineered trusses, etc. the weight isn't always as much of an issue.
5.) Shorter roof life expectancy
Most responsible roofing contractors agree that a lay-over roof will decrease the new roof's lifetime by about 25%. This fact alone means that any money you might have saved by doing a lay-over, as opposed to a tear-off and new roof install, was only a short term savings. In addition, you now have 2 layers of roofing that will need to be removed the next time your roof is done and that will also add more cost to the job