There are two approaches to installing new shingles on a home. The first approach is to do what is called in the trades a “rip off.” A rip off is where the old roofing is stripped off and the new roofing installed from the roof's wood sheathing outward. The second approach is to do what is called an “up and over.” In an up and over, the new roofing is installed right over the old roofing materials.
Pros and Cons
One big advantage to installing new shingles over old rolled roofing is that it saves a lot of time if you are doing the job yourself. The rip off is the most time-consuming part of any roofing job, and time, if you are having the new roof installed by professionals, means money out of your pocket. Roofing is very labor intensive. The disadvantage to installing new shingles over old rolled roofing is that you do not have the opportunity to inspect the roof's wood sheathing for possible damage. If your home has an attic or attic crawl space, you can perform that inspection from there, but some homes have no attic or easy access to the crawlspace. In the latter case, serious structural damage can go undetected.
Doing an Up and Over
Installing a new roof using the up and over approach also saves some money in material costs, because doing a rip off requires you to install new rolled felt before installing the shingles. When installing shingles over rolled roofing, you must first inspect the old rolled roofing for holes. Any damage to the old roof must first be repaired using roofing tar or a fiberglass roofing repair compound. This is an essential step to having a structurally sound roof.
Installing the Shingles
Shingles are installed over rolled roofing in a normal manner, but extra care must be taken with the ice dam, the first row of shingles installed at the roof's edge. Ensure that there is a solid seal between the ice dam shingles and the rolled roofing. Bubbles formed in the rolled roofing surface can make getting a good seal extremely difficult, but a good seal is needed to prevent winter damage to your roof.
If your old, rolled roof has been subjected to severe damage by wind, rain, snow or ice buildup, never install new shingles over the old roof. In these cases, a complete rip off is called for so that any structural damage to the subroofing can be repaired properly.