Doesn’t the building code allow two roofs? Why should I remove my old roof if I only have one layer now? Won’t I save money going over the old roof?
These are common questions we address with homeowners each day. What is cheap and seems like a good deal today will often come back to bite you in the future. Most building departments do allow two layers of roofing for composition shingle roofs as long as the total weight of the combined roofs is less that the maximum allowed by the building code. You have to know what your local building department allows before you proceed as they can vary greatly from one area to another.
#1) Dry rot. Unless the old roofing is removed there is no way to know if there is dryrot or fungus that may be present in the roof sheeting. Even if you do an attic inspection prior to starting the work there is still the chance that a slow leak has caused dryrot fungus to be present between the bottom of the roof shingles and the plywood or plank sheeting. This condition is often found in the roof overhangs where the wood is exposed to outside moisture as well as potential leaking from eves, gutters, etc. Just because the roof has never had leaks or stains present on the inside does not mean it was not leaking. Covering up this dryrot with another layer of roofing will not make it go away as fungus can continue to grow from the moisture in the air during the winter months in a cold attic or especially in the roof overhangs.
#2) Preparation: In the event that you have chosen to roof over that existing roof the proper preparation is needed to insure a good job. The roof should have all shingles which are overhanging the existing edge metal cut back flush to the edge and new edge metal installed on all perimeters. The vent pipe flashings will have to be taken up and the surface patched back in level as to not create a depression or void around the vent area, skylight flashings, chimney base flashings, dormer vents will all have to have the shingles pulled back and the flashings reinstalled or replaced for the new roof. And lastly the hip and ridge shingles removed to avoid the large hump which will occur if they are left in place. The fact is that most roofers will not take the time to perform these steps as it is labor intense.
3) Fit and Finish: Going back 30 years or more most asphalt composition shingles where called three tab shingles. They laid down very flat making them good candidates for roofing over. Back then you were allowed to have three roof layers and it was common to install composition shingles over old wood shingles and even shakes. The appearance was less than desirable. You need to keep in mind that asphalt composition shingles are rigid when cold and flexible and pliable when hot. They tend to form themselves over the shape of what is below. Todays dimensional style shingles are rough and uneven with a wood look pattern and a thick overlap. When you apply a second layer over it they may start out looking ok but over time and the heat of summer will mold themselves down on the old roof and look very wavy and uneven. It is almost impossible to get a nice looking job installing over these types of shingles.
4) Function and Longevity: Composition shingles are designed to be laid flat. When they do not lay flat water tracks into the side laps and saturates the nails that hold the shingles to the roof. This often leads to leakage from around the nails, as well as separation in the laminated portion of the shingles. The shingles also can lose their wind resistance since the seal tab or glue strip under the shingle is not making proper contact with the shingle above it. Now the roof that you expected to last 30 or more years is failing in 10 to 15 years and in some cases leaking within a few years.
5) Conclusion: It is always best to remove the present roofing to assure the best job possible.