Samuel F.B. Morse invented the telegraph in 1835. We mention that historical tidbit because we will use the word “telegraph” to discuss ways your old roof can ruin your new roof. When we talk about telegraphing, we are not referring to Morse’s clickety-clack device. When you ask, “Can you put a new roof over an old roof,” we would like you to understand why we say No, or dash-dot, dash-dash-dash in Morse’s language.
Outta Here
Long before you sign a contract with a roofer, you may be wondering if your old shingle roof will have to be removed before the new roof is put down. Once you begin discussions with a local, reliable residential roofer, you will discover that the old roof has got to go.
Removal of the old shingle roof accomplishes several things:
- It reduces weight on your home.
- It reduces the thickness of the new roof.
- It allows maintenance of the new roof to be efficient and relatively inexpensive.
- If your old roofing materials are recycled, it can reduce your home’s carbon footprint.
- Any repair issues with the new roof are minimized.
Nope
Asphalt shingles are North America’s most popular residential roofing material, so many homeowners in southeast Michigan wonder if a new shingle roof can be installed over an old shingle roof.
The short answer (thanks, Morse!) is No, for the simple reason that shingles are flexible. The new shingles are fairly thin and are designed to be installed on flat sheathing (the plywood or oriented strand board supporting them).
The thin, new shingles will deform and take on the shapes and lines of the old shingles beneath them. This is called telegraphing, and it quickly transforms a brand new roof into a hideous, strangely old roof. In addition to the cut lines and crisp edges you expect to see in the new roof, each shingle will also have odd creases and raised portions where the old shingles are pushing upward.
Wait, Weight
A square — 100 square feet of shingles — weighs around 270 pounds. A typical roof may need 30 squares of shingles, which means installing a static load of some 8,100 pounds just for the shingles. Add underlayment, ridge vents, valley flashing, drip edge, and all the other roofing elements and your Michigan home’s rafters are supporting upwards of five tons of dead weight.
If you do not have your local roofer tear off and cart away the old shingles, your rafters may have to support more than 10 tons of static load. That isn’t good for your home’s roof deck and rafters.
Add a typical Michigan snow load to that double-roofed house and you could face a structural emergency, complete with roof collapse and a snow-filled attic.
Municipal Codes
Some homeowners may be familiar with their local building codes. Sometimes, those codes allow for installing a new roof on top of an old one. This practice has two positive benefits:
- Installation and labor costs are reduced because the roofing crew does not need to tear off the old roof.
- Your carting and haulage costs are reduced because most of the construction debris in installing a new roof comes from removing the old, damaged roof.
Those two benefits sound good, but the negatives far outweigh any positives. Installing a new roof over an old one makes no sense economically or practically.
Economically, you immediately undercut the value and beauty of the new shingle roof through telegraphing and challenges to installation. In practical terms, two roofs are not better than one for several reasons:
- How could a roofer access a leak if it goes through the upper shingle roof, travels downhill a bit, and then goes through the lower shingle roof? No roofer could do that without effectively taking up both roofing layers over several days (at an exorbitant cost to the homeowner).
- All connections such as rubber boots, flashing around chimneys and vents, and other rooftop penetrations will have to be modified to compensate for the added thickness. This adds considerably to installation costs.
- Reroofing (adding shingles to an existing roof) mismatches the natural aging process of both roofs, so the underlying roof may crumble and decay, affecting the new roof and sprinkling shingle granules into your gutters for years to come.
Good roofers tidy up after themselves, but they must charge reasonable fees for carting away construction debris. The small amount you may save on labor and carting does not make up for the risks you take when you insist on having a new shingle roof installed over an old one.
In southeast Michigan, homeowners have come to rely on Victors Roofing for honest, efficient roofing work. Contact us today to learn about our methods, services, and commitment to our clients.
