Legal Limits: Local Roofing Code
Some building codes allow for two layers of roofing, or three layers on steeper roofs, but other locations could be more or less restrictive. The types of roofing material that can be layered is also regulated.
National, State, County, City, and other jurisdictions (like HOA administrations or historic districts) may have something to say about the construction of your roof. Check your Covenants and Restrictions document, if you have one, then if you are still interested, work your way up the governing agencies from there. Your local roofer should know the rules, which could save you some time.
Multiple layers of roofing materials may be allowed in some areas, but is rarely a good idea.
First Things First
Before you get too far, find out how many layers of shingles you have currently. Are you on your first layer now? If you are already at or beyond the limit then you will need to start with a complete tear off.
Pros and Cons of Layering Roofing Materials
The benefits of adding another layer of roofing without tearing off the existing layer are as follows:
- Save time and noise of tearing off the old roofing materials. This is a day, more or less.
- Save the cost labor and disposal of the old layer. This varies depending on the size of your roof and the cost of living in your area, but keep in mind that you are just kicking it down the road where inflation will likely catch up with you on the other end.
The drawbacks are much more extensive.
Leak Investigation and Repair Become Complex
Leaks in a single-layer roof are relatively straightforward to diagnose and repair for an experienced roofer. Many repairs are done in an hour or two with standard supplies that are probably already on the truck.
Not so with double-layer roofs. Water travels differently so subtle damage is harder to find.
Even with obvious damage like a branch through the roof there is so much more to deal with. Think about re-layering lasagna after it’s already been cooked. If you just fix the visible top layer, it will sag if there’s a void below it. If you fix the bottom layer, the top layers will be disturbed. Things get complicated, slow, and expensive, pretty quickly.
The Importance Of A Good Foundation
The wood between your attic and your shingles is called decking or sheathing. When your old roofing materials are bad enough to be replaced, there’s a chance that this wood layer has been damaged, too. Depending on your climate this means the sheathing could be some combination of saggy, brittle, spongy, rotten, et cetera.
When you don’t remove the existing roofing materials then you don’t get to see the condition of the decking. As you can imagine, new shingles over bad wood is less than ideal. Here are just a few reasons:
- When we apply your new shingles, the nails need good wood to grab on to.
- Inspectors, contractors, residents, and others on the roof could fall through a rotten spot injuring themselves and the objects below.
- If damaged wood is adjacent to pipes, vents, satellite dishes, or skylights then those systems can be weakened or stressed.
- In a storm rotten wood is less strong.
Less critical, but also a factor: you can’t see what kind of system components you have. You may or may not have the right system materials in the right places. Between the decking and your shingles you should have some combination of flashing, ice and water shield, and a felt or synthetic underlayment.
A Weighty Decision
One layer of roofing materials can weigh around 2 tons, depending on a number of factors like product type and roof size. Adding a second 2-ton layer over unexamined decking may be more than you bargain for, especially when you live in a seasonally cold area and have potential for the addition of snow loads.
Hot Potato, Cold Potato
On either end of the spectrum, excessive temperatures are hard on asphalt. With two layers of shingles you increase your roof mass and therefore magnify its ability to hold temperature for a longer period of time.
In hot weather this can raise the temperature of the asphalt causing it to soften and, among other things, be less impact resistant. This also causes extra heat in your attic, which has other cascading effects.
In cold weather snow may thaw more slowly and have more potential for ice damage. It also keeps them at a brittle temperature for a longer period of time, which obviously isn’t ideal. Your attic could be cooler, potentially increasing condensation, your heating bill, and your blood pressure.
A Decaying Sandwich
Your old shingles are probably already decomposing, or you wouldn’t be replacing them. That leaves a mess between your roof deck and the new shingles.
If some of your shingles are failing now and the others aren’t far behind, where’s the logic in putting good shingles over a layer at the end of its lifespan? The very little you save now will compound for a larger bill next time.
Here’s the Rub
Friction is a major enemy of asphalt shingles. While the two layers may be glued or nailed together they are still separate objects. You’ve basically installed your new roof over rough sandpaper. It’s not designed for that so you are putting your new materials at a disadvantage.
New shingles are also less likely to lie smoothly on an old roof. Textures as subtle as wrinkled felt underlayment can telegraph through to the top layer making a lumpy eyesore. Imagine the greater impact of row after row of old shingles, especially if they were chipped or curling.
The more defined your old shingles are the greater chance you’ll notice. It may not be immediate, but it is likely. It can take months to show up, but will occur more quickly in warm weather as the asphalt softens and forms to its environment.
3-tab has a lower profile but will make sort of a wavy pattern across your roof where each row changes. You might be able to overlay 3-tab with premium dimensional shingles, but we don’t recommend the risk. You definitely shouldn’t go the other direction with smooth on top of textured.
Under no circumstances would you want to change materials, for example putting asphalt shingles over a different material, like wood shakes. Aside from being a lumpy mess, and probably against code, they react to changes in temperature and humidity differently, which is a recipe for disaster.
Speaking of disaster, you may not be able to get insurance for your multi-layer roof. Many companies won’t cover your roof at all.
If they do, they are likely to just cover the new layer. Remember that falling branch scenario? When only the top layer is covered, you are out-of-pocket for the old layers: lower layer shingles, underlayment, and decking.
Plus, your premiums are likely to be higher. Less coverage for more money doesn’t seem like much of a deal.
Buyers and Lenders Look at Roofing
If you plan on selling or refinancing your home with multiple layers of roofing, be aware that certain lenders may not give you a loan. Loan programs with more strict home inspections will consider multi-layered roofing a red flag.
What’s Your Roof Warranty With A Second Layer Of Shingles?
Material manufacturers have very specific requirements for installation. A clean deck, usually with the underlayment removed, is a pretty common criteria.
If you can find someone to do your roof over you’re more likely to have more waivers than warranties with your contract. If you can get a warranty at all, read it very carefully for both labor and materials. Ask for clarification if anything is vague or confusing and remember a warranty is only as good as the company that stands behind it.
Limited Pool of Roofing Contractors
Many top roofers just won’t do it. They know its bad for you. They also know when your roof fails early then it is bad for their reputation, even though a warning was issued at the beginning.
Do you really want to remove the roofers with knowledge and integrity from your shortlist?
Contact us with your goals and concerns. We are happy to discuss options to best meet your needs.