Thread: why is 2nd layer of shingles a problem?

I've read a few times that shingles installed over a layer of old shingles have a lower life span, but never could figure out why that's the case. I feel like this is a silly question, but could someone explain it?

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Two Layers hold Heat longer and heat affects the life of Asphalt Shingles.
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I don't make a big deal out of it when I see it. I just let buyers know the pitfalls associated with 2 layers.

And just so you know Kristi, more than 1 layer is no longer allowed anywhere in the Twin Cities.

Originally Posted by Kristi Silber

I've read a few times that shingles installed over a layer of old shingles have a lower life span, but never could figure out why that's the case. I feel like this is a silly question, but could someone explain it?

Around here, the cost of stripping the shingles ends up to roughly equal the estimated cost of the reduced life. The cost to strip shingles from a roof is roughly $1,000.00 (more on larger homes). The reduction in life span in my area seems to be about15%, which works out to be about $1,000.00 (more on larger homes. The only real differences are the amount of stuff that ends up in landfills and where the money is paid (up front or at the end of its life).

However, many times the roofing contractor will not replace the flashings, which might end up leaking before the shingles are worn.

In addition to the problems mentioned above, there is weight to consider. The roof framing is designed to support a certain weight. Two or more layers (3 or more is not all that uncommon) can exceed the design weight and cause problems.

Also those older shingles probably don't have ice and water shield.

All the answers above basically sum it up. Two layers do lower the life expectancy of a roof and multiple layers make the situation even worse...

Being a person that has to learn things the hard way, I can tell you a leak in a multiple layer roof can be very difficult to find and fix.

CASMA - Technical Bulletin No. 6 - Re-Roofing

Also of note that here of late, depending on the geographical area you live in, adding a second layer goes against code. If you live in an area where the incidence of hail damage is at a certain level, then the layers must be removed before adding new. As it is here in GA and besides, it just makes sense. Remember, penny wise, pound foolish.

Originally Posted by Michael Bronner

Also of note that here of late, depending on the geographical area you live in, adding a second layer goes against code. If you live in an area where the incidence of hail damage is at a certain level, then the layers must be removed before adding new. As it is here in GA and besides, it just makes sense. Remember, penny wise, pound foolish.

Michael
I am also in Georgia, and have not heard of a requirement for a single layer of roofing. This is from the Georgia State Amendments to the IBC (2006 edition)

1510.3 Recovering versus replacement. New roof coverings
shall not be installed without frrst removing all existing layers
ofroof coverings where any ofthe following conditions occur:
1. Where the existing roof or roof covering is water soaked
or has deteriorated to the point that the existing roof or
roof covering is not adequate as a base for additional
roofing.
2. Where the existing roof covering is wood shake, slate,
clay, cement or asbestos-cement tile.
3. Where the existing roof has two or more applications of
any type of roof covering.
Exceptions:
1. Complete and separate roofing systems, such as
standing-seam metal roof systems, that are designed
to transmit the roof loads directly to the building's
structural system and that do not rely on existing roofs
and roof coverings for support, shall not require the
removal of existing roof coverings.

The way I read this is 2 layers of roofing are allowed when (see above).
If I am reading it wrong or using an outdated amendment please correct me.

Architectural shingle does not lend it self to be gone over due to the uneven surface. There are some things that can be done in the re-roof installation process that will cause the new shingle to not have a reduced life and create the appearance of a single layer installation.

• Weight on trusses is a marginal issue in older homes.
• Newer homes may present a weight issue depending on truss design and specifications.
• If the sub roof is 1/2" then re-roofing is a poor choice because of weight.
• Condition of existing shingle is a major factor, though often that can be overcome.
• Reduced life of shingle due to heat retention can be an issue but not a consistent factor.
• Ice and water shield is a minor concern if the roof has never had a problem, though not a guarantee in future.
• Like everything there are good and bad ways to install shingle. A bad job never lasts.
• 2nd layer require longer nails (not used sometimes)
• 2nd layer at times more susceptible to wind damage.
• 3 layer besides weight will usually not have long enough nails.
• Existing sub-roof damage is more difficult to determine.
• Roofers will push for a tear off at times to add to the job cost, a little more profit on the job.
• Shingle manufactures tend not to want 2nd layer as additional variables come into play with warranties.
• Not all roofs are candidates and not all roofers can do a good job on 2nd layer install.

Rick,

In answer to your question. 2006 IRC section R907.3 under reroofing, number 4 requires removal of existing ashphalt shingles if: ''For asphalt shingles, when the building is located in an area subject to moderate or severe hail exposure according to figure R903.5''. In this figure, metro ATL and much of N. GA into N and S. Carolina are in a moderate hail exposure area. Hope this answers your question.

Up to 2 overlays ( 3 total was allowed for years )
Some local communities by ordinance only allowed 2.
If you use the " nesting " method the life expectancy is not reduced. The problem is that very few roofers used this method. It is easy to spot if an existing roof used "nesting" as the bottom rake course is only a 3 inch exposure. This allows the installer to stack the shingles in and there are no gaps or bridges.
see attatched diagram which was typical for years. - This procedure has been in ARMA (Asphalt Roofing Manufacturer ) NRCA ( National Roofing Contractors Ass ) as well as printed on most shingle packages and or installation manuals.
As an installer I always recommended a tear off. If the owner had budget restraints and specifically wanted a layover on a reasonably flat existing shingle roof , I installed them .
I have installed 2nd layers on hundreds of roofs with no problems. With a "nesting " installation the warranty was intact . Remember 2nd layers required a longer nail. In all roofing applications the nail must full penetrate any sheathing or go at least 3/4 inches into the decking. This means that as an inspector ,you can see the nails in the attic to verify nailing pattern on most shingle roofs.

The post by Ken is correct as the IRC no longer allows 2 layers much less 3. This is for the reasons all the other posts mentioned. In Minnesota there are different zones. In some of the northern areas because of the design for snow load , multiple layers are still allowed. If in doubt visit your states web site for the specifics.
As far as an inspector it is a good idea to be able to identify this type of installation .
You can also look in the attic around vent openings to determine how many layers of shingles there are. Remember 2 shingles lapped = 1 layer of roofing.

Here is the figure: R903.5 Hail exposure

Chapter 9 - Roof Assemblies

Now I realize why it never was a concern for me. Intuitive is nice, fact is nicer.

Michael
Thank you
Looks like about 1/3 of Georgia (including Atlanta) falls under the requirement, while most areas of Georgia (including Columbus) do not.

Personally, I have never liked multiple layers of shingles; I don't favor "hiding" problems: common things like worn0out or sub-par flashings can be replaced , soft spots in the roof deck can easily be identified and repaired doing a tear-off, and like previously mentioned, your roof will look and wear better. It may cost a little more up-front, but I would rather do a tear-off and do the job right.

Wow, thank you for all the responses! That's awesome. All kinds of info here.

OK, now I see how shingles may have a shorter lifespan when over others. The importance of hail made it all click. It's the unevenness of the underlying layer that causes a lot of problems, making impacts or even a heavy snow/ice layer (or walking) more likely to cause damage, especially when very hot or cold. Even nailing would be trickier if the underlying layer is full of gaps. And an uneven bottom would be more likely the make the top layer not sit right, so wind could become more of an issue.

I can easily see how there are all kinds of other issues to think of, especially the integrity of the existing materials.

Nesting seems kind of an intuitive thing to do, if one is going to layer. Easy to follow the lines. I've always torn off when reroofing. I hate doing it, and am glad I'm busy this Friday when my uncle comes to start my roof. Typically, he just had to wait until it dropped into the freeze-yo-arsoff stage of winter before doing it. At least the carpenter ants will be dormant.

I'd hope a roof could bear two layers of shingles! People sheathe with 1/2"?! Boy, you guys must see all kinds of crazy stuff in your work.

And just so you know Kristi, more than 1 layer is no longer allowed anywhere in the Twin Cities.

Interesting. We've had a lot of hail and high winds these last few years, so I guess that's not surprising.

The only real differences are the amount of stuff that ends up in landfills

It'll all end up in a landfill eventually! Or maybe they'll find a way to recycle old shingles. Would that be nice? Make a car that runs on extract of old shingle or something.

Mike, great to have the perspective of an installer. That's interesting about building differences in northern MN. I wonder how better insulation affects houses that weren't designed for the amount of snow and ice that can build up these days, even down here in the Twin Cities.

Great info so far and exactly in line with everything I've learned. One point someone challenged me on once that I'd be interested in hearing responses:

The heat issue with multiple layers.... if it holds heat longer doesn't it take longer to heat up in the first place? So, the "net" heat retention is the same?

I'm not saying one way or another... just that it was brought up to me once and made me think. Any scientists out there have a take?

Originally Posted by Kristi Silber

It'll all end up in a landfill eventually! Or maybe they'll find a way to recycle old shingles. Would that be nice? Make a car that runs on extract of old shingle or something.

Kristi,

True; however, if a roof lasts 10% longer, basic math will tell us that over a long period of time, a 10% reduction in this type of debris in a landfill can be expected.

Up here, shingles have their own spot at the Recyclers (aka, the dump). They get chipped up, a magnet draws out the nails and then they get added to asphalt for paving. It would never breakdown in a landfill site.
From covering roofs to covering roads. Recycling works.

Wow, that's excellent that they reuse the shingles up there. I hope it catches on here, makes a lot of sense.

Gunnar, sorry, misinterpreted you...thought you meant shingling over a layer reduced the stuff going in the landfills, when you were saying the opposite.

The heat issue with multiple layers.... if it holds heat longer doesn't it take longer to heat up in the first place? So, the "net" heat retention is the same?

Matt, I would guess the problem is that the multiple layers means less cooling through the bottom, so peak at a hotter temp on top.

Originally Posted by Matt Fellman

The heat issue with multiple layers.... if it holds heat longer doesn't it take longer to heat up in the first place? So, the "net" heat retention is the same

.
My take is The Top Layer Heats up First then whatever is under it, if it;s felt and decking this mass loses it's heat faster than an added layer of shingles.
.
The Outside Layer Always get hot the more mass under this layer will radiate heat well after Sun down through the outer layer longer than a less substantive mass.
.
.

BSD-102: Understanding Attic Ventilation — Building Science Information


Effect on Shingle Life

In general, shingles installed on unvented attic assemblies operate at a slightly higher temperature. This has impacts on the durability of roof assemblies. A 2 or 3 degree F. rise in average temperature is typical for asphalt shingles and a corresponding 10 degree F. rise in average temperature for sheathing (Parker & Sherwin, 1998; Rudd & Lstiburek, 1998; TenWode & Rose, 1999)


All other things being equal, applying the Arrhenius equation (Cash et.al, 2005), a 10 percent reduction in useful service life should be expected. This is comparable to the effect of the installation of radiant barriers. What is more significant to note is that the color of shingles and roof orientation have a more profound effect on the durability of shingles than the choice of venting or not venting (Rose, 1991) – double or triple the effect of venting/non venting.

It is pretty simple for me.
Q: Why are shingles being REPLACED?
A: These shingles are shot. (Hail damage, age, etc.)
Response: If shingles are shot, why leave them on??

Originally Posted by Duane Nelson

It is pretty simple for me.
Q: Why are shingles being REPLACED?
A: These shingles are shot. (Hail damage, age, etc.)
Response: If shingles are shot, why leave them on??

That pretty much covers 1. below: (bold and underlining is mine)
- R907.3 Re-covering versus replacement. New roof coverings shall not be installed without first removing existing roof coverings where any of the following conditions occur:
- - 1. Where the existing roof or roof covering is water-soaked or has deteriorated to the point that the existing roof or roof covering is not adequate as a base for additional roofing.
- - 2. Where the existing roof covering is wood shake, slate, clay, cement or asbestos-cement tile.
- - 3. Where the existing roof has two or more applications of any type of roof covering.
- - 4. For asphalt shingles, when the building is located in an area subject to moderate or severe hail exposure according to Figure R903.5.
- - Exceptions:
- - - 1. Complete and separate roofing systems, such as standing-seam metal roof systems, that are designed to transmit the roof loads directly to the building’s structural system and that do not rely on existing roofs and roof coverings for support, shall not require the removal of existing roof coverings.
- - - 2. Installation of metal panel, metal shingle, and concrete and clay tile roof coverings over existing wood shake roofs shall be permitted when the application is in accordance with Section R907.4.
- - - 3. The application of new protective coating over existing spray polyurethane foam roofing systems shall be permitted without tear-off of existing roof coverings.

You can have a dry roof with no leaks that still needs replacement AND that has not "deteriorated to the point that the existing roof or roof covering is not adequate as a base for additional roofing".

"If shingles are shot, why leave them on??" Labor and disposal costs. Not ideal, I know.

Interesting that color can have such an influence on shingle life. Now I wonder if I should have chosen a lighter color. (Tear off starts Monday!)

Add to the fact the the first installation was most likely not done very well then you want to add a second layer of "not done very well". Doubling down is just bad in my book.

This is the homes first line of defense not having it done by a good roofing contractor is a poor choice.

My question to the crowd is when you see a roof-over how often do you see the flashings done correctly?

Many times on older homes I see composite shingles applied over the original wood or shake shingles which makes the whole roof appear wavy. Usually the top layer is completely shot within 10 years even with thicker interlocking shingles. Wondering how other inspectors call this out?

Originally Posted by Wade Ashby

Many times on older homes I see composite shingles applied over the original wood or shake shingles which makes the whole roof appear wavy. Usually the top layer is completely shot within 10 years even with thicker interlocking shingles. Wondering how other inspectors call this out?

Never seen it myself but everyone should call it out

See Jerry's post above

2. Where the existing roof covering is wood shake, slate, clay, cement or asbestos-cement tile.