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Roofing over dimensional shingles


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I see with some shingles, you cannot roof over another layer of shingles without losing a UL 2218 listing for hail damage.

Specifically, besides the UL 2218 thing, are there any restrictions regarding roofing over one layer of dimensional shingles with another?

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Right. Weight. Most municipalities around here allow 2 roof covers to be installed before a tear-off is required. Dimensional shingles weigh more than standard three tab so I was wondering about roofing over dimensional shingles. Is there anything in writing from a manufacturer or credible authority addressing this?

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I don't think the practice deserves such strongly worded damnation. I did a 2 roof tear off on my house in 2006 so the two roofs lasted a total of 42 years which seems about right. It was not leaking but it was time to go.

I guess half the shingled roofs I see are two layers and can't say I'm overwhelmed or otherwise concerned with any particular problems.

This is from ARMA who has a guide on shingling over an existing roof cover.

"In many cases, it is not necessary to tear off the old shingles before applying the new ones. If a roof has only one

layer of shingles, laying flat, and the decking is in good condition, a tear-off is not needed."

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I don't think the practice deserves such strongly worded damnation. I did a 2 roof tear off on my house in 2006 so the two roofs lasted a total of 42 years which seems about right. It was not leaking but it was time to go.

I guess half the shingled roofs I see are two layers and can't say I'm overwhelmed or otherwise concerned with any particular problems.

This is from ARMA who has a guide on shingling over an existing roof cover.

"In many cases, it is not necessary to tear off the old shingles before applying the new ones. If a roof has only one

layer of shingles, laying flat, and the decking is in good condition, a tear-off is not needed."

Yeah,

Well, they would say that. They are in the business of selling the product after all; but they aren't engineers. About fifteen years ago the engineers of the truss council (called something else today) published a paper that said basically, "Asphalt roofs are heavy, too heavy. Installing more than one layer works for a static load but when you factor in a heavy snow load the roof trusses are being stressed way beyond their design limits. Don't do it!"

Made sense then and it makes sense now. When folks want to know what I think of a roof-over versus stripping off and installing a single layer I use language just about as colorful as Kurt's.

By the way, if a roofer doesn't tear that first layer off, how is he/she going to be able to see whether that roof deck has started to rot or delam on the side that can't be seen from inside the attic?

ONE TEAM - ONE FIGHT!!!

Mike

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I don't think the practice deserves such strongly worded damnation. I did a 2 roof tear off on my house in 2006 so the two roofs lasted a total of 42 years which seems about right. It was not leaking but it was time to go.

I guess half the shingled roofs I see are two layers and can't say I'm overwhelmed or otherwise concerned with any particular problems.

This is from ARMA who has a guide on shingling over an existing roof cover.

"In many cases, it is not necessary to tear off the old shingles before applying the new ones. If a roof has only one

layer of shingles, laying flat, and the decking is in good condition, a tear-off is not needed."

Yeah,

Well, they would say that. They are in the business of selling the product after all; but they aren't engineers.

By the way, if a roofer doesn't tear that first layer off, how is he/she going to be able to see whether that roof deck has started to rot or delam on the side that can't be seen from inside the attic?

ONE TEAM - ONE FIGHT!!!

Mike

As far as rot or delam, I would start by considering any roof leaks and looking in the attic.

And I wouldn't argue that a tear-off is not best but I have not seen the problems that would warrant Kurt's or your position on this.

Do you include in your report when you see two roof covers a disclaimer or a remark resembling, "more than 1 roof cover on any building is dip squat moron shot." I know this is said to make a point but if I felt this way I would recommend to my client that they do a complete tear off of the roof no matter what I saw on the roof or in the attic.

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Depending upon the situation the IRC says no to roofing over.

R907.3 Recovering versus replacement. New roof coverings shall not be installed without first removing all existing layers of roof coverings where any of the following conditions exist:

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.

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"There are two layers of roof shingles on the roof. Putting more than 1 roof cover on any building is dip squat moron shot, for more reasons than anyone would want to listen to or I'd want to talk about.

When the current shingles are replaced, it will be necessary to remove all the shingles from the roof before installing new shingles."

Sounds good to me!

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If one is willing to dismiss the idea of doing the best job with the miracle materials and processes the roofing industry has given us, forgo what learned wise people have taught us, and accept second to ninth rate design and workmanship, well then, join the mouth breathers and put on two layers.

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Depending upon the situation the IRC says no to roofing over.

R907.3 Recovering versus replacement. New roof coverings shall not be installed without first removing all existing layers of roof coverings where any of the following conditions exist:

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.

Actually, it says yes to roofing over (with a few exceptions).

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Yes, I am a credible authority and it is in writing.

Putting more than 1 roof cover on any building is dip squat moron shot, for more reasons than anyone would want to listen to or I'd want to talk about.

I'll try:

* Subsequent layers increase the thermal mass, which cooks the life out of the upper layer.

* Roofers rarely use nails that are long enough, so the top layer's nails end up being nailed only into shingles, not into sheathing - or at least not into enough sheathing. I can often pull these nails out with my fingers.

* Roofers rarely install new flashings everywhere that they're needed. They, instead, rely on the flashings in the previous layer, ensuring that water will run between the layers. Around here, it's common to find water weeping out from between the layers all winter long.

* Roofers (at least in my area) install the rake metal on 2nd and 3rd layers on top of the shingles instead of below them. I don't know why, but it's a universal practice. The metal blows off in the wind and does nothing to prevent water entry between the layers. It's common to see sections of rake flashing hanging loosely off the roof like silver dreadlocks.

* The additional weight stresses the roof structure.

* The practice defers the cost of disposal to the next guy.

* Multiple-layer roofs are unilaterally more vulnerable to wind damage.

That's about it.

Other than those issues, I really don't see much of a problem with it.

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They put the rake metal( if there is any at all) on top to hide the multiple layers.

Clever, dont'cha think?

Yes, but if they put it over the penultimate layer it would also hide the previous layers. And it would look more nearly normal and work better in the bargain.

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I see things done a lot the same way Jim describes.

I've been here nearly 17-1/2 years and from day one I've never ceased to be amazed by the stupidity of what seems to be the overwhelming majority of so-called "professional roofers" around here. It's become a challenge just trying to find a roof cover that I don't have to criticize.

I know,....it pays the bills; but sometimes it's pretty frustrating.

Finally, the 2013 code will require drip edge flashings. That means that by about 2020 every municipality around here should have stopped using the 2009 IRC and we might see drip edge flashings installed on more than just the occasional 1 in 1000 roofs. Then it will be another ten years of writing them up for improper installation before they figure out how to install them correctly - if they bother to install them at all.

ONE TEAM - ONE FIGHT!!!

Mike

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