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Question DetailsAsked on 9/11/2013

Why leave shingles when installing a metal roof?

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8 Answers

2
Votes

Laziness. Find a better roofing contractor. The old roofing should be removed, underlayment installed and furring strips screwed down for the metal to be fastened to. Don't let the installer screw the metal to plywood sheathing. It won't hold over time.


If the old roof needs to be replaced why leave it there and not make sure there are no underlying problems before covering it over again? Also, the extra weight could be too much for your framing and cause cracking or other structural problems.


Todd Shell

Todd's Home Services

San Antonio, TX

Answered 4 years ago by Todd's Home Services

2
Votes

A couple of items further to Todd's comments -

1) some metal roofing contractors are trying to convince people that you do not need roof wrap under metal roofing - presumably so they can get their initial installed cost more competitive with other roofing materials. Don't fall for that - metal roofs do leak at times, and without the roof wrap you have nothing to keep the water from saturating and rotting the sheathing and rafters, destroying your entire roof.

2) also, if your roofing is flat sheet raised seam (flat panels except with bulges up for the seams) rather than corrugated, the batts or nailing strips (though roofing is not fastened with nails) should be discontinuous - small gaps are needed every few feet to allow hot air to flow up to the ridge vent (which you should definitely have installed), otherwise it just becomes an oven between the roofing and the sheathing, plus the sheathing gets much hotter and makes your entire attic hotter in the summer.

Answered 4 years ago by LCD

1
Vote

I am writing as someone who has been involved in the production of residential metal roofing, including tens of thousands of projects, for over 30 years. My perspective pertains primarily to metal shingle roofs which are more forgiving of going over uneven surfaces than are some vertical seam metal roofs. Please keep in mind that some uneven surfaces can be so bad as to affect the aesthetics and sometimes even the function of the metal roof and, again, this can more often be the case with vertical seam metal roofs than with metal shingles. Each roof should be evaluated on its own.


The vast majority of installations I have worked with have been over existing shingles. The low weight of metal roofing is one thing that encourages this. The aluminum shingles weigh about 1/8 what asphalt shingles weigh and steel weighs about 1/4. In many cases, we are probably adding less weight back to the home than what the current shingles have lost due to granule wear and oil evaporation -- this is important to keep in mind.

Now, one thing to keep in mind ... new asphalt shingles should not be placed on top of old asphalt shingles. With most asphalt shingle manufacturers, doing so will void the warranty on the new roof. This is because the old shingles will not allow the sun's heat to pass so easily into the attic so the new shingles end up staying at a warmer temperature, shortening their life.

That is not the case with metal -- temperature does not damage the metal or the paint finish. The warranty on your new roof will not be impacted at all by going over the old shingles.

In over 30 years and thousands of installations over old shingles, I have yet to go back on a job later and find myself saying "Hmmmm ... maybe we should have taken off the old shingles." It just never becomes an issue. I have re-roofed personal properties with our products five times now over the years -- every time was over the old shingles.

I like going over the old shingles for three reasons:

1) It saves having to fill up landfills with old shingles.

2) It ultimately increases the thermal resistance (R Value) of the roof assembly, actually increasing energy efficiency compared to tearing off the old shingles. What happens is that some of the heat which does pass through the metal shingles now gets stopped by the R Value of the old shingles, rather than flowing direct into the attic. Otherwise, that heat will flow direct to the roof deck and into the attic.

3) It allows the property owner to spend discretionary dollars on a better roof rather than on removing the old roof and disposing of it.

Going over the old shingles also avoids the potential issues that can occur, like unexpected rainstorms, when old roofs are torn off.

Still, ultimately, it is the property owner's decision but I would have no qualms about going over a layer of old shingles. It's not uncommon to go over multiple layers either actually. On older homes, of course, I do put the caveat out there that if there are signs of an existing weight issue or old leaks, those need to be addressed and that may require removing the old shingles but that is unusual. Keep in mind, of course, that building codes require no more than two layers of roofing of any type.

The question of how this impacts energy efficiency was raised. In a typical year, approximately 90% of our roofs are installed over old shingles. So, virtually all of the energy testimonials we have received over the years were for installations over old shingles.

As far as substantiation of the energy benefits if the old shingles are not removed -- Unfortunately, that specific scenario has never been tested. I will add a couple of things though. The energy savings documented here by this homeowner were on a roof installation over his old shingles: http://www.classicmetalroofingsystems.com/about-metal-roofing/energy-savings/

Additionally, a few years ago, we were having product tested at a major test facility and, before the test, they wanted to apply our product directly over the old asphalt shingles that had been tested previously. They assured me it would not make any difference. My feeling then, as it is now, is that leaving the old shingles in place is actually helpful (as I have described above) and I did make them remove the old shingles because I did not want our competitors claiming we’d cheated the test by leaving them in place.

Source: www.asktoddmiller.com

Answered 4 years ago by AskToddMiller

0
Votes

Cutting corners! Lots of roofs done that way over the years I have repaired many of them . taking off the steel and shingles installing ice and water and 30# felt and reinstalling the steel.. Do it right the first time.

Source: Ghent construction

Answered 1 year ago by Ghentconstruction

-1
Votes

I agree with Todd. I've done a few metal roofs this way and never got a call back. Why waste energy, time, and money? Nothing is going to get through that metal roof once it's on. One caveat: make sure you're using screws that will get through the shingles and the sheathing. (even better if you can hit the rafters) Two inch roofing screws, no shorter. And don't roof over two layers of shingles, as stated above.

Source: Aaron Combs Construction

Answered 1 year ago by AaronGB98

-1
Votes

Taking advice from a roofing CON-tractor is like being in a henhouse and asking a fox for advice. Use a little common sense in any project and you will be fine. The more you research a project beforehand the better off you will be in making the right choice when it comes time to hire someone to do the job. First off the weight of metal panels is not much so this would not be a concern. A metal roof should not leak at anytime and the only problem might be condensation, which is no big deal. If you’re worried about heat buildup under the metal roof then make sure your metal roof is light in color to reflect the sunlight. The lighter the better with white being the best. If you are worried about the condition of your roof under the shingles then go look at the attic side of the roof. If the wood is in good shape then why remove the shingles to see it? Before doing business with anyone make sure you check his references, background and be careful that he is not trying to charge you for more work than is necessary. So to answer this question I’ll use another question. Why remove shingles when installing a metal roof when there is no reason to do so?

Answered 1 year ago by useyourbrain101

-1
Votes

There is absolutely no reason to remove a structurally sound asphalt roof to install a metal roof other than to add to cost. Tens of thousands of metal roofs have been done this way with no problems. Any contractor telling you otherwise without due cause with your particular roof should be avoided. They just want to inflate your bill. Find another contractor!

Source: roofing professional

Answered 1 year ago by Guest_91583822

-1
Votes

Several of the contractor responses say why not leave perfectly good shingles under the metal roofing rather than remove them.


I would argue that putting sheet metal roofing over existing shingles if going to always look just at leat a little bit junky, because the surface they are starting with is uneven and somewhat compressible. Metal shingles I guess would not look any worse than a second layer of asphalt shingles, though because the asphalt ones are coarser and less exact in appearance any variations in lip location would be more evident with the metal shingles going over the variations - plus asphalt shingles, as they heat, lie down on the shingles under them whereas any high corners or tip curl on the underlying asphalt shingles could reflect through to the metal shingles as uneveness - I just can't see this being a workmanship like job. Granted if batten strips are put on the shingles preparatory to putting on the metal shingles (which some require if they have a lip-and-hook connection system) thjis might be less pronounced, but still sloppy in my opinion.


But the issue of putting metal roofing over "perfectly good" asphalt shingles - if they were perfectly good you would most likely not be putting new roofing over them, would you ?


There was also a comment that metal roofing will not ever leak - first, that certainly does not apply to metal shingles, which can get water blown in under just like any other shingle. Secondly, if they never leak, then why have I seen and been involved in so many tearoffs of sheet (interlock or raised seam or interlocking seam or whatever) metal roofs that leaked ? Granted, most of these were in snow country which tests the watertightness more than normal rain, but they DO leak at times and with age - at seams, around fasteners, at joints if the roof slope is longer than the material comes, etc. Sometimes they have significant leaking - like some brands of self-sealing sheet metal fasteners where it turned out the gasket on the screw broke down in a few years of sunlight or heat exposure. The "red-ring" silicone gasket ones (don't remember the manufacturer name) ones were a prime example - roofing screws with a silicone gasket that split and broke apart during the installation and leaked like a sieve.


And putting the new roofing over old shingles and roof underlayment means your secondary leak protection may be minimal or zero. Counting on a presumably normally 20+ year old water barrier AT TIME OF REROOFING (underlayment to some - tarpapear or maybe synthetic water barrier) to provide secondary protection for ANOTHER 50 years or so (presumed new metal roof life) is nothing but hopeful wishing - and since you are tearing down to it, you have no way of knowing what it's condition is !


Also - one comment said look at the the underside of the sheathing to see if it is in bad shape - granted if water has been leaking through for a long time, especially in frequent rain areas so it stays wet between leak events it will show on the underside, but I have seen some pretty bad sheathing on the top that looked decent and just somewhat water-stained here and there on the bottom. Not deteriorated to compost status, but delaminating, bulging laminate layers, rot, fungal growth, rusted off nail heads, etc.


Also - you should generally have ice and water shield along the eaves even with sheet metal roofing unless there is a lot of overhang - and the existing roof might not have this, or it might be deteriorated - and if applying over existing roofing you will not be able to put this the right place in the roofing materials layers.


One other factor - some of the cheaper metal roofing (which some homeowners buy based on lower cost) is not highly or even at all rust-resistant on the bottom, so put it down on existing possibly damp roofing, or on old composite roofing that might hold moisture from condensation, invites rust-through from the bottom.


For the 5-15% difference in cost, why not tear the old roofing off and do it right and get a really uniform, prefessional looking job without any concerns about having invalidated your warranty, having possible warranty arguments down the road about whether the metal roofing or the old roofing it was put on is the cause of the problem, worrying about appearance issues such as dips and waves, unknown condition of sheathing and water barrier under the old roofing, etc. If you are going to spend typcially about 2-5 times the money of a shingle reroof to get hoepfully 50+ years of service, why not do it right top to bottom ?

Answered 1 year ago by LCD




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