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Old 03-04-2011, 07:23 AM   #1
henrymartin OP
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Metal roofing over shingles??

Hey, I'll try the knowledge base here.

This winter has been tough on my roof (asphalt shingles) and I need to reroof the entire house. I was considering stripping the entire roof and doing new 30 year architectural shingles, but then a guy who works for a roofing company offered to do the entire roof in standing seam panels for only a few hundred more. Panels would be bent on-site to accommodate for my roof size from a rolled sheet metal (galvanized with Kylar 500 paint on top). He said the shingles don't have top come off, which would save me at least $500 for a dumpster, but I don't want the metal to come in contact with the shingles, as the granules would cause rusting down the road.

Anyway, I thought of nailing some strapping on top of the shingles to which the standing seam panels would then fasten with clips. Now, I thought of nailing the strapping all around the perimeter, plus run horizontal straps every 16-20", nailed to the rafters. But then I saw this installation in Europe where they ran vertical straps with horizontal straps on top of those, creating a grit to which the metal was then fastened. The idea is to allow for air movement between the metal and the shingles.

I already have soffit and ridge vents, along with proper vent nailed between rafters. My concern is the heat between the metal and shingles, eventually degrading the shingles to a point where they just crumble and fall down, accumulating in one spot and rubbing against the metal.

So, any professional roofers out here that can give me a piece of advice?

Roof pitch is 4:12 and 5:12 which put me away from metal shingles due to ice dams and backing up concerns.

Also, the rolled metal bent on-site, would there be rusting issues at the bends?

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Old 03-04-2011, 07:31 AM   #2
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Not roofer myself but a firefighter so I've torn up a few roofs. I've seen metal over shingles often. It'ss a nightmare when you need to tear it open in a hurry but I don't see any drawbacks to it otherwise. If anything you have a little better protection with both. Nailing through the shingles and adding wood strips seems like you could have more go wrong. The wood will definitely deteriorate before the shingles and the metal roof plus you are adding a potential entry point for water.
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Old 03-04-2011, 10:57 AM   #3
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I've never seen or heard of any problems

I grew up in snow country. After one particularly bad year, the insurance companies decided to pay for material if folks wanted a metal roof. Long story short Many of us put the metal directly on the shingles, and 15 years later, absolutely zero problems. I then moved to an area where they run narrow wood slats across the shingle and then apply the metal, anchoring to the wood slats. The snow loads warp the metal where it is free spanning.

As for rusting issues. Metal roofing material should come with a very good warranty against oxidation and fading.
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Old 03-04-2011, 11:07 AM   #4
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personally if it was me, and I've look at doing jobs like this for people before, I'd put 1" foam over the whole roof to quiet the steel and level the surface. this will cost more. but i think you will be happier with the results.
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Old 03-04-2011, 02:25 PM   #5
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Done it, it works great, you will have no problems. If you are in doubt just have a #30 felt paper installed under the metal, then go directly on top.

And by the way, save $500 for the dumpster? What about the two days of labor saved ??? Should be a lot more than $500 saved.

P.S. I was roofing contractor for 17 years.

Get three estimates!!! Then ask to see the last three closets jobs to your house, drive by them, maybe even knock on the door and ask if they were happy with you, and a quick call to the local building dept. to ask if there has been any problems with the contractor you choose.

Also, ask if the owner will present on the job site??

Spend a small amount of time doing a little research.

In my opinion.... standing seam metal roof is the best you can get!
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Old 03-04-2011, 03:50 PM   #6
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I put metal right over my old shingles a year ago-zero problems. Several contractors said it would be fine. We went around and broke off all the overhanging shingles flush with the fascia boards, cut in ridge vent and screened it, and started laying the metal. I wouldn't recommend stripping with wood-extra time and cost, plus the metal could bend under load. Make sure your decking is good under the shingles; if it's decked with 1x's, drill your holes to avoid the cracks between boards.

The old shingles also deaden the noise. Putting metal roofing on was fun; I really enjoyed myself, working outside with a great view, in some damn fine weather last fall.
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Old 03-04-2011, 05:47 PM   #7
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At a minimum you should use a slip sheet of 30# felt. You need that for standing seam over any substrate. The shingles could possibly cause some oil canning in the metal depending upon the gauge of the metal and the condition of the existing roof. You shouldn't have any problems, but then again, how much are you going to spend on the roof, and do you need to cut corners that badly.

I'm a homebuilder and have seen all kinds of shortcuts. Usually they work out, but occasionally they bite you in the ass. I usually pay between $10-$15 bucks a square to strip a low pitch roof, plus the haulaway. A lot of roofers will have a dumptruck, or a buddy with one, a little cheaper than $500.

The older I get the more inclined to skip the shortcuts and just do it right the first time, comes from some lessons learned... Just my humble opinion

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Old 03-04-2011, 07:21 PM   #8
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Thanks to all for the responses.

This is the scoop. The roofer is doing this on the side for me, and he did another roof for me years ago when I built my addition.

I buy the materials, he charges me to install, I help. Tearing off would be my job, so the only cost is the dumpster, which, per local trash company is $600.

If I do shingles (I like Tamko Heritage 30), I'll do a complete teardown, new drip edge, Ice&water, titanium underlayment, Ice&Water in valleys, new rolled ridge vent, Tamko's starter shingles and ridge caps. Plus, I'll bend some 4x8 sheets of aluminum for the two valleys I have, which always end up with icedams. (currently weaved shingles).

If I do metal, it is glavanized, painted Kylar, BUT, bent on side. My concern is the bending process which is likely to produce microscopic cracking in the paint, which would then eventually lead to corrosion at the bends. Also, unlike shingles, any roof vents are just caulked and screwed, vs in shingles those are actually tucked underneath and caulked. I have four vents and a stainless chimney.

Since I'm getting a really good deal, the metal would run me only about $1k more vs the shingles. If strip to bare decking (plywood) and put on new ice&water, plus titanium underlayment, it'll add about a thousand more for shingle removal and materials.

My SO likes shingles. I like the look as well, but hate clearing the snow. yet, with a 4:12 pitch I may still get snow even on the metal.

A guy offered me Decra metal shingles cheaper than asphalt, but they are more likely to leak on my pitch, plus they are mineral coated, so snow will stick to those as well.

As a side note, I may be selling my house once the market changes, so there is the potential of that. Realtor is telling me to go with Arch shingles due to looks.

Chimney sweep has to climb up once per year to clean chimney, so roof has to remain walkable. Heck, if it didn't, I would just PVC and weld the sucker and be done with it forever.

henrymartin screwed with this post 03-04-2011 at 07:39 PM
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Old 03-04-2011, 07:22 PM   #9
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I'd get the install instructions from the roofing material manufacturer. Should be easy to find online. See if there are any warranty issues with covering an old roof. There also might be some other things in those instructions that you need to consider. Nothing worse than having a problem a couple of years from now and being told the warranty claim is denied because the product was not installed correctly. Happens all the time with asphalt shingle roofs where enough attic ventilation isn't installed.
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Old 03-04-2011, 07:37 PM   #10
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Put the metal over the shingles. Sleep well. Manufacturer will say it is fine and done all the time. We had metal put over asphalt shingles and researched the pooh out of it.
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Old 03-04-2011, 08:20 PM   #11
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I used firring strips (1x3") over my rubber roof as it is supposed to keep the ice formation down but I probably should have just put it right down on the rubber.
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Old 03-05-2011, 06:08 AM   #12
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4:12 and 5:12 ice guards are a must. nothing like 10 tons of snow unloading all at once.
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Old 03-05-2011, 06:14 AM   #13
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I have a metal roof installed over shingles and have not have a single problem with it at all. Should be good for the next 50 years!
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Old 03-05-2011, 06:59 AM   #14
henrymartin OP
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Originally Posted by stainlesscycle View Post
4:12 and 5:12 ice guards are a must. nothing like 10 tons of snow unloading all at once.
The roof does not really pitch towards any entrance, except the deck. I have a pergola over the deck, so that should be okay.

I've seen plenty of ice dams this year on metal roofs with 4:12 pitch.
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Old 03-05-2011, 11:32 AM   #15
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Felt between the metal and shingles, and off you go. Standing seam roof metal even painted, is designed for bending. No flaky paint/rust issues there.

My auto shop has a 5:12 pitch with screwed down channel drain (ribbed) roofing, and it sheds the snow. 10 years, no shoveling.

On my barn I'm building at the house, I called the bubs to do my roof with painted metal standing seam.

They did the whole job in two days, left at 2pm the second day.

best $3800 I ever spent.

you should see this thing shed snow!!!!
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