New metal roofing over asphalt shingles. Good or bad ?

[Gitche Gumee Kid]

SIL wants to roof his hunting shack with metal roofing over existing shingles. Has had two different schools of thought impressed on him. #1 must tear off old shingles or #2 apply metal directly over existing shingles.

What say the knowledgeable members here? TIA

GGK

 

[RedNeckRacin]

Personally I'd want the old roof torn off. I also wouldn't want all the waves from the shingles showing through. I don't have any proof but I think the metal might rub enought on the shingles to start rusting from the back side.

 

[jfh28]

take the old shingles off........the heat buildup under a metal roof is much higher then what the asphalt shingle is designed to handle and they will degrade....that's the reason roofing manufacturers have developed a specific roofing underlayment designed for the heat of metal roof panels.......standard underlayment breaks down from the heat when metal roofing is installed on top of it......old asphalt shingles will do the same thing........I know it's a pain to remove the shingles but I seriously doubt the metal roof manufacturer will honor the warranty if they don't........Jack

 

[John Fitzgerald]

It's SOP around here to put new metal roofing over shingles, as long as shingles are not curled or moldy. I have had two houses done that way in last 25 years. No problems so far. All the houses in my neighborhood that have replacement metal roofs were done over shingles as far as I know.

 

[the old grind]

IMO, the less slope, the more he needs to remove the asphalt shingles. (uh, snow load, too?) I'd also say, the larger the roof the more he needs to remove the asphalt shingles, and that's because to R & R them later when the deck goes to s__ the metal panels wouldn't get mauled in the process. Compare the risk of failure to value of labor saved. Is it even close in the long run? If the shed won't last another ten it's not the same as 'for my grandkids .. and theirs'. (Should it be just black & white to decide or advise?)

btw, I suggest it hardly matters what you skin over .. the top of your shed, your shack, your trailer floor, or your deck, .. if you leave unneeded stuff under the waterproof topmost layer that can absorb/hold dampness or condensation you are at best 'water repellent'. Human breath, minimal ventilation, dirt floor, there are lots of moisture sources/causes.

I'd tell honorable SIL not to allow moisture more place overhead to collect, to be sure of good vapor barrier, and to provide 'adjustable' ventilation. :2cents: t o g

 

[Deere Dude]

Around here metal right over shingles and seems to hold up well. The only thing I would add is to nail down purlins to the roof trusses or joists, right to left, every two feet so tin can be screwed to solid wood and not just OSB or plywood.

 

[EddieWalker]

I spoke to the rep at Mueller's last year and asked the same question. He said it didn't matter to them if you left the shingles on or took them off.

I personally like to remove shingles because of the weight, but also to make sure the decking is in good condition. If neither of those things matter to you, then go ahead and put the metal on over the shingles.

The advantage to leaving the shingles on is that they can handle any moisture from condensation that might form under the metal.

 

[woody]

Around here metal right over shingles and seems to hold up well. The only thing I would add is to nail down purlins to the roof trusses or joists, right to left, every two feet so tin can be screwed to solid wood and not just OSB or plywood.

this is what I did about fifteen years ago and it is still performing with no problems.

 

[Rustyiron]

The advantage to leaving the shingles on is that they can handle any moisture from condensation that might form under the metal.

This, ^^^^ especially on what I'd guess was an un or poorly insulated "hunting shack".

 

[TemporaryKubotaOwnerMark]

I'm not aware of any structural or longevity problem with leaving the asphalt shingles in place. (And how in the world did hot mop get into this discussion????)

Nobody has mentioned the biggest issue - fire. If you put a metal roof over asphalt, and fire starts in the asphalt, you have kissed that structure goodbye.
Because of the layered nature of asphalt shingles, here is enough airspace in there to keep the fire going, and there's no way to get water to it. Fire departments & insurance companies don't like it for that reason.

On a hunting shack? Maybe doesn't matter, but I thought I'd mention it.