New metal roofing over asphalt shingles. Good or bad ?

[KYErik]

Perfect timing - I was just putting a metal roof over shingles this afternoon (a pump house roof).

I have seen "double bubble" foil/bubblewrap laid over shingles, then the metal roofing directly on top of that (no purlins). I prefer at minimum using horizontal 1x4 southern yellow pine purlins (oak 1x4's are preferable). I was using 1 x 4 oak purlins this afternoon.

As for the location of the screws- I am am a huge fan of "through the rib". I have done two pole barns (one was done 15 years ago) and two houses this way with no leaks. Contractors dislike doing it this way because it is harder. You have to be very careful when tightening the screws through the rib- too tight and you will flatten the ribs and make the sheet wider/throwing it out of square. Too loose and the washer doesn't seal tightly.

The largest building is a four year old 72 x 38 pole barn- no leaks during a rainstorm. If there was a loose washer on a screw through the rib, the tiny amount of water that splashed around the screw would land on the purlin in an open place (directly under the rib) so it would have some air circulation and could dry out. I have missed the purlin a time or two (or three or four) on the large building and if I forget to immediately caulk the hole through the rib, it is hard to tell that any water is leaking through that hole in during a rain storm (I can use a ladder and caulk it from the underside later on). More commonly, I will notice it by seeing daylight through the hole while standing in the building.

The flats between the ribs serve as water channels. All the water from further up the roof will be flowing past and there is much more chance of water running in. Any leaking will get the wood directly sandwiched under the metal (no air circulation), and it will take longer to dry and be more likely to rot.

Those are my 2 cents

 

[John Fitzgerald]

The vast majority of us have to take what the contractor does, because we would either botch the job or get killed from a fall. I know of no contractors around here that screw into the rib. If you forced it, they either would not take the job, or would not honor any leak warranty, and would charge more on top of no warranty. I would roof a low shed with up to a 4/12 pitch, but I am not getting up on metal roofing on a 6/12 pitch or more.

 

[TomSeller]

I had a standing seam metal roof installed last Spring.

I talked to ~15 roofing contractors and got bids from 14 of those. Everyone one of the contractors required pulling off the old asphalt singles for them to warranty the installation of the roof.

I feel the same. To the underwear analogy, if you put clean underwear over dirty underwear, you still have dirty underwear on. Why wear dirty underwear?

 

[s219]

I think the gauge of the roofing matters a lot. I have worked with thinner metal that would likely not support screws through the ribs without causing other problems. But I used a heavier gauge PBR (purlin bearing rib) metal on my boat house roof. It's made to walk on and the ribs are a lot closer and a lot stiffer. I would have no concerns screwing through those ribs (although I still put all screws in the flat valleys).

 

[drizler]

I personally think conventional 26ga metal roofing installed with washer head screws or nails is grosley over rated. Virtually 100% of all the fasteners are exposed to the weather and over time the sun will deteriorate the gaskets and leaks will happen.
If I were to have a metal roof installed it would be a standing Seam with concealed fasteners.
Reality is however, standing seam roofs very expensive but, last generations.
As far as installing any new roof over an old, I would compare that to the groom wearing dirty under ware to to the wedding.
B. John

I just tore the roof off my old garage I built in 1990. I'm going to put it back up in another location in a year or so. When I yanked the screws the washers were all just like new. The only deterioration was rust coating on the screw itself , just a light brown coating. When I put it back up I'm going to use the new screws but it's not like I really have to. There are structurally fine.

 

[woody]

The vast majority of us have to take what the contractor does, because we would either botch the job or get killed from a fall. I know of no contractors around here that screw into the rib. If you forced it, they either would not take the job, or would not honor any leak warranty, and would charge more on top of no warranty. I would roof a low shed with up to a 4/12 pitch, but I am not getting up on metal roofing on a 6/12 pitch or more.

when we had ours done I asked the contractor to put the screws through the ribs and he said that's not the way they do it after explaining to him why I wanted it that way and telling him I would help he agreed and did not raise the price. he measured and laid the metal across my trailer to pre drill five sheets at a time then I handed them up while he drilled more. has done good for over ten years now.

 

[cdaigle430]

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

20 years with metal over shingles no problems at all. Had 4 feet od sow and ice on it for several of them years.

 

[mykleh]

When I did my house 15 years ago I put tar paper over the shingles then the tin over that.. works great.. biggest thing as mentioned before can only be one layer and they have to still lay flat no curling or mold, moss what have you..

 

[civilian]

When I reroofed an area of my house with metal, I planned on removing the shingles. When I bought the metal, I told the lumber yard(where metal roofing was coming from) that I planned on putting down ice and water guard under the metal. They sold me the ice and water without granules on it. Manufacturer would not allow metal direct over shingles or granular ice and water shield. Jon