Looking to get a standing seam metal roof put on my house. I've been getting estimates. Today, a salesman told me something I haven't heard before. He said that you can't put a standing seam roof over an existing shingle roof. The shingles will need to come off. If not, you can get something called "oil canning" which is when the metal becomes all warped. Any truth to this, or is the contractor looking for more money?

I've heard a flat substrate is best.
Doug Lee
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^this

In my experience, standing seam is a lot stiffer than regular 29 gauge metal roofing and as such should not "oil can". But I also would not install it over asphalt shingles because the brackets that hold the roofing need to be screwed securely to the underlying substrate. Shingles will cause small differences in bracket height. Don't do it.

What gauge is the metal you are considering ?

sop here is lay down 1x4 2' on centers on the old shingles and lay the standing seam on them. have not heard of any problems with this method. this is using the newer snap togather version of SS that does not use cleats

We sheeted over felt, ice and water shield, or rosin paper. Never shingles.

The metal is 24 gauge. This "oil canning" is the first I've heard of it, and I've gotten 4 estimates now. More are on the way.

I thought that's what the gauge was on SS but it's been a while. Still , I wouldn't put it on over shingles

I agree with all the above. Im no roofer but have done a few roofs. My opinion is you tear those shingles off. If you can get the old underlayment off easily do that to. Put down new underlayment (which should be high temp because of how hot the metal will get). Then metal away

Ive done tons of metal siding and oil canning is the popular word. The metal can shrink and grow considerably. Some of the south faces ive done on houses can be sub 32 and in hours be hot to touch. It seems crazy but a piece of metal will grow, and when its fastened securely it buckles or oil cans. Generally the roofing material is heavier stock and doesnt oil can as bad as small gauge fascia and siding pieces

Oil canning as it's called can happen even on a flat surface like plywood. I think it is just one of those things that are almost impossible to avoid with daily temperature changes. Not to mention seasonal changes.

Remove the shingles and put ice and water shield on the entire roof.
Just how I do them

The asphalt shingles will also be like sand paper under the metal, as the metal expands a contracts. I’d have to remove the shingles for no more cost than it would be

I would also remove the shingles, as others have mentioned. The only oil canning I have seen was from roofs that had the metal formed onsite from a continuous roll.

DEFINITELY TAKE OFF SHINGLES FIRST, DON'T FORGET TO PUT SNOW BARS ON THE STEEL ROOF SO YOU DON'T SMASH ANYTHING WHEN THE SNOW/ICE LET GO

Putting metal roofing directly on shingles is a terrible idea. But, as Ridge Runner mentioned, fastening purlins or lath over shingles then screwing metal panels to that is very common here. I am not aware of any issues doing it that way.

Depending upon slope of roof, using 1x4’s or 2x4’s 2’ on center as mentioned above is an absolutely acceptable and widely used technique. I have personally sold hundreds of this style of application of steel roofs with no issues whatsoever. If you choose to go to bare roof, remember you must meet a couple requirements for acceptable standards. The roof substrate needs to be a minimum of 5 ply 1/2” CDX, or 5/8” OSB to meet holding standards. On top of the chosen substrate you must use a high temperature underlayment. Or strip the roof with # 3 1x4’s and go to town. Many also claim that the gap over the shingles helps to cool the attic in the summer heat…..

Most around here add insulation between the shingles if nothing more than blue board.

Just asked a friend that installs a lot of standing seam and he said strip the roof. If you are completely against stripping it you could put 1X4s on the roof over shingles, but he would not recommend doing this or warranty the work if asked to do so.

I've seen a lot more steel roofs leaking after 5-10 years than asphalt shingled roofs, especially in severe climates. I just don't see the appeal for a steel roof. I've installed many of both, but prefer shingles if I'm living under it. Not so concerned about my lawn mower.

Maybe just run another comp roof over it and call it good , you'll be old or dead before it matters again