Putting a metal roof over asphalt shingles

[Kcwoodbutcher]

I'm about to put a metal roof over our existing shingle roof and have been told it is wise to lay 2x4s between them. That's a lot of 2x4s as the roof will take about 150 square of metal. I'm assuming you would do this to prevent moisture accumulation between the metal and shingles.  First, is it necessary and second, if so can it be thinner than a 2x4.

[LeeB]

I used1x4's.

[paul case]

i put them on right over the shingles. i have helped do serveral this way that are 10 years or older. no problems. some worry about moisture and the shingles wearing the metal from underneath but it hasnt been an issue. i would be more worried about the space between the shingles and metal being a home for rodents spiders and wasps. not to mention that hail can ruin a metal roof with the spacer boards in there.
it is imperative that you have good decking under the shingles and use screws that are long enough to reach it. pc

[Bro. Noble]

My folks had a metal roof put on their house recently.  The contractor said the old shingles needed to be removed.  I'm not sure what the reason was, but I know that there were at least two old layers.

[jab73180]

I would use the cheapest boards you can find.  Its true about the shingles wearing through the metal.  If the metal is going to expand and contract a lot it will rub through quicker. Some days I can hear metal moving around quite a bit.

A trick to getting straight screw heads is to pre drill through a stack of roofing but measure carefully.  

[LeeB]

Don't use treated wood. It will eat the metal up.

[beenthere]

For a longer lasting metal roof, the ventilation would be helpful. But who wants a roof that lasts a long time?
I like a roof to last 30+ years myself.

[SamB]

In my neck of the woods most people that put metal roofs over asphalt shingles use ¼” Styrofoam, commonly called fan fold, between the metal roofing and the shingles. The reasoning is that over time the sand on the shingles will cause the metal to wear out from the bottom side due to expansion and contraction. The Styrofoam allows the metal to move without the sandpaper effect, if you need additional insulation use thicker foam.

[Bibbyman]

 



We had the same question and got all kinds of answers.  We went ahead and put sheet metal right over the shingles using longer screws.    It looks great after 4 years.   

We roofed another couple more sections of roof this spring - again, right over the shingles.  But we have one more major section of roof to do that has a long span.  We will probably remove the shingles just to save weight.

They do make as special and pricy rubber sheet to put down under the metal.  But we opted not to use it.

One tip I've learned to make the roof look better is to layout and pre-drill the screw hole before putting it on the roof.  Leave them in a stack and that way the screw heads will all line up.

[D Hagens]

(Image hidden from quote, click to view.)

We had the same question and got all kinds of answers.  We went ahead and put sheet metal right over the shingles using longer screws.    It looks great after 4 years.   

We roofed another couple more sections of roof this spring - again, right over the shingles.  But we have one more major section of roof to do that has a long span.  We will probably remove the shingles just to save weight.

They do make as special and pricy rubber sheet to put down under the metal.  But we opted not to use it.

One tip I've learned to make the roof look better is to layout and pre-drill the screw hole before putting it on the roof.  Leave them in a stack and that way the screw heads will all line up.

 Going to agree with Bibby on this, I've done lots of metal over shingles. You don't need strapping as long as you the right mini-lags. As for the metal rubbing againts the shingles it's a myth if installed right.

[Banjo picker]

Glad to see this topic, as I am goning to be in this boat pretty soon.   So lets hear some more pros and cons...Tim

[cutterboy]

A builder friend of mine told me to lay down felt paper over the shingles so they can't make contact with the metal roofing, so that is what I did. However, after reading the responses to this thread and sitting here thinking about it, I don't think that was nessessary.
  How much and how fast does the metal roofing move? I think it would take a very long time for the shingles to wear through the metal.

[Kcwoodbutcher]

Thanks for all the input. I think I'll just lay it over the shingles. It doesn't make much sense that the shingles could wear through the metal, at least not in my lifetime.

[Banjo picker]

KC your are two years older than me ...so I need more input...   Tim

[SamB]

I suppose it could be a regional issue, but the supplier where I buy most of the metal I use voids the warranty if it’s placed directly over asphalt shingles. It also voids the warranty if  you cut the metal with a saw. Five to ten years is only 10-20% of what is advertised to be a fifty year product. I guess time will tell who’s right, but I will continue to use the foam not only over shingles, but under any metal roofing it also prevents condensation problems.

[Larry]

I used the Dow fan fold stuff on a shed.  Great for new construction but I wouldn't use it on old.  Put tin on the new house and pre-drilled all the holes on the ground.  Made it lot easier putting the tin down.

Give these guys a call for a price.

http://www.gravettemetalsales.com/  They sell Central States tin.  Good prices and good metal.

[WDH]

I am also getting a metal roof installed.  The roofer is planning to put down 1x4 nailers over the singles.  He said the air space helped cool the roof, and in Georgia, that is a big deal.

[D Hagens]

Glad to see this topic, as I am goning to be in this boat pretty soon.   So lets hear some more pros and cons...Tim

The pros that the metal roof will last a very, very long time The cons......if you don't like hearing the rain then don't install it
 

I am also getting a metal roof installed.  The roofer is planning to put down 1x4 nailers over the singles.  He said the air space helped cool the roof, and in Georgia, that is a big deal.

Your roofer is right, it will keep things cooler with strapping, also keep in mind that the colour of the tin helps lots

I suppose it could be a regional issue, but the supplier where I buy most of the metal I use voids the warranty if it’s placed directly over asphalt shingles. It also voids the warranty if  you cut the metal with a saw. Five to ten years is only 10-20% of what is advertised to be a fifty year product. I guess time will tell who’s right, but I will continue to use the foam not only over shingles, but under any metal roofing it also prevents condensation problems.

So if you can't cut the tin what do they expect you to cut it with Take a skill]saw, turn the blade around and go for it The noise will kill you but a perfect cut every time
 

A builder friend of mine told me to lay down felt paper over the shingles so they can't make contact with the metal roofing, so that is what I did. However, after reading the responses to this thread and sitting here thinking about it, I don't think that was nessessary.
  How much and how fast does the metal roofing move? I think it would take a very long time for the shingles to wear through the metal.

The movement of the tin on the shingles is minimal, like if you think about it how can a tar based material with a few rocks go through metal over the years.
 

[pineywoods]

I've done it both ways. Be aware of weight, it can be a problem if you apply over the shingles. Metal roofing comes in 2 thicknesses. The thinner gauge needs a clean solid support, else walking on it will bend it. On my house, I removed the 2 layers of shingles, cleaned up the decking, and laid down 1 layer of tar paper, then the thin gauge metal. didn't pre-drill, used a battery powered drill driving hex head, self-tapping screws. I used long screws through the ridges, not down on the flat. If you do that, be careful not to over tighten the screws. That flattens out the ridge and messes up the sheet spacing.

[SamB]

Painted steel siding/roofing should be cut with snips or shears per manufactures instructions. Sawing with any type power saw heats the metal too much and causes premature oxidation. The same premature oxidation can occur when said material is placed directly over sand coated shingles and the underside of the material is compromised. Whether from expansion and contraction or simply being walked on. That being said, I guess it’s up to the one paying the bill if they want to follow the manufacture’s instructions and have a  warranted product or not.