Expert: Dan Merrill - 6/30/2009

Question
QUESTION: I am looking to replace my three-tab asphalt shingles with a metal roof.  In particular, I'm looking at a product that is a sandwich of 26 ga metal on the bottom, 2 to 4 inches of polyiso, and 24 ga Kynar 500 painted steel on the top.  Would it make sense to tear off my shingles down the existing 7/16" OSB deck (excellent condition) and put this insulated metal roof panel right over the deck?  I live in North Texas, so want hail resistance and hot summer sun resistance.  I'd like to have a good chance of getting at least 30 years out of this roof.

ANSWER: The IRC code does not require a tear off of a single layer of Asphalt shingles if you are installing a metal roof.
But please check with your local code enforcement office to verify. Some building departments are supposed to follow the IRC but pick and choose which codes they will follow and which ones they will not.

If the asphalt is laying flat, there is no reason to remove it. In this case we would remove hip & ridge shingles, cut back all edges, then cover the whole roof with a synthetic underlayment. The underlayment helps prevent any chemical reactions between incompatible materials, this is money well spent.

A steel or Aluminum metal roof panel with a 70% concentration of Kynar or Hylar paint should easily last 50 years or more. The newest versions with EnergyStar rated "Cool Roof" pigments should last even longer.

Good luck,

Dan

---------- FOLLOW-UP ----------

QUESTION: I left out a possibly important detail - sorry.  I'm going for a "hot roof", due to the practical impossibility of adding insulation below the deck (space within scissor trusses inaccessible) and considerable ease of fully insulating this above deck.  The intra-truss "attic" space would then be ventilated with the air conditioned living space, removing all roof penetrating vents to that "attic".

And I didn't state a key part of my question very well either.  These are not ordinary metal panels. These are 3 inch thick sandwiches of metal, 3" polyiso and 24 ga cool roof metal.

So my real questions are two:
1) What do you think of a hot roof in this case, warnings, advice, experience?
2) What do you think of such metal-polyiso-metal sandwiches in this case, warnings, advice, experience?

Thanks for your original answer to what I actually asked, and my apologies for not asking what I really meant to ask.

Answer
We have not used these types of panels yet. But we have done several of the SIPs panel projects.
The problem with the ISO sandwiches is that they do not allow the roofing material to radiate any of the sun's heat inward, so asphalt roofing for example will have a quite short lifespan due to higher than normal surface temperatures.
Instead of the asphalt shingles (so-called "40 year")lasting 6-8 years as they do on average, they tend to last maybe 3-4 years.

A metal roof with "Cool Roof" pigments (Kynar) really excel under these conditions because they do not, and do not need to radiate heat inward through the roof deck.

The main concern with these installations is making absolutely sure your contractors follow the manufacturers installation instructions exactly. Pay extra attention to seal all seams (top & bottom) and edges exactly as directed by the manufacturer.
Many, maybe even most of contractors are unwilling and/or unable to follow manufacturers instructions. We see this ALL the time. Almost every single product or system failure I have seen has been due to product installations where the installers did not follow the simplest of instructions.
It goes without saying that you need to pick a manufacturer that has been around a while and is large enough to stick around for the duration.

If you can not follow the installation hands-on and eyes-on, find a nit-picky person you can trust to follow the project through for you.

When installed correctly, these installations are a near perfect solution to many of today's building issues like energy usage, mold growth, etc.

Good luck,

Dan