Metal Shingle Roofs - to batten or not to batten

[nrbatnzn]

I am looking at putting metal roofing shingles on my house.  The ones that are about 18 inches wide and 52 inches long.  I currently have a 25 year old shake roof in Arizona.  I have been told about the benefits of three different installation methods by three different contractors. 
  The first is to configure battens on top of the existing shake.  Since the system is supposed to be water tight, I shouldn't concern myself with the deterioration of the underlayment.
   The second suggests removing all shingles and paper, install new underlayment and use the batten configuration to create an airspace for greater insulation value.
   The third suggests removing shingles and paper and the install new synthetic underlayment and then install the new metal roofing shingles directly to the plywood/underlayment.
    I need help!  What do you see out there? 

[nrbatnzn]

What is the BEST way to install 18X52 metal shingles?
  One says to batten on top of my shake roof and apply it on top.
  One says to tear off the old and batten the roof before installing the metal.
  The last says to tear it off and install the metal shingles on top of new synthetic underlayment and plywood.
     What do the experts say??

[MountainDon]

I merged these to here as they were much the same; easier to track the subject when limited to one topic.

[MountainDon]

I would do what the manufacturer of the metal shingles says to do, in order to maintain their warranty.

[MountainDon]

I am in favor of removing the old roofing. Uneven old shingles can make for a wavy, irregular appearance with the new roofing. Some roofers will want to recommend leaving the old roof as they can then quote a cheaper price.

I also do not believe battens or furring strips are necessary or that they even improve the roof, UNLESS the shingle manufacturer stipulates to use them.


Is the existing roof over a non conditioned attic or is the space under the existing roof conditioned or a cathedral ceiling in any portion?

[Alan Gage]

When I was looking into metal shingles the manufacturer instructions I saw all called for installing over sheathing/underlayment. I'd check some manufacturer websites, I'm sure they can give a recommendations for existing roofs as well.

Alan

[nrbatnzn]

Yes, I have cathedral/vaulted ceilings under about half of the roof.  In your opinion, does this make a difference regarding installation?
Regarding warranty, I am told that ANY method is accepted under warranty and I plan on getting this in writing before proceeding.
Finally, I am told by a couple of these contractors that the reason they like the battens, at least in AZ is that when it is 120 degrees in the shade in the summer, the air underneath the shingles allows for greater heat dispersion.  Make sense to you?
Thanks for your input.

[MountainDon]

I asked about the cathedral ceiling as with a new long lived roof being planned it seems to me to be a perfect time to improve the insulation on that section with several inches of added rigid foam. We live in a high desert area of NM and noticed an improvement in utility costs when we added insulation to the attic of our mid 80's home.

Can you get to talk to some people who have had the air spaced metal roofing for a couple of summers? It would be interesting to hear what they say.

Older Land Rovers offered what they called a tropical roof, an extra air spaced metal roof above the normal one. It was supposed to have made the interior cooler. So maybe the roofers are onto something there. It would be nice to have some quantitative evidence though. The LR had air circulation between the layers assisted by vehicle forward motion. I would think the air space on a house would work best with greater pitches?  What thickness furring? The norm here seems to be 1x (3/4"). I wonder if 2x (1 1/2") would be better. In recent years some believe that a 2" space for air circulation in a roof is better than the old rule of specifying 1 inch.

Just thinking out loud and giving opinions.

[firefox]

The military has electronic shelters which are essentially aluminum boxes.
If they are left in the sun, they get somewhat warm to say the least.
They are normally equipped with "shade Tarps" which have grommets in the corners.
There are little posts that stick up in corner brackets to match the grommets in the tarp.
The system works very well, so I suspect that doing the equivalent with the firing strips
is a workable solution.

If you can place some copper tubing under the tile and above the strips you could preheat your water if you are so inclined.

Bruce

[MushCreek]

Up until recently, I knew the answer to this one. Funny how the older I get, the less I know!

I was always told to batten a tin roof. The theory is that 1) Condensation WILL form, and this way it has an easy exit. 2) Eventually all roofs leak somewhere, and this way, the leakage has an easy exit. In either case, the added air circulation speeds drying instead of keeping the water trapped in place. Lately, rain screens and drainage planes are all the rage with siding. The difference is that many siding materials absorb and hold moisture, which of course metal does not.

Some of the cons:

Cost
The air gap may actually increase condensation in the first place.
The roof may be more susceptible to hail and wind damage.
The added thickness to the roof profile may look odd, depending upon the style of house.

Firefox- I LIKE the idea of putting copper tubing under the tin for hot water! I wonder how much you would get compared to having a collector on the outside? I also wonder if there would be a way to increase the gain by thermally coupling the tubing to the tin? Looks like I need to run some experiments! Someone needs to develop a roofing system that incorporates this, so you can have solar hot water in snooty neighborhoods that look down on that sort of thing. I envision copper roofing sheets with tubing soldered to the underside, connected with soldered loops at the ends. Can you imagine the heat output? (Not to mention the COST)

[firefox]

Not sure, but there might be some kind of heat sink type epoxy that you could use to
attach the coils to the tin.
Bruce

[UK4X4]

Landrover tropical roof

The idea behind this is heat comes through multiple avenues- ambient and radiated

a black steel roof in the sun can easily be 60 deg C when measured- although ambient may only be 25oC

The outer skin gets hit by direct radiation
air in between the outer and the inner skin becomes an insulator- as air is a bad conductor of heat

I have used this method multiple times on electrical enclosures where we can't install AC systems due to power restrictions.

Its the same when used as a fire back- it helps to stop radient heat transfer.

The greater the distance the better the effect.

In many tropical countries you'll see tin roof's set 8ft up above the real roof of the building, so basicly the building is in shadow