Metal roof over Avantech question

[Arky217]

My roof is going to be 1/2" Avantech zip roofing panels over 24" spaced trusses.
I was planing to cover the panels with 30# felt and then put metal roofing over the felt.

But I begin to wonder if the 1/2" Avantech panels would be thick enough to properly hold the metal roofing screws. I called the supplier where I plan on purchasing the roofing and they said probably not. They recommended over the felt, that I first nail at least 1" thick laths to the trusses at a minimum spacing of 4'.
I then wondered if any possible condensation under the metal would be greater with the air space between the felt and metal. The supplier said the air space would actually lessen any possible condensation.
If I used laths, I also wondered if the laths should have any cutouts at the bottom where they rest on the felt so that any possible condensation accumulation would not puddle.

Will the 1/2" panels really not be thick enough to attach to ?
And if I use laths, is condensation really nothing to be concerned about ?
What say ye roofers ?

Thanks,
Arky

[DirtyLittleSecret]

Arky,
Did this last season but used a 5/8" I think (they certainly werent 1/2" like the wall panels).  I actually consulted an engineer at Huber who told me no need for tarpaper as the Zips have a self sealing skin that meets or exceeds standards.  I did however use the roof vent foam top and bottom to allow for ventilation and reduce any potential for ice bridging/pooling.  Will have to see whether the 50 year warranty ever needs to come into play.

[Arky217]

Arky,
Did this last season but used a 5/8" I think (they certainly werent 1/2" like the wall panels).  I actually consulted an engineer at Huber who told me no need for tarpaper as the Zips have a self sealing skin that meets or exceeds standards.  I did however use the roof vent foam top and bottom to allow for ventilation and reduce any potential for ice bridging/pooling.  Will have to see whether the 50 year warranty ever needs to come into play.

Maybe they're making the Zip roofing panels 1/2" now ? I'm also using their 7/16" Zip wall sheathing panels for the walls, but they're closer to 1/2" than 7/16". The Zip roof panels that I got are definitely right at 1/2".

On the foam strips that go at the end of the panels, you say you're using them to 'allow' for ventilation. Seems like with the foam strips, the air under the ribs would be 'dead'.
I've wondered about that, whether or not to use the strips and have 'dead' air spaces under the ribs, or not use the strips and allow the heated air under the ribs to rise and flow out the continuous ridge vent that I'll be using.

About the 30# felt, I was told the same thing by Avantech. I'm tempted put the metal directly over the Avantech as well, but many say to use the felt first. Could be, that many that promote using the felt are thinking plywood/OSB and not Avantech. Right now I'm thinking felt, but may change my mind, considering that Avantech says the roof should be weather tight for at least 3 months after taping the seams with their 4" rubber tape.

What I'm really pondering on right now, however, is the screw placement on the metal panels. I think I'll start a new thread to get some general input on screwing down metal roofing panels as there seems to be two camps of thought on how to do it.

Arky

[rmcewen]

I just bought some Zip roof panels and mine are definitely 5/8" (the red ones).

I plan on using 2x4 purloins between the zip panels and the metal roof, mainly so I'll have something to stand on while attaching the roof.  Not planning on using felt, that's why I bought the Zip sheathing in the first place.

[MushCreek]

The best way I know of is to run double furring strips. After sheathing and tar paper, attach strips parallel to and on top of the rafters. Then, a second set of strips runs horizontally (purlins). The metal is attached to the purlins. This way, if any condensation drips, it runs right down the roof and out- there's nothing to create a dam. If you go with an un-vented attic, the extra air space helps keep the roof deck a bit cooler, too. The purlins make it a lot easier to scamper around the roof until the tin is installed. I plan to do mine this way, but I'll probably put 2" foam over the sheathing, then run my various furring strips.

[Don_P]

I sure prefer a solid deck to purlins, it looks better over the long haul. I believe most manufacturers call for 5/8" minimum for adequate screw withdrawal strength. I prefer paper under anything. I've done the zip system and prefer synthetic tarpaper over regular osb or ply if you need a longer lasting cover.

[Arky217]

I just bought some Zip roof panels and mine are definitely 5/8" (the red ones).

I plan on using 2x4 purloins between the zip panels and the metal roof, mainly so I'll have something to stand on while attaching the roof.  Not planning on using felt, that's why I bought the Zip sheathing in the first place.

Now that's interesting on the thickness. It prompted me to go out and measure mine.
My Zip roof panels are red also, rated for 32" span. The Zip technical dept. told me before I ordered them that they were 1/2" thickness. Well, the actual thickness is 17/32", which is a lot closer to 1/2" than it is to 5/8".
I measured the green Zip wall panels as well. Was told they were 7/16"; they actually measure 15/32". Both of these measurements are a little surprising since usually sheathing is slightly thinner than it's nominal thickness, not slightly thicker.

As far as being thick enough to hold the screws, I got to thinking about that and remembered that I've seen shingles nailed on sheathing as thin as 3/8". Well, if nails will hold in 1/2" or even 3/8", surely screws would hold even better in 1/2".

Here's my revised installation plan so far:
Put the metal directly over the Zip panels. (the sheathing, after taped, is supposed to be weather tight for at least 3 months with no roofing on it, that would seem to indicate that the felt would be unnecessary)
Screw down the roof around the perimeter on the flats at 12" spacing.
Screw down the rest of the roof (the total portion actually over the attic) only on the outer ribs. It other words, only where the ribs overlap. Double thickness ribs will keep the screw from squashing them when tighten down. Also, if a screw becomes loose and/or the rubber gasket cracks/deteriorates, water penetration should be at a very minimum as opposed to the screw being in a flat.
I figure with a tight, close fastening on the perimeter, the screws in the double ribs should be entirely sufficient to hold the metal down.
Anyway, that's the plan at the moment, can't say that it won't change based on more info.

[DirtyLittleSecret]

Sounds like a good plan.  BTW: the foam I was describing is the open cell foam used for ridge vents (allows for plenty of ventilation).  Come to think of it, my red roof panels were definitely 5/8".  Lovin' Huber.
Make sure you post us up some photos!