Metal roofing systems.
Roger Wallace
> The Meads wrote:
> Dear Mr. Wallace,
> Thank you for answering my request for information on
> painted metal roofing systems. I am looking forward to
> the information you are sending me.
===>It's on the way as you read this - Thanks for your
===>interest in metal roofing systems for residential use!
> I am trying to learn about this technology for residential
> roofing applications. You are correct that there is a lot
> of different products out there, but also a severe lack of
> information on durability and application methods. The
> two main questions I have are:
>
> 1) What gauge metal is necessary for residential roofing.
> There is everything out there from unknown guauge, 30 ga.,
> 29 ga, 28 ga, 26 ga.
===>How true! The material gage (thickness) depends on
===>what *type* metal roof you put down, for example:
===>
===>"barn tin", usually bare G30 galvanized: 32, 30 & 29 Gage,
===>very thin, like heavy foil, almost.
===>
===>"screw-down" metal building ribbed type panels, most
===>are painted with a polyester finish (poor fade resistance)
===>and are run in 29, 26 and sometimes even 24 Gage.
===>
===>concealed fastener "standing seam" type, usually run
===>with a Kynar-500 paint finish (excellent fade resistance)
===>are typically run in 24 or 22 Gage; rarely 26 Gage.
>What do you recommend?
===>If it where my own house, definitely a concealed fastener
===>standing seam type. "barn tin" (nailed down) and "metal
===>building" (screw down) type roofs require as much as
===>FIFTY-SIX (56) screws/nails per square = 0.56/SF.
===>Thats a lot of holes to be drilling into an expensive roof.
===>
===>Many manufacturers will harp on how their screws have
===>"gasketed" (neoprene washered) heads to prevent leaks;
===>however, think on this: ALL materials, whether it be wood,
===>rock, metal or glass, thermally expands and contracts in
===>relation to the outside (ambient) air temperature.
===>
===>Metal (specifically steel) has a high expansion coefficient
===>(elongation of the material). Basically, a 30'-0" long by
===>24 gage thick piece of steel can "grow" (elongate) by as
===>much as 3/8" in length on a normal day (50 to 75-degree
===>air temp), which doesn't sound like a big deal, but every
===>screw, bolt, nut, nail, *will feel* that stress, every day.
===>
===>Thats a LOT of movement and stress being applied to
===>each and every nail or screw in the roof! I have sold these
===>types of systems for other manufacturers, and it is my own
===>experience that the nails/screws will last about two (2)
===>years before they start "wallowing out" and leaking.
> 2) Application method to existing roofs. Some dealers
> recommend putting it on over the old shingles, some say
> to strip the roof first with 1x3, others say put it right
> down on the shingles. Some recommend stripping the
> old shingles off and applying it to the roof sheeting. Some
> say strip the roof sheeting first. Confusing !!!
===>Again, how true. A lot of installers, (even manufacturers)
===>are willing to tell you what you want to hear just to sell
===>you their system. Here's the truth:
===>
===>"barn tin" and screw-down "metal building" type ribbed
===>panels can be installed right over the existing substrate,
===>provided it is (a) strong enough to hold the additional .
===>roof dead load (about 2.5#/SF, typically) and (b) if the
===>existing substrate is still structurally sound enough and
===>smooth enough to apply a new roof on. Shakes and
===>shingles are pretty forgiving; they flex and bend over an
===>area of 12" x 24". Metal roofing has to flex along the full
===>length of the panel.
===>
===>Most (good) contractors will felt over the shingles/shakes,
===>add 1x4 lathe strips at 3'-0" on center, then nail or screw
===>their product down right over it. I think the felting is a good
===>idea because it takes care of any condensation dripping off
===>the back side of the panels.
===>
===>Standing seam systems are more particular. Most systems
===>lay down directly on top of the existing substrate and will
===>conform to that existing substrate. If the existing roof is
===>warped, bowed, etc, then the panel will behave accordingly.
===>
===>A few roof systems (like AEP-SPAN's "Snapseam" and
===>"SpanLok" type panels) are what I'd define as a "Structural
===>AND Architectural" standing seam panel. Those systema
===>are engineered by design to structurally "span" across open
===>spaces (like nailers or metal building purlins) as much as
===>4'-0" on center.
===>
===>Even so, my recomendation would be to remove the
===>existing shingle/slate/shake/tile roof and begin again with a
===>good substrate; at least a 5/8" CDX plywood base and
===>cover it with 30# felt paper before installing any kind of
===>roof. This way, you're always positive of the structural
===>integrity of the substrate you're attaching to; and lets face
===>it - that substrate is what's holding it all down!
> Any help will be appreciated.
> Charlie Mead
> Wood Products Co.
> 136 Emerald Cove Lane
> Rockwood, TN 37854
> 423-354-1993
===>I hope this lesson in "sheet metal" helped you
===>out. If you have any further questions, please feel
===>free to E-mail me at: pers...@airmail.net
===>Good Luck,
===>Roger Wallace
===>District Sales Manager
===>AEP-SPAN
SAUNDRSMNH
One thing that was not pointed out is the fact that many light-gauge steel roof
systems do not have diaphragm ratings. If you live in an earthquake zone, the
diaphragm is an impostant component of the lateral force resisting system.
Some of the steel roofs do have approvals for diaphragm capacity - be sure to
get the right system, accounting for all loading conditions.
-Rob
Roger Wallace
===>Rob:
===>Very true. Almost all standing seam metal roofing systems
===>utilize a "slip-fit" or "floating clip" connection to respond to
===>the normal thermal expansion and contraction of the sheet
===>material; therefore negating any "assumed" shear or diaphram
===>capabilities. Repeat: standing seam metal roof systems DO
===>NOT provide shear & diaphram! This is why most mfrs
===>recommend a solid substrate, either plywood or metal deck.
===>
===>"Screw-down" metal building roofs, on the other hand, do
===>supply "some" shaer and diaphram strength, however, we're
===>talking about a very light gage member with fasteners spaced
===>usually 6" to 10" apart. These panels depend on these "point
===>load" attachments, which are also not a good structurally
===>engineered conection. I would definitely recommend that the
===>owner/architect/contractor or design professional consider
===>the condition "null" as far as taking care of shear & diaphram
===>resistance (racking of the building & wind/dead load stress
===>from the sides. I'd take care of the diaphram loading in some
===>other method.
===>Roger Wallace
===>District Sales Manager
===>AEP-SPAN