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Corrugated roofing

felloes | Posted in Construction Techniques on December 2, 2002 11:41am

We are re-roofing a garage/workshop with corrugated tin.

In order to prevent condensation, I intend to first lay down some sterling board covered with roofing felt, then putting the tin on top.

The question is, do I need to have a ‘space’ (how much) between the felt layer and the corrugated tin?

Would  rigid insulation sheets between the sterling board and tin help reduce condensation?

Any advice would be most welcome

Cheers

B&G

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Replies

  1. Ragnar17 | Dec 04, 2002 03:04am | #1

    I'm not an expert on roofing systems, but I figured I'd give you my two cents since no one else has posted a reply yet.

    I think you're going to end up with condensation on the underside of the corrugated metal no matter what you do in terms of sheathing and insulating.  This is due to the fact that the corrugated section will hold some air between itself and the roof deck, even if none comes from the warm shop space below.  The trapped air will get hot during exposure to sunshine, and then cool rapidly at night, resulting in condensation. 

    The point is that the felt paper will be necessary to keep the condensate from damaging the sheathing, so make sure you put in on well.  You might even want to make it into a more of a waterproof layer by possibly using some roofing cement at the laps, and staple penetrations, etc.

    Ventilation should help to reduce the amount of condensation that occurs.  There will naturally be venting at the eaves, and if there's some way to do it with a corrugated roof, venting at the ridgeline would complete the path.  The monitor roofs of old barns comes to mind. 

    The main function of the insulative layer will be inhibit heat loss from the warm shop space beneath. 

    Hope this is helpful,

    Ragnar

  2. Piffin | Dec 04, 2002 05:10am | #2

    Here's my two cents.

    I'll disagree with Ragnar a little. It's not the heating and cooling of that space that brings on condensation. It is when warm moist air hits a cool surface. Keep the moisture from getting to the bottom of the tin and you've got your solution. Foam board is good but put it inside of the sheathing for insulation and vapour bar.

    Over the sheathing, put the felt ( tarpaper ) and then nail horizontal 1x4" furring strips at about 16" or 24"OC depending on your snow load requirements. Fasten the corrugated tin to those. This creates an air space that will ventilate itself and keeps the roofing from wearing on the felt with thermal expansion in the heating/cooling cycle.

    .

    Excellence is its own reward!

    "The first rule is to keep an untroubled spirit.

    The second is to look things in the face and know them for what they are."

    --Marcus Aurelius

    1. Ragnar17 | Dec 04, 2002 09:32am | #3

      Thanks for chiming in, Piffin.  I'm assuming the furring strips should be installed vertically, so that any condensate will not hit them and get stuck, like multiple rows of dams...... What would you recommend in terms of venting the ridge?

      I'll disagree with Ragnar a little. It's not the heating and cooling of that space that brings on condensation. It is when warm moist air hits a cool surface. Keep the moisture from getting to the bottom of the tin and you've got your solution.

      Maybe I was a little unclear with my explanation.  At any rate, the point I'm trying to make is that you're not going to be able to prevent condensation entirely.  Even with the best ventilation possible, there's going to be condensation forming on that cold metal.  Think about what happens when you leave a piece of metal on a picnic table overnight.  It's literally in the air, and it will get to both sides of the corrugated roofing.

      I see two options: allow runoff channels for the condensate, or spray icynene to the corrugated metal so that there's no more air space at all.

      Any other thoughts?

       

      1. Piffin | Dec 05, 2002 04:30am | #4

        vertically????????

        Then you couldn't fasten the corrugated to them! How much condensation are you expecting anyway? A few drops here and there won't leak but I doubt there would be any condensation between the tarpaper and the metal roof. Heat would bent out naturally at the ridge. Most corrugated on shops and barns doesn't get a neoprene filler strip..

        Excellence is its own reward!

        "The first rule is to keep an untroubled spirit.

        The second is to look things in the face and know them for what they are."

        --Marcus Aurelius

        1. Ragnar17 | Dec 05, 2002 06:05am | #5

          By "vertical" I mean parallel to the rafters, as opposed to something akin to skip sheathing.  

          It's been so long since I've seen a corrugated roof that I just can't remember how close the nailing has to be, etc. -- so take it easy on me!  ;)  What's a typical installation if you're not going to use solid sheathing? 

          Ragnar

          1. Piffin | Dec 05, 2002 06:29am | #6

            I've never used it except on a barn

            No, wait a minute. In Telluride, I did one with Cor-ten which is a really heavy ( ten gauge) corrugated iron roofing intended to look like old mining camp roof. It gets a rust patina for a wear layer instead of paint. This was on John Nasbitt's house in about '85 or '86

            I can't remember the fastening schedule.

            On plain old barns, We put a nail on the hump every three humps..

            Excellence is its own reward!

            "The first rule is to keep an untroubled spirit.

            The second is to look things in the face and know them for what they are."

            --Marcus Aurelius

          2. Ragnar17 | Dec 05, 2002 06:47am | #7

            I built a few animal sheds with my dad when I was a kid, but that was a few years ago.  ;)  On a typical barn, it sounds like you'd frame the rafters, put on some skip sheathing (at maybe 24" centers?), and then put on horizontal rows of those fancy nails with rubber washers under the heads.

            Now that I think about it, the panels are pretty weak when you bend them along the long axis, so you would need to have solid support in the horizontal direction. 

            Do you really think it's necessary to fur them up if you're using solid sheathing?  You might get a little better ventilation flow, but the furring strips would interfere somewhat with that, too.

            I do think you're going to get a fair bit of condensation -- at least enough to worry about.  Think of how much condensation you see on the insides of windows -- it's not a lot, but enough to cause dry rot after a while.  The "skip sheathing" across the solid sheathing would just trap it.

            Regards,

            Ragnar

          3. Piffin | Dec 05, 2002 07:18am | #8

            The moisture would have to travel through the taped foam ceiling, the plywood, the tarpaper, and the vented airspace all before it had a chance to condense on the underside of the metal..

            Excellence is its own reward!

            "The first rule is to keep an untroubled spirit.

            The second is to look things in the face and know them for what they are."

            --Marcus Aurelius

          4. Ragnar17 | Dec 05, 2002 08:12am | #10

            Piffin,

            Condensation doesn't just come from the warm air within heated spaces; water vapor is in all air.    Any time a given air mass cools, the relative humidity increases.  Since there is an air space between the corrugated metal and the tar paper, there is a potential for condensation.

            Again, think about a piece of metal that gets left out on a picnic table overnight.  It is "vented" to the umpteenth degree, but it will still see condensation on all surfaces.  It is not directly above some heated space, but it still sees condensation.

            Regards,

            Ragnar

          5. Piffin | Dec 06, 2002 02:37am | #11

            That's all true, but the air must be warmer than the surface for consensation to occur - or the surtface must be colder than the air, depending on your point of view..

            Excellence is its own reward!

            "The first rule is to keep an untroubled spirit.

            The second is to look things in the face and know them for what they are."

            --Marcus Aurelius

          6. Ragnar17 | Dec 06, 2002 03:10am | #12

            That's all true, but the air must be warmer than the surface for consensation to occur - or the surtface must be colder than the air, depending on your point of view

            There's another case, too: ambient air will precipitate when the temperature plunges below the dew point (i.e.fog).

            I'm glad we got this roofing thing all figured out now!!!  ;)

            Ragnar

          7. DaveRicheson | Dec 06, 2002 04:00am | #13

            Piffin, is right on target. My shop is framed walls and roof trusses 4' o.c. with the 2x4 purins let in between the trusses at 2' o.c.. Directly over this is the metal roofing. I strapped the bottom cords of the trusses and hung 5/8' dw. Walls are 2x6 with f/g unfaced f/g insulation, followe by dw. No vapor barrier anywhere, and the attic area has not been insulated yet, nor is there any heat in the building. the roof is vented with soffit vents and a continous ridge vent. I run a 150,000 btu torpedoe heater when I am working in there, so it warms up pretty quick.

            I was concerned about condesation on the underside of the metal roofing, and posted a vp question here about it last spring/summer. Consensus was I did not need one. Throughout the fall and early winter I have checked the attic for signs of condensation. None found thus far. The combination of soffit/ridge vent and self venting feature of the metal seems to reduce the probability of condensation to near zeroe. This is standard ploe barn metal roofing with four ribs on per sheet. It is not as self venting as corragated steel.

            I live in the lousville Ky area and we have some pretty good swings in temperaure and high humidity level here. Bottom line is that Piffin was correct in his reccomendation on my vp question and he is correct here also. Theorectical is good to kick around, but the proof is in the building.

            The package described in the original post will probably work just fine if he can locate a ridge vent for the roof.

            Dave

          8. Ragnar17 | Dec 06, 2002 05:36am | #15

            Dave,

            If I understand the construction of your shop, it's different than what Piffin and I are talking about here.  However, I'm a bit confused by your description.

            For starters, I'm not sure why the purlins are mentioned in respect to the roofing system.  Per my understanding, purlins connect rafters to ceiling joists. 

            At four-foot centers on your trusses, it sounds like you'd need some sort of skip sheathing across them to adequately support the roof panels to prevent bending along the weak axis.

            Not trying to be bristly here --- just need a better understanding of what's going on so that I can learn from your experience.

            Thanks,

            Ragnar

          9. Piffin | Dec 06, 2002 04:36am | #14

            Always good vollying with you. I like the mental workout..

            Excellence is its own reward!

            "The first rule is to keep an untroubled spirit.

            The second is to look things in the face and know them for what they are."

            --Marcus Aurelius

          10. ian | Dec 06, 2002 07:36am | #16

            Piffin, Ragnar

            Just looked up my steel sheet reference book.  Corrugated sheeting should be nailed (using roofing nails designed for the sheeting) or screwed (again using the right roofing screws) to batterns laid at right angles to the corrugations.  Any condensation that forms will be above the batterns and so shouldn't be much of a problem in respect to the batterns.  The recommended battern spacing varies with the corrugation profile (depth of rib) and thicknesss of the sheeting. 

            As an example, the following is taken from the material published by the BHP Steel, the Australian producer of corrugated sheeting, all dimensions are mm (25.4mm = 1in)                                                                                            Max spacing of batternsProfile               thickness       rib depth         min roof pitch      internal spans      end spansCustom Orb          0.47               16                  1 in 12                  1200                 900                           0.53                                                                 1600               1200Custom Blue Orb   0.66               17                  1 in 12                  1200                 900                           0.86                                                                 2400               1800Screw spacing is every second rib at the ends of a sheet and every fourth rib for internal batterns.  Side lapps are always made so the free edge faces down into a valley, rather than up the valley side and the lapped edge is always screwed down to every battern - this may result in screws every third rib for internal spans.   Ideally roofing is a single sheet between ridge and eave. 

            Also watch how well the batterns are attached to the rafters.  There are two common failure modes with steel sheet roofs - individual sheets blow off because not enough screws were used or the whole roof comes off because the battens were only just nailed down.

            hope this helps

            Ian

          11. Ragnar17 | Dec 06, 2002 09:56am | #17

            Ian,

            Thanks for the info.  The original poster asked about putting corrugated over a solid sheathing.  Do you have any experience with that?

            Ragnar

          12. ian | Dec 06, 2002 10:56am | #18

            Ragnar

            For a roof I would suggest that the sheathing be insulated, and a vapour barrier installed before the roof is fixed.   Battens for the steel roofing would then be placed on top of the sheathing - this will give an air gap and help avoid moisture damage to the sheathing and battens.   As I remember the thread, one of the key questions had to do with condensation on the underside of a corrugated steel roof.  If moisture vapour can get to the roofing steel then you will get condensation and icing if you have snow on the roof.   This would suggest the need for a vapour barrier - and I would think it should be before the insulation to minimise condensation.  If it is between the insulation and the roof, then I would expect that the condensation would occur within the insulation which I can't see as ideal.

            Ian

          13. Piffin | Dec 05, 2002 07:20am | #9

            Horizontal is right.

            If you have installed some, you know what the most common word heard around it is.

            Rrelated to the purple thumb syndrome.

            Excellence is its own reward!

            "The first rule is to keep an untroubled spirit.

            The second is to look things in the face and know them for what they are."

            --Marcus Aurelius

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