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Dryer vent options: how to roof flash with metal panels and/or how to protect studs from heat in wall?

lightnb | Posted in General Questions on

I’ve been unable to locate any information on what type of jack/boot to use and how to flash it for a dryer vent penetration through a corrugated metal roof. The roof does not exist yet, so it can be done at the time the panels are installed. What product and procure is recommenced for sealing this penetration and making it water tight?

Alternative routes for dryer vent are:

1. 13′ run through an interior wall. Meaning, drilling 4-1/4″ holes through the middle of every stud. About ten studs. I have concerns about pyrolysis with repeated exposure of studs to high temperature. the only way to do this would be a type of pipe or technique that was cool to the touch on the outside at all times.

2. 12′ rise, a ninety elbow, then 23′ horizontal through attic to gable exit. That’s about 40′ equivalent, and too long, I think.

So as much as everyone says “don’t do the roof”, I think the only option here is to go up, 16′ rise between studs, only drilling the double top plate. But that leaves the problem of how to terminate and flash this with a corrugated metal roof, something I can find no information on after hours of searching.

Does anyone have a boot/jack they recommend for this? Or a detail on how to use a standard jack like the DryerJack with a metal roof and make it water tight?

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  1. Expert Member
    Akos | | #1

    I would use a standard roof vent and flash it similar to a skylight: pg22

  2. Expert Member


    I gave you the same response as Akos in your other thread:

    However both threads beg the question as to why you are using corrugated panels for the roof instead of snap-lock with concealed fasteners? I use a lot of corrugated galvalum as siding. Even then the shape makes it a bit problematic to detail things like the upper corners at window head-flashing so that moisture stays out of the cavity behind. On simple gable or shed roofs things re a lot easier, but penetrations will always be a vulnerable spot.

    Alternately, why not run a bulkhead or drop a hallway ceiling to run the duct? I agree boring 4 1/2" holes in studs isn't an option, although that's for structural reasons, not heat concerns. The temperatures involved are too low to worry about.

  3. lightnb | | #3

    @Akos: Thanks for the PDF link. It helps when there are pictures. It looks like this flashing can be done later, after the roof is installed.

    @Malcolm: I asked as a new question because this is actually a two part question.

    The other half is, if running duct through a wall (horizontally) is acceptable and if there is a type of pipe and/or mounting method that keeps the heat away from the studs. I have renamed the question to reflect this. I have no problem venting the waste stack and the bath room fans through a gable, as longer runs are OK for these.

    We opted for corrugated as it is half or less the price of the standing seam. We can take another look at standing seam, but I think it was cost prohibitive.

    There's also another type call 5V that has flat sections at least 10" wide. Is it sufficient to mount the metal vent to the underlayment, cut a square hole in the panel the same size as the vent cap, then place butyl rope and/or sealant around the flange? (something like "Through the Roof!")

    1. Expert Member
      MALCOLM TAYLOR | | #4

      Fair en0ugh. Cost is certainly a valid reason to choose the corrugated. The standard details work very well for ridges, valleys and overlapping sheets. Large or small penetrations are problematic. I would do everything you can to avoid them.

      Edit: With panels that have a flat section, you can use a standard roof termination and bed it in caulking with lots of gasketted screws. Ideally you also overlap the roof panels to cover the top of the flashing as opposed to just cutting a hole.

      You don't need to worry about the heat. Dryers in many places are still ducted with plastic flex-pipe and the terminations are often also plastic. The main reason metal is specced is to prevent fire damage if the lint ignites. Dryers just don't generate enough heat to affect framing. The air they exhaust is about 120F. Attics can reach 150F from solar gain.

  4. gusfhb | | #5

    few seconds on google

    I would agree with those that question an exposed fastener roof.

    It will certainly keep out bulk water, but fasteners will weep occasionally and possible leak over time. Not a friend to an insulated structure

    I would also tend to not try to put the penetration in the center of a panel but rather in the overlap, allowing the upper panel to become part of the 'flashing'

  5. brp_nh | | #6

    Do you already have the dryer? If you don't or even if you do, what about just going for a ventless dryer.

  6. lightnb | | #7

    For reference, at the local metal roofing supply place, the price to cover the roof with 36" "Tuff Ribb" panels is $2,750 vs 16" Standing Seam snap-lock panels, which are $8,015.63. Panels only. That's almost 3x the cost. I don't know if it's just this place that's high, or if there is an industry "conspiracy" to mark up these standing seam panels a lot because someone decided they are a "premium product". (The way every glasses shop charges the same $300 for the same $6 pair of glasses, because they are all actually supplied by the same company).

    I'd love to do a standing seam roof if someone knows how to get panels without going through DeBeers Roofing, but I really have a hard time with the giant leap in price for the same area coverage. Do the standing seam panels never leak or weep?

    Regardless of roof panel type, I think it would be better to explore the through-the-wall option more. Does anyone know for certain if it is IMC compatible to run through-the-wall with bored holes? It would be a 12' run. And what methods of heat shielding are required, or recommended? I was thinking we could bore 5.5" holes in a 2x10 wall and use metal duct strap to hold the 4" pipe in the middle of the holes, so there would be 3/4" gap to the studs on all sides. I'm trying to determine if that's code compliant. The hole size is OK, as a 60% bore hole is allowed in non-bearing walls. I'm primarily concerned about fire safety.

    1. Expert Member
      MALCOLM TAYLOR | | #8


      Where are you getting that heat in the duct is a problem?

    2. Expert Member
      Akos | | #9

      Maybe there is a confusion with how dryers operate. The air is heated before it gets into the tumbler area, here the water in the clothing is evaporated and carried out through the exhaust. This means the exhaust of the dryer never really gets above 100C, if it would, your clothes would also melt.

      All dryers also have a thermal cutoff in the exhaust that disables the heat if it ever gets too hot.

      The temperatures we are talking about here are well bellow what wood can tolerate, no need to worry about insulation. This is not like a furnace exhaust pipe.

    3. MattJF | | #22

      Go through the wall.

      What is the 2x10" wall you are talking about? Do you really have an interior 2x10" wall?

      No heat shielding needed. 4" metal duct is all you need. Taped seems, no screws as they catch lint. The best fire protection is as short and straight of a run as possible as the cause of fire is excessive lint buildup. Long sweep elbows are good if they fit:

      The most common setup for going from an interior room would be either going up or down and running between the floor or ceiling joists to the back of the house, then a 4.25" hole through the rim joist. Air seal the hole. Flash to the WRB per any other penetration.

      Venting near the ground makes occasional cleaning with one the brush kits much easier.

  7. lightnb | | #10

    I had a dryer once where there was a short in the heating coil and the coil was always heating, ignoring the thermostat. (The coil was basically hardwired in the "on" position). I don't recall it shutting off. It just got really, really hot until I fixed it.

    According to FEMA, there are 3,000 home fires caused by dryer vents each year.

    While I can't find a source, I imagine the issue is contact with combustible materials. A dryer vent pipe with a fire inside it sitting on a concrete slab is probably not going to burn down the house. If it's inside a wall touching wood framing members, then it probably will.

    I'm trying to do what is safe. Is there a consensus that running the duct through studs creates no more chance of fire than running u through the roof?

    1. Expert Member
      MALCOLM TAYLOR | | #12

      Dryers are well known hazard and are identified as such by Fema, fire departments, codes and insurance agencies. They all have issued advice on how to install and maintain them to mitigate the risk - and none of them suggest isolating them from framing.

      Would it be safer? Maybe. So would using 2 hr rated electrical outlets, conduit, type X drywall throughout the house, installing sprinklers, building entirely out of non-combustable materials etc. It's prudent to know what the risks are, and to use best practices to mitigate them, but it also is important to know what deserves attention and what doesn't make much difference.

  8. Expert Member
    Akos | | #11

    All units should have a thermal switch set around 150C and a thermal cutoff around 190C. I don't know enough about gas units, but I'm sure they would have similar safety setup.

    Dryer fires normally start from lint getting on the heating element directly. There are very few things more flammable than lint. Keeping lint out of your dryer ducts is much more important than trying to insulate them.

  9. lightnb | | #13

    "It's prudent to know what the risks are, and to use best practices to mitigate them, but it also is important to know what deserves attention and what doesn't make much difference."

    Which is why I'm asking these questions, doing research and trying to determine what is required by code, and also what is best practice even if it exceeds code.

    So code allows dryer ducts to be installed through bored holes inside and parallel to a wall and there are no requirements for thermal protection of framing members?

    This would be IRC/IMC 2012, I believe, in the state of Georgia.

    All I could find in the IMC on dryer vents is male fitting end away from the dryer, and make joints with metal tape, no screws.

    1. Expert Member
      MALCOLM TAYLOR | | #16

      This may not be the version of the IRC your state uses but it will be similar.

      You can't find clearances to combustible framing addressed because they don't consider it an issue.

  10. Expert Member
    Peter Engle | | #14

    There is no requirement in the IRC/IMC to insulate dryer ducts from wood framing members, and single-wall metal vent is acceptable. As others have mentioned, the vents do not get hot enough in operation to damage the wood. Even pyrolysis requires higher temperatures. Keep the vent clean and there won't be any problems. Don't use screws on the seams, as the points catch lint. Clean the metal and use a good-quality foil tape instead.

    If you've got room for a 2x10 wall, do you have room to simply make a soffit to contain the vent? Might be easier. Or a double stud wall with the studs flatwise would save space and provide an internal cavity for plumbing and wiring as well. If you're worried about flexibility, the studs could be connected across the gap with blocking. Slightly more expensive to build, but very handy for the utilities.

  11. STEPHEN SHEEHY | | #15

    Installing a heat pump, ventless dryer will be cheaper and more energy efficient.

    1. cussnu2 | | #17

      Not to mention that the long runs being considered here in any direction are seriously problematic.

      1. Expert Member
        MALCOLM TAYLOR | | #18

        What's wrong with a 13 ft run from either practical or code standpoint?

  12. rockies63 | | #19

    Um, since you seem to be about to embark on spending a lot of time and money for this vent, why not get a non-venting dryer? Pricier than a venting dryer, but I'm sure the money you save from not having to accommodate all that pipe would cancel out the upcharge.

    1. Expert Member
      MALCOLM TAYLOR | | #20

      I find a lot of this discussion mystifying. There is a straight path through a 10" wide interior wall - although why the wall is that depth is unclear. There is neither a lot of time or money involved in venting the dryer. It's about as simple as it gets for one that is not located on an exterior wall.

      1. Expert Member
        Peter Engle | | #21


        I'm pretty sure he is making the wall 10" wide specifically for the dryer vent, so that he can bore holes through the studs while remaining within the 60% hole size allowed by the IRC.

        1. MattJF | | #23

          Ahh, I was trying to figure that out above.

          I would do a normal 2x4 wall with a small 5" or so soffit along the ceiling line. Small soffits like this can be made like a plywood box instead of full 2x framing to minimize the profile even more.

          1. Expert Member
            MALCOLM TAYLOR | | #24

            Lot's of possibilities. That wall could become an architectural feature with deep shelves inset into the stud bays, or provide a niche for a piece of furniture.

            The bulkhead in my dining room used for drains is what gives interest to the ceiling.

  13. lightnb | | #25

    This is new construction, so there is little cost, just decisions to be made. The wall houses plumbing and the dryer vent. A 2x6 or greater wall can go on 24" centers, so a 12' section is just 6 studs getting upsized. They would be 2x6's anyway for the plumbing. So we're talking $2 extra per stud. About $12 total to go 2x10.

    A soffit/box is an option, but across the ceiling would require the vent to go up 12' before going over, then back down again, which doesn't make a lot of sense, since going 12' horizontal is all it would take to go out the wall. We could keep the whole thing low to the floor, about 3' or so high.

    Since it looks like there is no code/safety issue with heat, I think that option (horizontal though 2x10 studs) makes the most sense.

    1. Expert Member
      MALCOLM TAYLOR | | #26
    2. MattJF | | #27

      Sounds like you have a workable plan. At 4.25" hole, you can just clear through a 2x8. You might need something else to go vertical in there though.

    3. STEPHEN SHEEHY | | #28

      Don't forget you need to provide for access to clean lint out of the pipe.

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