GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

How to make penetrations in preexisting corrugated siding?

DamionL | Posted in General Questions on

Hi all, 

Are there any drawings, photos, or guides for how to make penetrations in preexisting corrugated siding that is attached directly to the plywood sheating? I need to make some penetrations for a bath fan, ac line sets, electrical outlets etc, but it’s not feasible to remove the panels since they are large and caulked to each other. The siding is also fiber cement, not metal but I assume the process should be the same.

My thought was to trace out a sufficiently thick mounting block, cut the siding as close as possible to the dimensions of the block, attach the blocking to the sheathing using sealant and screws, drill for my penetrations in the mounting block, then caulk all 4 sides of the mounting block to the corrugated siding. 

Does this make sense or is there a better way?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  1. Malcolm_Taylor | | #1

    Damion,

    That sounds about right, although you may want to insert a head-flashing over the mounting block rather than caulking the top.

    1. DamionL | | #3

      Malcom, how would that work since I can't tape the head flashing to the underlayment because the siding is already on?

      Would it make sense to instead not caulk the top of the mounting block, but caulk the the other 3 sides so that water can escape from the top but can't enter from the sides or bottom?

      1. Malcolm_Taylor | | #5

        Damion,

        You slide the vertical leg of the head-flashing (with end dams) under the existing house-wrap before you mount the block.

        1. DamionL | | #6

          Malcom, sorry I should have mentioned this, the housewrap is peel and stick, Grace Ice and Water Shield. Yes I know, completely inappropriate but luckily it doesn't seem to be failing in my moderate climate zone. I didn't know this when I purchased the house, I found out later.

          Anyways, I wouldn't be able to slide the vertical leg underneath the house wrap in this case, so do you have another suggestion? Could I just do what I mentioned in post #4 below?

          1. Malcolm_Taylor | | #11

            Damion,

            Ah - makes sense. At the head I'd taper the top of the block as Frank suggests.

  2. Expert Member
    DCcontrarian | | #2

    There is a drip edge flashing that is made for the tops of doors and windows, that's what you want to use. Just treat your block like a very small window.

  3. DamionL | | #4

    DC, something like this? https://images.thdstatic.com/productImages/e5152334-b102-45b3-b5b8-14eb5ca64390/svn/gibraltar-building-products-drip-edge-flashing-re23ch-64_600.jpg

    So I just attach this to the top of the block with sealant and then attach the block to the plywood with sealant and screws and caulk the 3 remaining sides?

    Malcom, I assume this is what you're talking about as well, and I see now why I can use it, it doesn't have a vertical lip that gets taped to the underlayment.

    I hope I got that right.

    1. Expert Member
      DCcontrarian | | #7

      You want one with a vertical piece that goes up the wall.

      You want to be prepared for the contingency that the siding leaks slightly and there is water running down the back of the siding. You want it to hit the flashing and kick out over the drip edge.

      Ideally the vertical part of that flashing is taped to the sheathing. If that's not possible, second choice would be to smear the back with sealant and slide it between the siding and the sheathing.

      1. DamionL | | #8

        DC, As I clarified above, the underlayment on the wall is peel and stick.

        So, caulk and slide the vertical leg between the peel and stick underlayment and the siding, then caulk the other 3 sides of the mounting block after installation? Also caulk around the penetration and the back of the mounting block before installing with screws?

        Does that sound about right?

        1. Expert Member
          DCcontrarian | | #10

          Sounds good. I would caulk behind the mounting block in a horseshoe shape with none at the bottom, so that if water gets behind the block it can drain to the bottom.

          Basically try to have as many layers as possible and try to make each layer so that if water gets in it has a way to get out harmlessly.

  4. FrankD | | #9

    I would drill the hole in the wall first, push the wire or duct through, and make a good water and airtight seal to the peel and stick. Only then install the mounting block along with the electrical box, vent terminal or whatever is going on it. Ideally would have head flashing as mentioned, but you could use a PVC block with a sloped top edge.

  5. AD_in_AK | | #12

    Great advice given here. I have done this quite a bit by drilling hole from inside to situate the block, trace and cutout for block, run hole saw through sheathing , wiggle in drip flashing under siding, mount block, caulk. The penetration sealer I have been using is the M.A.P. system pipe sealer from Interra (the small one is great for hose bibs and the 4inch one is great for bathroom fans. )

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |