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How to Install Window on Exterior Wood Siding?

ataylor2009 | Posted in Construction Techniques on April 16, 2014 09:59am

We are in the process of building a pole barn, and the exterior siding is all 2×12 Douglas fir. The siding isn’t lapped – there’s a half-lap milled on each edge of each plank, so they interlock but lay flat. This was originally intended to be a workshop / storage building for my toys, but my wife has decided she wants to finish it out and live it in temporarily while we’re building a house next door.

So, I’ve got to figure out how to install real windows (as opposed to the cedar+plexiglass units that came with the kit) in the building. Since there’s no exterior plywood, no house wrap, etc., do I just caulk+nail the window flange directly to the wood siding and then trim? What kind of flashing would be best over the head trim?

Thanks in advance for your input. I really don’t want to muck up my brand-new building.

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  1. calvin | Apr 16, 2014 10:22pm | #1

    That's going to take some thought as it flies in the face of usual building practices that work.

    Horizontal or I'm assuming vertical siding nailed to purlins as you called it a pole barn?

  2. DanH | Apr 16, 2014 10:33pm | #2

    So this is dutch lap/shiplap siding with nothing behind it.  And you're installing a vanilla window with "modern" nailing flange.

    You obviously must somehow seal the grooves in the siding, to prevent infiltration from the side.  I suppose just caulk, but that seems less than ideal.

    Head flashing is a problem.  If the top of the window is not too far below a joint I'd be inclined to run flashing from inside the joint down to lap over the nail flange.  But that would get a little weird if you had to run flashing up 11" to the joint.  An alternative might be to make a 3/8-1/2" saw cut above the window and set the top end of your flashing in that groove, with some caulk.

    1. ataylor2009 | Apr 16, 2014 10:52pm | #3

      I hadn't considered sealing the grooves/joints. When I ordered this kit I was told the planks were all grooved on both edges and went together with splines, which actually would've been better than the shiplap I actually received. Nothing I can do about it now, though. I'm wondering now if I'm going to need to caulk between every one of the planks as they go up?

      I think I can position the windows such that the head flashing can go in a horizontal joint between planks, although I'm planning on just using a Festool plunge saw to cut the window opening out of the solid plank siding after it's all up, so technically I can put it wherever I need it.

  3. DanH | Apr 17, 2014 07:37am | #4

    Yeah, given the thickness of the siding, letting in a drip cap is a good idea.  The mortise should probably be treated with the wood finish (and allowed to dry) before inserting/caulking the cap, though.

  4. ataylor2009 | Apr 17, 2014 09:20am | #5

    It's actually horizontal 2x12 plank siding, with board & batten  above.

  5. AndyEngel | Apr 17, 2014 10:26am | #6

    However you detail the flashing

    I'd treat the wood around the window, particularly the endgrain, with some copper napthenate like Wolman CopperCoat.

  6. User avater
    Mike_Mahan | Apr 17, 2014 02:00pm | #7

    If there's no interior finish or insulation, why not install the windows from the inside? Caulk the nail flange to the siding or building wrap from the inside. Screw to the siding. At the bottom make a saw kerf just below the flange, sloping down to the outside. Slide a piece of flashing through it and caulk it to the inside of the bottom flange.

    1. AndyEngel | Apr 17, 2014 03:27pm | #8

      That makes too much sense!

      That sound you just heard was me face-palming.

    2. DanH | Apr 17, 2014 07:39pm | #9

      It seems to me that leaves you with a lot of "lip" to collect precip.

      And I don't see how your bottom flashing is supposed to work, to drain to the outside if the window's on the inside.  Unless you're saying you make a cut all the way through the 2x material to make a drain channel.

      1. User avater
        Mike_Mahan | Apr 17, 2014 08:42pm | #10

        The bottom of the cutout for the window should slope outward. The saw kerf should, indeed go all the way through the 2x siding, sloping outward also,  so that the flashing material would be continuous from behind the inside of the bottom nail flange to the outside of the siding. The window flanges should be caulked to the inside of the siding.

        If there is building paper or Tyvek behind the siding then it could be done this way.

        Make the cutout for the window through the siding and wrap. Fold the wrap down at the bottom of the window. Put sticky flashing paper on the outside of the bottom nail flange, stuck to the outside of the flange with the sticky side in. Install the window from the inside, screwing it to the 2x siding from the inside. Fold the bottom of the wrap back up onto the bottom sticky flashing. Seal the other three nail flanges to the wrap by using sticky flashing on the inside. This installation comes pretty close to what it would be if the window had been installed before the siding. Care would have to be taken at the bottom corners to make sure all the laps and seals would be redundant enough to prevent leaks.

  7. Rswp1 | Apr 25, 2014 08:57am | #11

    installing a window in a 2x wall

    I'm not sure if this is the best foolproof way of installing your windows so they won't leak, water is pretty un-predictable. But, here is what I would do.

    First off, purchase your windows with the nailing fins. If you can purchase them with removable fins (likely) then do so. Purchase window seal tape, it usually is sold 6" wide in a roll, very sticky on one side. Also purchase metal flashing, preferably "Z" flashing with one edge having a drip edge and the middle section which will be the horizontal edge when installed, to be 1 1/2" wide.

    Whether the windows are aluminum, vinyl or wood/metal clad, really makes no difference. 

    One question I would have is. In what sort of condition is the siding? is it fairly flat or has it cupped with time? This could make a difference in the end.

    Cut your openings approximately 1/2" larger than the frame width and height of the window (not the tip to tip measurement). Remove the top nailing fin.( I'm going to assume that you are going to finish the inside of this new living space with some sort of framing to install insulation.) At the top of the window opening cut on each side of the opening horizontally about 2" long extending out from each side. Now for the "Z" flashing. You'll need to purchase or have shaped by a metal supplier, a piece that when installed has a horizontal measurement the depth of your siding (1 1/2") plus the distance the window frame will extend out beyond the outer surface of the siding. This where the drip edge of the flashing will lay, just beyond the wind top frame. Cut the flashing in a length equal to the length of your window opening width plus the 2" inches you cut at the top sides giving you an extra 4" in length plus the opening width. 

    Cut with a utility knife 2" wide strips of the window seal tape and apply this around the outside of the window opening so the edge of the tape is at the opening edge and there is a full 2" around the opening on the siding.  Now install the flashing under the top of the opening up against the bottom of the siding and secure to the inside of the wall with pan head screws, (there should be about 2" of metal extending upward on the inside to screw into) Dont' put any more screws in other than the inside (don't want to create more holes for water to wick into). So now you have your flashing in place, your tape in place which will give the window a soft cushion effect to somewhat seal between the nailing flange and the siding.

    Now install the window, plumb and with some shim space on the sides. Preferably, screw not nail the window into the siding. Apply 3" strips of the window seal tape over the nailing flange on the bottom first making sure that the tape extends 3" beyond the window opening on both sides. Next apply it to the sides lapping over the bottom tape. Next run a bead of caulk between the drip flashing and the wall siding all the way across from one end of the flashing to the other, sealing the gap. apply the tape to the top of the window opening overlapping the side tape. Your window is now sealed. Now apply a 3 1/2" wide trim board with the top horizontal piece extending out 2" beyond the window to match up with the 2" extra flashing. To hide the tape around the opening (PVC) is a great material, it won't warp and will stay in place, this will help in keeping water out as wood may and probably will warp or simply decay over time. Hope this helps.

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