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Thoughts on installing large mulled windows into 2×6 bucks, then installing entire unit into rough.

MikefromtheMountainsofUtah | Posted in General Questions on

I have a unique situation with my home which is leading me to think I should insist our largest windows are replaced in a unique way… Please let me know your thoughts.

2×6 walls. Siding and exterior window trim has already been removed.  Prosoco Rainscreen MVP liquid “tyvek” has already been applied to the osb.  The original windows were recessed and I’m changing that, bringing the new window flanges out flush with the osb.

So I’m pondering how the new enormous windows are going to go in, and thinking that because there will be new bucks anyway, the following installation method might be smarter.

1. Remove the old windows (rough opening is 6′ wide and 14′ tall), remove the old 2×4 bucks.
2. Build the new 2×6 bucks around the mulled window units with a perfect 1/4″ around.  Cross brace the newly built buck to keep shape during test.
3. Test fit the new buck inside the rough opening, prepare shims.
4. Apply PROSOCO liquid flashing to the 2×6 buck.
5. Mount the entire window unit to the prepared buck, while it’s all on the floor.
6. Place the complete window unit/buck in the rough, and secure the buck to the rough.

I know this is a bit unique, but my concern is both that the Andersen Rep says my windows (3 windows mulled vertically) is too large to install as one unit.  And my separate concern that these windows are so large and heavy one mistake from the installation crew and something is going to get damaged.  The way I see it, I can tightly control the most important details of the installation if I do it this way.

Final step here is that once the window/buck is installed, the PROSOCO reps have no problem with me sealing the entire window flange/buck/rough/osb from the outside using their products, so that I have a monolithic waterproof unit. 

Sorry for the long dialog.  I’ve been thinking about all this since I ordered these windows. 

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  1. jamesboris | | #1

    It can work if a) you can manage to do it without the unit flexing its way out of true/square during the period in which you hoist it + the buck into the opening, and b) you pay attention to water/air-sealing details, some of which you'll likely need to improvise on-site... this sort of thing always yields unexpected little interfaces that need to be sealed. I'm not sure that this approach will save much time/effort in the end, but hey, you're not putting in 20 windows like this, so if you're excited to try this approach, no real downside. You'll need a lot of cross-bracing... but be sure to design it so that it's not in the way of lifting up the window. Grab 4 suction cups and a couple friends, won't be too bad. Block it up to get it off the ground if needed.

  2. Expert Member


    When window reps tell me their windows need to be glazed on site, I defer to them. Your plan may work. My concern is the consequences of it not. If the window installer puts it in in several pieces, they will be responsible for it. If you put it on bucks and try and lift it in one go, and things go wrong, that will be on you. It might end up being an expensive experiment.

  3. Expert Member
    Michael Maines | | #3

    I think this could work; Anderson is likely at least as concerned with shipping as with the actual installation. In theory, doing a structural mull on site should be possible, though there are certainly risks.

    Have you installed windows that size before? You will probably need equipment, not just muscle.

  4. MikefromtheMountainsofUtah | | #4

    Good points on the liability falling on me. I guess in my mind I'm imagining that the margin for error will be quite a bit larger if it's built horizontally. Seeing the quality of installation by the local window company for the previous window left much to be desired.

    My plan is to have an installer on site, but to have them installed in bucks and ready to go up for him. A block and tackle could easily lift the 300 pounds up into the opening, but I want him to take out the old one which is probably more sketchy in my mind than installing the new.

    1. Expert Member
      Michael Maines | | #5

      Are you sure it's only 300 pounds? I would have guessed about twice that much, but I usually use triple-glazed windows.

  5. user-6623302 | | #6

    That unit will be too heavy to get installed correctly. Just make sure the new framing is done correctly.

    1. MikefromtheMountainsofUtah | | #7

      Why would it be too heavy to install correctly?

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