I have andersen 400-series windows throughout the house. I'm renovating the attic so the trim and wallboard are off revealing this situation:

photo of window in rough opening

The window is not attached to the rough opening on the sides or top. It just sits on that stack of 2x4s at the bottom and the flange is nailed to the sheathing on the outside. I checked Andersen install instructions and they don't specify that you have to nail to the rough opening but they do say to shim them; no shims anywhere (the thing on the left is a cut strap). If I push on the window frame, the whole thing moves a bit as the flange flexes. The stack of 2x4s that it's sitting on seems to be nailed together top to bottom but not toe-nailed to the sides of the rough opening so that rocks slightly too.

Fiberglass insulation tucked in on the sides but not the top. This window has maybe 1/2" space there but the other one (other side of house, same situation) has about 1-1/2" of empty gap. I see some sunlight through the gap and flange but I don't see any signs of leakage. I had some of the other windows replaced and I don't think there's flashing tape over the flanges on the outside. No flashing over sill. Wimpy looking header, and those are collar ties above, not joists, so there's only a few feet between the top of that window and the ridge of the roof.

All the windows were installed at once around 2000 under previous owners so it survived like this for nearly 20 years. My guess all the windows are installed about the same. I'll toe-nail that stack of 2x's to the trimmers on the sides and foam the gap around the windows. Should I be concerned or is this ok? Does the window need to be attach the inside of the rough opening somehow? Does there need to be a proper header above the window?

Edit: The house was built in the early 50s. These windows were installed around 2000. The dark brown wood (full studs and top header board) are original construction, old growth douglas fir lumber, generally very solid. The lighter wood inside that was added during the window replacement. Reasonably certain the frames originally held larger windows. They built up the sides of the rough opening to hold smaller flanged replacement units. The outside was wood shakes originally and vinyl siding was installed over that at the same time as these windows went in.

  • I would suggest that the stack of 2x4 on the bottom of the opening is there because the rough opening and the window size didn't match up. Whether the opening wasn't framed properly to begin with, or the window was too small (due to mismeasuring, incorrect spec, or mismanufacture/misshipping) and they didn't want to bother with returning the window and waiting for a new one, or possibly a change order was issued after the opening was framed but before the windows arrived. The original carpenter who installed the stack probably figured he had it tight enough to not be an issue.
    – FreeMan
    Jun 26, 2020 at 11:15
  • Not saying that said carpenter was correct, but that was probably the thought process. The other option is that this is a "new construction" "replacement window". The 2x4s immediately around the window are much lighter in color than the surrounding lumber. It's possible that the house is much older and they put in smaller windows as part of a major renovation which included stripping off the old siding and maybe even sheathing. To adapt to the smaller windows, the framing was padded inward, then "new construction" windows (with that mounting flange) were installed instead of...
    – FreeMan
    Jun 26, 2020 at 11:18
  • replacement windows which are normally installed with roughly zero outside work and are attached to the wall by screwing through the sides of the window frame and into the studs around them. The stack of 2x was quicker and easier than building a little cripple wall to set the window on. If this was a "new" replacement, I'm not sure of the code implications of leaving the "header" as it was originally built (assuming there were codes at that time) or if that required bringing it up to then std. code, but that step was skipped. (Wow, that got long, but it's really not an answer.)
    – FreeMan
    Jun 26, 2020 at 11:21
  • 2
    The nailing fin or flange IS how the window is SUPPOSED to be held in place - nothing sketchy about that. Not flashing it is sketchy. Shim if shims are needed - I tend to get the sill framing level and not need shims as a result. Be careful with the foam - it's quite possible to bow the window to the point that it won't operate via foam pressure.
    – Ecnerwal
    Jun 26, 2020 at 11:57
  • I've never used the "replacement windows". On the handful of windows I've replaced (getting ready to do two more) I used new construction windows with the flange, added 1x or 2x lumber to the existing RO to provide a nailing surface for the flange. This reduced the size of the glass a bit, but was still OK in my situation.
    – SteveSh
    Jun 26, 2020 at 13:56

2 Answers 2


There are a lot of problems with that install. But I do know that you do NOT want to nail the window frame to the rough opening on the sides. The flanges that are nailed to the sheathing provide all of the support. Usually there are shims underneath the window at the bottom to 1) bring the window up to the right height in the opening and 2) ensure that the bottom of the window is horizontal. I used these windows myself.

Note that most of the movement you noticed would be prevented when the inside trim is installed, which would tie the inside of the window to the walls.

EDIT 1 - Other Shortcomings

A header, of the appropriate size for the opening, is definitely needed if the the wall is load bearing, Even if that's not the case, I would frame the window with a minimum header just to strengthen the opening. I notice that there is a header of sorts - two 2x4s laid on their sides. I have to defer to those with more experience as to whether this meets my "minimum header" need.

I would also have done the standard king-jack stub framing for the window.

There definitely should be flashing at the bottom of the rough opening, the sides, and the top of the outside of the window. The Anderson installation instructions provide details on this, and there are lots of on-line resources you can consult also.

Finally, 20 years is not a very long time for a window. I still have 65 year old original windows in parts of my house, and many historic structures have windows going back hundreds of years.

EDIT 2 - Added picture

Here's a picture with the low-expansion rate foam used.

enter image description here

  • I don't think that wall counts as load baring. The window is right below the ridge of the roof. I'll ask a structural engineer when I call him in for other stuff. The sill flashing might have to wait until the siding is getting replaced.
    – user116817
    Jun 26, 2020 at 23:28

+1 For getting the Andersen instructions. I installed 21 Series 400 replacement windows a few years back. Not sure what you have here.

The basic flow for them, is to level the sill, caulk it, and drop the frame in. Then you start with screws from the top, within the track, from the frame into the rough in, shimming and checking for plumb and square. I think there were 4 screws per side, you also have to check to keep the width even.

When you're all done, then you insulate the gaps and cut the exterior trim to fit.

  • I'm pretty sure they're 400 series tilt-wash double-hung windows. They do have flanges on the outside up against the sheathing. Sounds like yours didn't have the flanges.
    – user116817
    Jun 26, 2020 at 23:19
  • There are several flavors of 400 Series windows. I bought the Replacement type. They are made to order (to the sixteenth) They are intended to be mounted into an existing jamb. New Construction windows would come with the sill & jamb and be installed in the rough opening. So the process is different, so I was giving you a different look at a similar problem. Seems that you have New, but many parts are the same. It sounded like you didn't like how "loose" the window frame seems to be mounted. Try looking in the sash channel, if the holes are there, you could try screwing and shimming.
    – DaveM
    Jun 28, 2020 at 3:07

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.