The outside pane of a double-glazed window in my house is broken, and I'm curious to know why. Picture below.

I've done some Googling to investigate, and as I it understand the possibilities are:


This is a second floor window so it would have to be a projectile. My research suggests that this would appear as a series of points originating from a central point-of-impact. I don't see the typical spiders-web pattern, so I don't think this was an act of vandalism.

Heat induced

At the time of the breakage, it was 70F in my house and 39F outside. I don't think this is a sufficient gradient to cause a spontaneous breakage.

Mechanical stress

I'm less sure about this - I can't find consistent information on how this looks.

Edge failures

I don't think the breakage appears to emanate from the edge, so I can rule this out.


All in all, I'm at a bit of a loss, and was hoping some expert here might be able to help me figure out why my window broke.

Photo from the inside Photo from the outside

  • Do you have kids? Or neighbor kids? My son took out 1 window and years later the neighbor kids got a window in my shop with an air soft gun, it only cracked the outer panel so I did not worry about it, to me it looks like someone leaned on it or a wide force from the center but I am no expert I probably have replaced only about a dozen windows.
    – Ed Beal
    Commented Mar 28, 2020 at 5:19
  • 1
    Perhaps the window and frame are being twisted - the semi-circular patterns seem to suggest that.
    – Solar Mike
    Commented Mar 28, 2020 at 5:26
  • Is the window moveable? The window dropping might be a reason. Commented Mar 28, 2020 at 5:55
  • @Harper-ReinstateMonica Yes, it can move - it's a casement window with a crank handle. Commented Mar 28, 2020 at 6:03
  • 5
    This question can only be authoritatively answered by someone who has broken a lot of windows via many different methods. I cannot find any reference to any such study. - I am applying for a grant. Commented Mar 28, 2020 at 16:19

2 Answers 2


That pattern indicates that a blunt object hit it, something relatively soft and larger. A rock or other hard and small object would leave a clear indicator like the picture below, with a central point and circular lines. What larger and blunt object would hit a second level window? Probably a bird. I think that this is a bird strike in the middle of the glass

As an interesting side note, here's an eagle that flow through someone's window last month

enter image description here

  • 2
    Eeks - thankfully I can report after extensive search that there is not an Eagle in my house at least! Commented Mar 28, 2020 at 16:59

Not a rock or small projectile.

Two possiblities, or both. Break upon opening/closing or ball impact.

Note the smooth section in the middle at the base where the crank attaches. See the stress fractures to the corners. This looks like the pattern obtained when the push/pull force is different where the crank attaches compared with the rest of the frame.

First test the tension on the crank handle to see which way "someone" has recently tried to move it. Check if the moveable window is jammed in its frame. If so, the window breaks when either trying to close it all the way, or open from a jammed closed position.

Second option the window was not strongly closed (or forced closed under tension) and then hit by a big ball, with the middle-lower point held firm and the rest of the window frame flexing.

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