A Guide to Casement Window Sash Sag

A Guide to Casement Window Sash Sag

Like any structural component of a home, windows require proper installation, upkeep, and, sometimes, a little TLC.  A common issue with casement windows, those that are attached to their frames by hinges, is sash sag.  This occurs when the bottom of a window begins to droop. An all-too-common conundrum, sash sag can be prevented as well as mitigated, and we’re here to provide tips for both instances.  

Though the general principal of downward movement is the same, there are four varying types of sash sag with unique principals.

1.    Sash Settling 

Sash settling is when the whole sash has shifted down toward the sill.  This is usually not a problem, unless windows are very large and heavy.

2.    Frame out of Square 

Casement windows must be installed with a 90-degree angle where the sill and the jamb meet on the hinge side.  When not done properly, sagging can occur.  Other reasons why a window frame may not be squared is that one jamb is longer than the other or the sill and head are different lengths.  Also, if a building has shifted on its foundation over time, the angles and alignment of a window may move.

Homeowners can check if their window frames are square by measuring the angle of the frame’s lower hinge corner.  If it’s less than 90 degrees, sagging is occurring.  Another method is to measure the frame diagonally in both directions.  When the measurement that includes the upper hinge corner is shorter than the other, the frame is out of square. 

3.    Sash out of Square

Casement windows hold the sash’s weight at the lower hinge corner.  The majority of unglazed sashes are not completely rigid, and therefore gaps between the sash and glass can allow for the weight of the glass to push the sash out of square.  Glazing blocks (also known as settling blocks) are used to hold the glass in place within the sash.  Sag is best reduced when the blocks are placed in the corners, as close to the hinge as possible.  

Since the lock side of a sash is unsupported, only the hinge corner is supporting the weight of the glazing.  The strength of corner joints and the stiffness of the sash rails and stiles will play a factor in how square the sash will remain.  To check if a window sash is square, homeowners can use the same steps listed for frame out of square, but keeping the measurements within the limits of the sash.  The only difference is that if the angle of the lower hinge corner is greater than 90 degrees, sagging is occurring.  

4.     Sash Not Held Square in the Frame

When a sash is placed within a frame, both the top and bottom hinges need to be accurately located on the sash and frame, otherwise the movement of hinge joints coupled with the weight of the sash can cause sagging.  

Other factors to consider with sash sag are: tall/narrow windows are less inclined to sag; mounting screws should be secure, but not overly tight, to prevent hinge movement; and adjustable hinges can be used to deal with any movement or sagging.

This all goes to show just how important the details are in window upkeep.  Have additional questions about sash sag or restoring your windows in general? Let us know!

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