Shims are used to wedge the window into position. They allow the installer to ensure the window is level, square and plumb as well as providing solid support for the window.
Shims should be about 2” (50mm) wide and there are two at each shimming point, one from the inside and one from the outside for level support across the entire depth of the frame. Cedar shims should be minimum Grade #1 cedar to minimize splitting.
The first step is to use shims under the sill to make it horizontally level. Generally the shims are placed at 16” (400mm) intervals.
Once level horizontally, the installer must level and secure the window vertically by placing shims on each side between the frame and the rough opening. Shims should be placed near the top and bottom with one in the middle and no more than 24” (600mm) apart. Shims should not be placed where they could compress the frame.
In most cases shims should not be used at the head, or along the top of the frame as normal building movement may affect the proper operation of the window.
One Last Check
Once the window is secured in place with shims, but before any fasteners are installed, the window should be checked for squareness, plumbness, horizontal and vertical levelness and that all movable parts and locking mechanisms operate properly and freely.
Anodized, or rust protected, screws are the fastener of choice. They should be driven in through the centre of every shim or through the nailing flange if one is supplied.
Once again check to ensure that the window has remained square, level and plumb, and that all movable parts and locking mechanisms still operate properly and freely. At this point cut off the protruding ends of the shims and insulate.