Shim Trim With Screws
Using screws instead of wood shims is an easier way to hang and adjust preassembled window trim.
Using screws for backing/shims is a great way to hang preassembled window trim. It’s fast and allows you to microadjust quickly. I like to cut my trim so the back of the jam extensions line up perfectly with the outside edge of the window, which means all I have to do is line up the screwheads with the edge of the window.
However, vinyl windows are not always straight (especially if someone else installed them), so I throw a level up to make sure my screws are straight even if the window is not. When installing really wide extension jambs, you may want to use wood shims on the bottom in case someone decides to use it as a chair or set a 100-lb. planter on it.
— James Martin; @detailsmatter
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This tip is good for a number of situations. When drywall is installed horizontally the bottom edge is tapered, which can throw your baseboard miters off. A drywall screw installed near the corner, with the head in plane with the drywall, will correct that problem.
If the rough opening of a window isn't level or it's twisted, installing the stool can be a problem. A few drywall screws can create a level plane for the stool. Likewise if the drywall around a rough door jamb is throwing the casing off. In some situations, low expansion foam or some caulk to give the trim more support is a worthwhile.
This ISN'T a good idea in a structural situation, say a bottom plate or a door sill, where large or long-term forces will be acting. There, grout, several shims or sometimes caulk ( wet shims) is needed. However screws can be useful to establish a plane to work to.