Sizing The Keystone for Your Window Trim

Keystones undoubtedly add some character to your window trim, but some care does need to be taken when selecting which one to have installed. A keystone that is too small won’t fit properly into the trim, leaving a portion of it exposed. A keystone that is too large (as seen below) on the other hand, can be disproportionately large and also look out of place.

A keystone that is obviously too large for the trim that "contains" it.

The key question (get it?) then is, how do you select the right size? As a rule of thumb, keystones should be 2-3″ larger than the trim they’re being fit into. This becomes difficult with smaller window trims that are only 4″ in size like Trim 001, and in such cases I have seen contractors actually cut the bottom 3″ off a keystone so that it is height proportionate. Don’t forget to re-apply the cementitious coating after doing this.


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