In this October 2012 thread, contractors provide a variety of solutions for building up exterior trim to better complement the thickness of cedar shingles.
I have used plenty of 5/4 trim with shingle siding, but would rather use thicker trim. I was thinking of going with 4/4 material (3/4" net) boosted out with some scrap 7/16" OSB. With this arrangement, my casings … would be about 1/4" proud of the shingle butts. What do you think? — IamTheWalrus
I’ve done buildups like this before. Normally rip cedar instead of using OSB. — tjbnwi, Northwest Indiana
Bad idea. OSB swells and rots like crazy. Good idea: Space 1/4" plastic shims along the back of your trim pieces and tack them in place with stainless staples. Orient them so that the inside 1" of your trim does not have any shims on it ... Presto, you have padded out your trim, made a rain-screen detail, and corrected for the material buildup on window flanges. — NW Architect, Portland, Ore.
We commonly use 1/2" treated plywood as a packer behind window surrounds. — Nate E, New Jersey
I would add strips at the edges only. This gives the finish piece of trim the ability to dry on the back side. — Calvert, Dallas, Pa.
We used to rip one side of the 1x4 corner trim down to 23/4" so that the corner was 3 1/2" wide on both sides once you butt the ripped piece to the full piece. Then we would [rip the waste] into two equal pieces and use that as a buildout shim. All of the jobs we did were lapped siding that would come out with beat-up 1x cedar on the bottom for packaging, so we made use of that stock as well. — Dlhunter, Chester County, Pa.