Do you look at the house strait on or most often view it from the side?

Thanks snoonyb. I don't see the side profile much. But my bigger concern is the cedar sticking out past the window trim. I'm concerned that will cause water issues. I'd have to build out a trim close to 2" thick and that might look goofy.

Would it work for you to just add a thicker back band around the existing trim? It can be simple, or have a profile. Depends on your taste.

If you are correctly flashed, moisture won't be a concern.

mikelob-
Let's talk terminology. It's possible you bought a bunch of cedar shakes meant for roofing. Cedar shingles are generally thinner and this is to prevent the problems you appear to be encountering is doing. You may not have the right product for the job.

Here is a link that might be useful: Shakes/shingles

kudzu, my thinking exactly as 18" perfections should only have a butt thickness of 1/2" of just under. What do you actually have mikelob as 7/8" tapersawn sound more like a handsplit and are designed for a much larger exposure if used on a sidewall?

I think he knows he bought the "wrong" product (shakes):

"I made a classic mistake of buying a ton of cedar shake..."

And those are indeed usually used for a roof. They can be used for siding, but as he's finding out, the devil is in the details...

Shingles versus shakes, the nomenclature certainly has been blurred over the years. Even by the manufacturers. The 7/8" butt tapersawn perfections can be installed on walls, I even see them spec'd out sometimes.

Some people crave the shadow lines, others think it looks too chunky, especially at eye level. On walls, 18" are usually installed with 7-1/2" exposure.

Good luck with your project.

Minimum if not more with shingles with that thick of a butt.

The toughest detail is the sill since few PVC windows even have a projecting sill and drip and you need a big one.

A big sincere thank you to all of you. This is helpful. I know I should be using 1/2" but I have a huge investment now and I'm going with it. It's encouraging to hear it can be done and if the flashing is proper it will work. I'll mock up a window and see how it looks. Hopefully not too "lumpy" Thanks everyone

If your going to use these thicker butt shingles I would suggest using a larger exposure thus reducing the thickness where they overlap to help eliminate the problem otherwise properly flashing may even be a tough task.

If your going to use these thicker butt shingles I would suggest using a larger exposure thus reducing the thickness where they overlap to help eliminate the problem otherwise properly flashing may even be a tough task.