Modern "nail-fin" window frames project too far from the sheathing because they are designed to project the same distance as the sills (which means the sills project too little) for reasons only know to their shipping department I suspect. The fact that window manufacturers have never know anything about installing their windows is a bigger problem than ever.

The solution is to add a special PVC "sill nosing" below the short (and often dripless) factory sill and let it extend at the sides under the jamb trim (just like a traditional window). Attach it well to the sheathing and seal it well to the sill.

The jamb and head trim can either be 5/4" thick (a strong inch thick) or 3/4" thick with a thin band molding along the outer perimeter to balance the excessive projection of the window frame.

To avoid this trouble you would need to buy an all-wood window with factory installed flat trim which is not necessarily cheaper and is not as maintenance free (although my wood windows are doing fine after 120 years).

I'm not sure that fiber-cement is a good material for window trim since it seems so difficult to work with, seal, and join. I would use cellular PVC unless the fiber-cement is free and someone highly skilled is installing it.

Here is a link that might be useful: pvc trim

We are using Hardie plank and have used Miratec for the trim. We are actually using Miratec for all the house trim because it is so much easier to work with (so they tell me). The bottom piece directly under the window (sorry but I don't know the correct term for it) is a piece of treated wood that was wrapped in metal with a flange that extends under the very bottom of the window. The top trim piece of the window is 7 inch Miratec with a thin piece of "drip cap" on top of it. Trim on the sides is 4 inch Miratec. Our windows are Marvin (wood on the inside, and I think fiberglass on the outside). Hope this helps some.

MiraTec (from Masonite) is a wood fiber and phenolic resin composite material with zinc borate preservtive and a water repellant.

We have Hardi with MiraTec trim and have been unhappy with the MiraTec.
It is not holding paint as well as the Hardi and the top layer of the MiraTec is peeling off .

lsst, MiraTEC has a 30yr warranty against delamination. You might have gotten some defective product. Check their website.. "during the first five years of the warranty period following installation, CMI will compensate
the Owner for correcting the affected Trim limited to twice the original purchase price of the affected Trim. Starting with the sixth year following installation, the amount
payable (limited to twice the original purchase price) will be reduced by 4% each year until the end of the limited warranty period for the affected Trim". They do recommend using premium paint, which will hopefully help with the color retention.

Thanks stayn2busy,
I will check into it.

The MiraTec warranty is a very poor one and the product relies on "water repellent & zinc borate" treatments to prevent deterioration. It expands in length greater than wood requiring expansion gaps and needs special treatment for cut edges. I can see no advantage over wood or cellular PVC other than cost which is not enough to justify its use IMHO.

If MiraTec composite nominal 1x4 trim boards cost $.65 per lin. ft. and cellular PVC trim boards cost $1.40... for a house with 35 36x60 windows, the additional cost for PVC would be about $335. It makes no sense to me to spend $33,500 on windows and balk at an additional 1% to make the trim as low maintenance and reliable as the windows.

Wood windows used to have the trim attached at the factory and the small cost of it was part of the window cost. Now that trim is a separate line item in a budget, it has become a big deal to spend 2 or 3 times as much for better trim when, in reality, that amount is usually well worth it.

Unfortunately, modern windows are not designed well for trim installation and sometimes require 5/4 stock which increases the cost of trim even more. But I would never let $300 to $500 stand in the way of good trim work; it's not the best place to save that kind of money.

Macv,
Your post explains what probably happened with my MiraTec.
I know there was no treatment done to the mitered corners.The edges are so rough.
I put the problem on my list for the builder to correct during the first year punch list but he thought I was being picky.
Five years later and with the product failing, I wish I had had him redo it.