Doors and Windows - rough opening

I have a 13 year old house with Weathervane windows that need to be replaced.

I am finding out that the sizes do not match standard Andersen, Jeld-wen, pella or marvin.

Rather than get a window that is 3 - 4" smaller I hope to go with one that is a bit larger than my current frame.

If I have to increase my rough opening, how much can I safely increase with out major rework of new header.

generally, the answer would be "none." Headers are usually supported on both sides by a 2x4 jack stud. This stud cannot be removed without compromising the structural support of the header. Thus, window rough openings might be lowered, but not raised or widened. If you plan on changing your R.O's, plan on changing your headers too.

A local lumber yard told me if my new window was 1/2 - 3/4" too large, I should be able to shave 1/4 - 3/8 " inch off each jack stud.

What do you think?

If you're really in a pinch you can do that, but it's certainly not recommended to shave anything off the jack studs. You also need room around your window frame so that it can be shimmed and squared, so you likely need to remove more than what you think.

In such a situation, you might consider removing the jack studs altogether and replacing them with 1x4's. That would gain you 3/4 on each side. It's against code, but if you live in an area where you don't have to worry about codes, that's your easiest solution.

Why not just order replacement windows that are made any size you want? It would be much less headaches and save you money in the long run by saving you labor.

He likely wants to go with a standard size because it's cheaper.

But you're correct that it will save money in the long run to get something that fits correctly. If a wood interior is what you are after, I know that Pella makes custom sizes- they just cost extra.

I think custom sized windows look better once they are installed, because the trim inside and outside will remain symetrical to the rest of the siding. Not having to change the siding, or drywall or paint is well worth the extra charge for a custom sized window.

You are right,
Cost is the reason I want to stay with standard sizes. Pella will size to fit, but there is a steep charge for that.

I am considering Norco made by Jeld-Wen, they will also custom size but it costs aprox 25% more to increase width and another 25% more to increase height.

Anyone have opinions about Norco vs Pella or Marvin.

Thanks,