I have a ‘70’s era home in Canada with cedar beveled siding. Original wood windows are rotting. I need full frame replacement Windows.
I removed the interior trim and the rough opening is 30 by 48. They are single hung windows. The cedar siding butts up against the exterior window frame. The exterior window frame measures about 1/2 inch wider than the rough opening.
I would like to avoid cutting back the siding if possible. Can I build up the rough opening with 2x and then use new construction windows with a nailing flange and then put some cedar trim around? Or am I missing something?
I assume I will need custom sized window as there is nothing close enough from the stores. HD has one window that measures 30 by 48 but requires a larger RO

 

This is the situation for which replacement windows were designed. Am I missing something in your desire not to use them?

 

This is where you have to make some decisions.

Possibly can do as noted, replacement windows but going that route you will have a less than perfect install.

New construction with nailing flanges, cutting back the siding, installing proper flashing is then possible and much better/complete construction. Pluss the new cedar trim will give you a little more detailed look, it's the option I have always done, when available.

 

Maybe my ignorance.
I thought replacement windows fit inside the original frame. My frames and sills are rotting. Is there a replacement window with frame and all? Can they be installed without a nailing flange? Do they screw into the sides instead of the exterior?

 

Those are called inserts, I have never had any experiance with them.

Sounds like you need complete windows, either new construction (with nail fin) or replacements (mount inside of rough opening)!

 

Yes you can. Ideally you would measure your window brickmould width and subtract 3 1/8, which would be 1 1/2" per side for the new nail flange plus a little wiggle room. For your height, subtract about 2" , which would be 1 1/2" on top for your nail flange, and 1/2 on bottom because you have a sloped sill on bottom instead of a nail flange.

You WILL have to trim back your siding on bottom, which is quite easy to do with a multi tool. When you install the new window, take care to center it perfectly, use some 1 1/2" scraps of trim to space the exterior perimeter correctly before you fasten the nailing fin so that your trim will fit.

After the window is in... you would then just add a 1 1/2" trim to cover the nail fin. This will result in a smaller window, and you will need new trim, and that trim won't cover the old paint lines inside unless you get wider casing. Another trick that sometimes helps is to make your jamb bigger than the window, then add another stop inside that, and a baaseshoe of quarter round inside that. My standard way to trim vinyl windows is a jamb plus baseshoe... the baseshoe hides any gaps where to jamb meets the window.

With some windows you can also get jambs that come attached to the new window but they are often a problem or arrive damaged.

As you mentioned, you don't want to put insert (also called retrofit or pocket replacements) into an old rotten frame.

You could also drop your opposition to trimming the siding back, in order to just trim another 3/4 - 1" or so off the sides. It's not that hard, you just have to be precise. But there IS a limit to how much you can trim back because the new end of your siding has to be renailed (to the studs). So for example, you cannot trim 3 1/2" off each side to use 1x4 trim, or your siding won't be nailed to the studs anymore. A big problem when you don't have wood sheathing behind.

When you are going between existing wood siding, it's pretty hard to use any flashing tape, so you would want to caulk behind your nailing fins instead. Try to put a new z flashing above the top window trim (slip it behind the siding on top before you put the window in). And you would need to caulk your trim to the new window and caulk the outside perimeter of the trim to the siding.