new window and layers of old siding

forresth

I picked up a huge 6' x 10' 5 pane new construction window used and am putting it into my house (much more used) with a couple of layers of siding. I have removed basically everything inside the sheeting lumber to rebuild it for several inches to either side of the planned window opening. Outside the 1" sheeting, there is a layer of 3/4" thick and large asphalt shingles, then aluminum siding with some sort of thin particle boards slid in as backers.
I figure just cut off the side flanges. The bottom flange is pretty much broken from being set upright. I can reuse the top flange with the beam I cut to go above the window. I figure I can get trim pieces similar to what surrounds other opening in the aluminum siding.
Does all that sound right?
Is there anything I need to do to make sure it is all water tight?

Just Bill

Caulk the flange where it meets the house. Then install self stick flashing(6") over the bottom flange. Then up both side flanges, overlapping the bottom. Then across the top overlapping the sides. You will also need a metal flashing the goes over the edge of the window and up under the housewrap/felt. Install your exterior casing over that, caulking at the joint. New siding up to the casing and caulk.

Windows on Wash

+1

Only think I might add would be a layer of a flexible sill pan flashing prior to setting the window (if you can get it in there).

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forresth

I can do anything for structure I want. I tore all the existing out since the window is replacing a door that went full height in one section, and I found rot below all of it. For the header, I am putting in a 3 1/2 x 8" beam I cut from a standing dead hemlock. It will be supported by a pair of pressure treated 4x4s that will be sitting on the foundation. I was planning to use the 4x4s as the side structures. The bottom will just be built up how ever high I need to to make it all fit.

I don't think getting side flanges under the siding and behind the felt (if there is any) is a real option. this isn't flimsy vinyl siding, it thick asphalt shingles, boards and aluminum siding. and the window is real heavy to be trying any real gracefull moves with. I probably wont be able to get very far with flashing either with all the nails. I suppose I could run a sawzall behind the siding though, but there would be no real way to replace what I cut.