Supporting a large bow window in wall with cathedral ceiling

I have a question about the strength of a wall which a large bow window is supposed to go into and about how to support the bow window. The original double pane window is flat, with one square picture and two casements.

Here is the senario:
1. Bow window; fiberglass, 126" long x 66' high, 5-section of which 3 are pictures (ends and center) and 2 are casements.
2. Wall is an END wall of a room with a cathedral ceiling. Lannon stone is on the exterior below the window. I don't know the interior structure of the wall.
3. Installer said the window will be attached via the two side jams as well as the bottom jam and...
4. Supporting the cantilevered bow will be cables (they don't do chains) attached to the studs of the wall above the window and...
5. NO knee braces will be used.

A different remodeler said the wall/ceiling arrangement of a cathedral is not designed to support the cantilevered bow and thus it would flex and the bow would sage and the casement panes would then jam. I have also heard that the CABLING tends to stretch, requiring additional adjustment in the future. I have been told a chain supporting system will not stretch!

In your opinion(s), is there a strength issue with the end wall of a room with a cathedral ceiling if a large bow window is installed into it?

Do the cables stretch, requiring adjustment in the future?

If it is installed, should I demand both cable (or chain) supports and knee braces?

Any other comments would be greatly appreciated... since this window will cost $10,000.

1 no
2 ussually
3 I would
The knee braces if installed correctly will giv ethe most support. The unit should be set a touch high 1/8", to adjust to just a pinch of settling.

I would certainly prefer knee braces as well, as that's one very large bow window.

I agree with both comments above. We often install bays/bows without knee braces (only chains or cables), but I'd suggest both in your case.

Most manufacturers require one or the other for warranty purposes. What is the wall thickness, depth of the seatboard, and projection of the unit beyond the outside wall?
All of those factors should be considered in determining how much support would be needed. I would use knee bracing and allow for settling as Anthony suggested.

Does the one contractor not want to tie the knee braces into the stone for some reason? Or, is it just not his normal practice with a cable support system? On a bow that size if you don't mind the appearance of the knee braces, I think it's a good idea if the braces are done tastefully. Many guys do the pre-finished braces while others will fabricate them on site. What I don't prefer appearance wise is when the guys just angle a 2" X 4" and wrap it in PVC/aluminum and leave it hollow. It does the job for support, but usually doesn't do an expensive projected window justice cosmetically.

If I fab the knee braces I use 2 x4's to construct L , then double 2 x 4 diagonal, Then cap the whole thing solid. When secured properly this will hold an elephant.
Pella/Gunton used to supply a nice 2 x 4 and scrolled small or med knee brace. The large knee brace was not scrolled.

The way that Anthony fabs them looks nice and is what I prefer when I'm not utilizing pre-finished ones. It takes a little more time and skill which is why a lot of the speed installers don't do them this way.

2x4 wall or 2x6?

Most cases will not require any special precautions beyond the turnbuckles and knee braces, however, that is a bit window.

I am not entirely comfortable with that arrangement and no knee bracing.

Thank you Anthony for your reply.
Are you saying that when the braces are installed they should be 1/8" lower than the window... so that shims would be used between the window and the brace to fill the gap and allow adjustment?

Wayne is referring to having the knee braces set a tiny bit higher so that as they settle out under the weight of the bow that they finish level.

Have the floor of the bow set about 1/8th bubble up on the level.

Thank you Ellwood for your reply.
To answer your questions,
- Wall thickness is 4".
- The salesman said the seat board would be, oh, about 18". That was ONLY his guess.
- The salesman said the projection "would be out to about here" as he streched out his hand to show where.

I will demand for knee bracing for the extra support.

Thank you Mike for your reply.
I don't know why he doesn't want to add knee braces. Funny thing though, the instructions for the Marvin Cable Support System says "The Cable Support System provides adjustable support for projecting window units when used in conjunction with platform boards, structural support members of the building and support brackets. With proper assembly and installation, the cable support system will help reduce the weight carried by the platforms. A maximum load of 500 pounds per pair of cables is allowed with additional support required for assemblies that exceed a 12″ projection." Note that it says "when used in conjunction with"...

Thanks for the tips about making the knee braces. I do want braces that are strong and look good too.

Thanks Anthony for these tips.
I work at a metal fab. shop. (draftsman/designer) and am thinking about possibly making some knee braces out of stainless steel.

Thank you Windows on Washington.
The wall is 2x4.
From the replys I'm getting it looks like knee braces would be an additional requirement besides the cables/chains for this big of a bow window... just wish the installer had the same intuition/knowledge as the rest of you!