Should we really install window this way?

I’m going to help a couple buddies install a wide bow window in an old opening. They have more experience, but I’m wondering about part of their plan and would like some advice on that.

The wall beneath the opening is 4” thick concrete block, with brick facing. There’s currently a 2” concrete slab sill under the existing window, extending from inner edge of wall to beyond the brick. Plan is to leave that, which sounds fine to me, since most big window replacements in that neighborhood have done that. But the slab is set at a pretty steep pitch, and the window base is flat, and the question is how to deal with that wedge-shaped gap when giving the window solid support over the wall. (They say we’re going to be adding braces and insulation underneath, but I’ll be ignoring that here, for simplicity.)

Their plan is, I hope, clear in the diagram:



Three boards will be added under the window seat, running across the window bottom: A 2X6 sill plate, laid flat on the concrete slab, so it has the same steep pitch as the slab. A thinner, narrower board on top of that, to provide a flat surface for some of the seat near the outer part of the wall. And a narrower board between the wood sill and that added board, to force the board level. Shimming as needed. The plan is to use no bottom fasteners at all to secure the seatboard, aside from some wood screws driven at an angle through the seatboard edge and into the 2X. That way the seat board finish won’t be marred at all, and the screws will be covered by the casing.

Before I heard that, I had assumed we were going to cut thick wedges to fill the gap between concrete and wood sills, and space them periodically perpendicular to the wall. That way the whole 2X6 would be flat and resting on flat-topped wedges (or nearly so, there would still be thin shims needed no doubt), which sounds a lot firmer to me. The way they plan to do it, the seat doesn’t have any true flat base anywhere over the wall, because when you look at the diagram it’s basically boards resting on board corners only. Are just board corners strong enough? (And straight enough? Little strips of board edge corner that have pulled off would change the straightness quite a bit, I’d think.) And it seems like it’d be easier to fit in some insulation using wedges instead of boards. I’d also like to see more seatboard fasteners than just the diagonal wood screws, some nails near the outer edge of the wall.

What would you do?

where your 2x6 is laid flat with a fall, can't you just take a 2x4 and stand it vertical and cut your angle on the bottom to match sill fall? looks like it would do the job and let the front part of the window seat rest on the high side of the sill.

i mean to rip the 2x4 long ways. just rip what height and angle you need all in one cut. this one angled cut should do it and pack the hole that's created with insulation. budro

Thanks, budro. You're talking about not using the 2X6 at all, right? I keep thinking that leaves us without a sill, but the concrete sill is a sill.

see the line you have drawn that runs vertical through the "b" of the word board. let that be the outside part of the 2x4 or 2x6 that will have a long point cut at the bottom. if your window is 2' long it will be 2' long, if 3' it will be 3' long and rest under the sill board for however wide the window is. look at your drawing and see the three pieces of wood you have placed in the area. the blank space to the right whete you have nothing drawn is where this support i am talking about would go. budro

While budro's idea sounds good, it really won't offer any long lasting support.
The weight of the window will cause the bottom edge of the 2x that you angle cut, to kick out and slide on the concrete sill.
There's just nothing there to prevent it from kicking. Unless you add some sort of fastener or edge strip mechanically fastened to the concrete sill on the lower side of the 2x angle.

The main support of a Bow Window is through the mull posts between each of the individual windows that make up the unit. The seat board really only finishes off the inside of the bottom. Adding the boards underneath are just to close off the gap between the concrete sill and the seat board. Your main focus should be how you attach the side jambs to the jack studs in your opening and then supporting the mull post.
New construction window units usually have a nailing flange and then replacement units are screwed through the side jambs to the opening. I would recommend purchasing a cable kit from the window supplier that runs up through the mull post and attaches to the header above the head board. That takes all of the stress off the outside of the bow and prevents it from sagging over time. Some of them can even be adjusted if any settlement occurs. The manufacturer usually has detailed instructions available to go by that will show how it should be properly installed.
Good Luck,
Jim

the person who said my idea won't work probably knows little or nothing about installing windows. i have put replacement windows in for years and stand behind what i said. the window would have to be 10' wide and 30' high to exert enough pressure to "push" the piece of wood i recomended out. put a bead of liquid nail on under the angle cut if you want for good measure. this piece is only a filler. your support will come from how it is nailed or screwed in. the window will also be supported by the concrete sill at the front of the window. when enough pressure is exerted on the front of the window to "crush" this concrete sill, then yes, the piece of wood will come out. read the post from precision above mine. he has offered good advice on support. thanks, budro

Precision is correct. I have installed a number of bow/bay windows and every one I put in had cables to support it from the header. I incorrectly assumed that this was some sort of different window that required support under it.
Don't bother with the supports under it as this is a waste of good time. The cables will do all the work and then you can insulate under the window after installation.

hello budro..
i like your suggestions here..
can u please tell me that how can we install a uPVC window?