I'd like to include a half-hex bay (not a cantilevered bay window, but a full projection from foundation to second story roof). Still weighing the alternatives of AAC vs. ICF, so the question applies to both: how far from an inside 45 degree corner should the window opening be?

It appears to me that in 2x6 stud wall construction, the width of a window in a bay is limited primarily by the width of one common stud that forms the corner, plus a jack stud on each side (looking from the inside out, of course). That allows for pretty tight window spacing.

With ICF or AAC, should the bay corners be wider? I realize both products are going to be strengthened with vertical rebar (encased in a 3" grout columm for AAC). Is there a rule of thumb that says "windows should be ___ far from any corner"? I don't mind going with narrower windows, but I do want aesthetically appealing fenestration. (been waiting years to use that word...)

One option with ICF is to just frame out the bay window area with standard lumber. You aren't gaining much by having small wedges of ICF wall between the bay windows and it makes it difficult to install the ICF and difficult to consolidate the pour in such small spaces.  By the time you installed the window bucks for each opening, you would have very little room left for concrete unless you had large sections of wall between the windows which kind of defeats the purpose of having bay windows.

With ICF, the general rule in the prescriptive method is to have 2' of horizontal rebar extended on either side of a door or window opening. If the window happens to be within say 1' of a wall corner, that 2' of rebar can bend and wrap around the corner. With bay windows or anywhere you have multiple openings in the wall closely spaced, you would just extend one continuous piece (or multiple pieces depending on the spans and loads) of horizontal rebar across the openings and then extend at least 2' on either side of the outermost opening.

AAC requires 3 inch concrete cores flanking windows and doors. Add a minimum two inches to each side gets you to 7 inches. Add 3 inches more for 2x bucks on either side, and you arrive at 10 inches for the thinnest outboard column. One dimension would have to be more in order to trim an inside face to 45 degrees. In a second approach, you could use steel posts for the outboard columns and then box them in AAC to save a couple inches. The bucks would have to bolted to the post. In either case, it is super easy to trim the bucks with AAC so the whole thing looks like masonry.

Lintels are a separate issue. The normal AAC kind require an 8 inch bearing surface on the block below. That would require a 14ish inch column in an L shape. Narrower columns would require a bond beam lintel, using U blocks, where the concrete and steel would run from the walls around the bay window in a continuous beam.

I hope you realize that you'll need a structural engineer's stamp for either AAC or ICF. You might ask him/her about using pressure treated posts (PVC posts?) boxed in AAC. I did that between a pair of 6-foot patio sliders in a walkway, and the column is only 8 inches wide. But the bond beam lintel in my application is structurally sound without the post.

It would be a dramatic touch if you could step into the window and look 180 degrees. Code would require tempered glass if you take the windows to the floor.

Posted By AAC-curious on 23 Apr 2013 10:49 PM
I'd like to include a half-hex bay (not a cantilevered bay window, but a full projection from foundation to second story roof). Still weighing the alternatives of AAC vs. ICF, so the question applies to both: how far from an inside 45 degree corner should the window opening be?

It appears to me that in 2x6 stud wall construction, the width of a window in a bay is limited primarily by the width of one common stud that forms the corner, plus a jack stud on each side (looking from the inside out, of course). That allows for pretty tight window spacing.

With ICF or AAC, should the bay corners be wider? I realize both products are going to be strengthened with vertical rebar (encased in a 3" grout columm for AAC). Is there a rule of thumb that says "windows should be ___ far from any corner"? I don't mind going with narrower windows, but I do want aesthetically appealing fenestration. (been waiting years to use that word...)

ACC - If you are using ICF with a 45º block, structurally it doesn't really matter as you can get the rebar etc that you need into the straight portion of the wall. The big think is as you say "aesthetically appealing". You need to make sure that on the outside there is enough room for the brick mould or what ever finishing element you have on the window to be far enough away from the siding on the straight section that it doesn't look cramped. i.e. it is usually nice to have the siding rotate around the 45 before it hits the window. You don't want to wind up with just 1/2" or so of siding coming around. Also if there isn't enough room you will have some issues with sealing the edge of the window.
If you are using corner details you need to move the window farther away from the straight wall. Also if you are using a brick or stone veneer.
Take a minute to sketch out the intersection and you will soon see what looks appropriate. Also consider how far out the bay will go. Does it need to stay under the existing soffit or will it have a roof of its own?

Posted By toddm on 24 Apr 2013 10:29 AM


I hope you realize that you'll need a structural engineer's stamp for either AAC or ICF.

Interesting comment. I know we do not need an engineers stamp on our ICF if we stay with in the manufactures spec's. The AHJ reserves the right to question any lintels that appear questionable but as a rule we don't stamp a standard type residential construction. This is possibly because our codes treat ICF the same as any other concrete or masonary structure and addresses it in each section of the code that pertains to to it i.e. when the code spells out prescriptively how wide a footing should be it includes ICF.
Wonder how many AHJ do require stamps??

Dunno what an AHJ is. Must be a Canadian thing. In the US, if it isn't prescriibed by the building code then it takes an engineering stamp to get a building permit. Correct me if I am wrong.

Got to thinking, hey there is pressure treated behind every window and door in most concrete houses anyway, so why not do it with PT post and beam? I have 10 windows in a row. I built a stub wall, bolted a PT plate to it and then switched to post and beam, using my wood trim to finish it outside. You can also a bay window sold as a unit with mullions joining the three.

I am assuming you want stucco because either of the above would be easier and cleaner looking than siding. With AAC, you box the beams using thinset and a few screws to hold it in place and apply stucco directly. With ICF, you box it with foam and then wire mesh. You'd want a brake to get crisp corners on the mesh. In either case I'd use chair brackets to keep the posts up off the concrete even though they are PT.

But again, you need to lay out what you want and let an engineer tell you how to build it.

Posted By toddm on 24 Apr 2013 01:03 PM
Dunno what an AHJ is. Must be a Canadian thing. In the US, if it isn't prescribed by the building code then it takes an engineering stamp to get a building permit. Correct me if I am wrong.

Authority having Jurisdiction.
That's what I was trying to say. It is Prescribed in the Canadian Codes so I'm a little surprise that it would not be in other jurisdictions. ICF is pretty mainstream.