framingwindows
I'm looking to build an outdoor shed with a lean-to roof, and I'd like to have windows all along the top right beneath the roof, like this picture:
What additional engineering considerations are required for something like this? Do special windows need to be used? I don't see any room for a header, and there doesn't look to be enough space between the windows for more than one trimmer.
No, no and again NO. Not the way you suggest it, anyway.
This is an exterior wall. It is therefore a load bearing wall. Those two 2x4s (called jack studs) support the header over the window.
The weight of the rest of the house above the window is protected by the header, which in turn transfers the force to the two jack studs (as in they jack up the house) carry that weight to the underlying structure. If you remove one of the jack studs, too much force may be transferred to the other one, causing it to bow or even possibly fail. Either scenario translates to having weight of the wall above the house coming to rest on the window itself - and that is a BAD THING (TM).
If you wish to restructure for a larger opening properly, you must open the wall up further and relocate the king and jack studs, and install a longer (and possibly wider) header as well. Otherwise you can cause many tens of thousands of dollars of damage to your house.
Do not fiddle lightly with structure.
Yes, there is a rule-of-thumb for determining shear walls and how wide windows should be to have the posts between the windows transfer the loads adequately to the foundation.
You're in one of the highest rated seismic areas in the U.S. Critical loads are moving the building up-down and side-to-side. We try to tie the roof to the walls and the walls to the foundation for vertical and horizontal loads.
I don't know the width of your house (I'll explain why that's important later), but as a rule-of-thumb, for seismic loads, you need a minimum of 4' of shearwall at each corner (before any windows) and you want no more than 60% of the length of the wall for window / door area. This allows you about 40% of the wall to tie to the roof and foundation.
For your 25' long wall, without structural calculations, I'd use : 25' x 40% = 10' of shear wall (Sorry, I know you want about 20' of window area.)
As a rule-of-thumb, the house length to width needs to be no greater than 1:3. So, if the 25' is the width, then the length should not be greater than 75'. Likewise, if the length is 25', then the width will be no greater than 25' (1:1) and no less than 8'-4"...you don't want it too skinny either.
Now the width of windows: As a rule-of-thumb, the width of each window should not be greater than twice the height of the foundation wall (stem wall plus depth of footing). So, if the stem wall is 18" high and the footing is 6" thick, then the windows should not be any wider than 4'. (As a rule-of-thumb, loads transfer from the wood posts between the windows to the concrete foundation at a 45 degree angle. So you don't want to be further apart than what can be transferred through the foundation to the ground.)
Yes, you are correct, Simpson has some great holddowns and pre-made shearwalls that we often use for garages because the garage doors exceed the 40-60% rule-of-thumb. However, they're expensive and you'll need to calculate the tributary loads. Keeping your house fairly rectangular, installing 4' shearwalls at all corners and keeping window widths about twice the depth of the foundation, you'll be safe.
By the way, use 1/2" minimum plywood (not OSB board for wall and roof sheathing) and nail at all supports with 10d at 4" o.c. at all supports...roof and wall. (If you nail any closer, you'll split the studs and have no resistance.) This all assumes the shearwalls are no less than about 1:2. If you have a vaulted 16' high wall, then the corner shearwall needs to be about 8' wide.
If these rules-of-thumbs don't work for you, you'll need to get it calculated. I'd get a hold of a structural engineer (NOT a civil engineer) in your area. Tell him you just want the calculations, not the plans. It'll cost you about 4 hours of his time (assuming he's got the software) and you can install it as per his design.
Best Answer
This answer is pretty much the same as Jasens, I elaborated a little bit. Each post, at the corners and between the windows are supporting a rafter, sized properly for the roof load. Set between those rafters are purlins which make up the rest of the support for the roofing. The roofing is attached to the purlins.
So to answer you first question, as long as you follow this post and rafter example, you could have as many as you prefer as long as you have enough wall space for racking resistance.
In other words, don't make the windows too tall.
I added a pic to show what is going on with the roof.