Shed Dormer on Older House

[Simba13]

Afternoon everyone,

We've been asked to design a full shed dormer across the back of an older house. I took a look at the roof, the owner cut a hole so I could see the framing. It's a rafter roof, rafters are 2x6's at 24" O.C. attached to a ridge board/plate. I'm trying to keep the renovation under 30% of the roof area so that the rehabilitation code doesn't kick everything in the house to IBC (I'm sure there would be no end to the number of code violations this house would have if you really dug into it, those rafters I mentioned for starters). But it's a nice family that just wants a bit more light and room upstairs for their kids so I soldier on . I guess the way we would go about this, is that we cut the roof at the edges of the dormer, something would have to be done at the ridge before jacking the roof up and then framing a wall under the edge of the raised roof... Can a ridge board, not beam, even accommodate this kind of thing? We'd be decreasing the pitch of the roof in that area by a significant amount so there are probably some snow load implications. I'll obviously consult my boss before going forward but I wanted to wrap my head around this a bit before I do.

I have an idea for a TV show if anyone wants to capitalize on it, "DIYers do the craziest things"

Thanks,

 

[XR250]

Most likely need a structural ridge across the dormer-ed area with supports down to some floor beams or foundation. These are never fun jobs.

 

[Simba13]

Quick shot of the ridge board in case anyone was curious.

 

[phamENG]

You have to keep in mind how a traditional rafter and tie system works. Gravity loads on the roof are translated into vertical reactions and thrust at the supports. The trust is resisted by the rafter tie (typically a ceiling joist at or very near rafter bearing). Then throw in a collar tie to help minimize destabilizing rotations at the ridge under unbalanced loading conditions (wind and/or snow). It's also best when the rafters are symmetrical, though it's not entirely necessary. If you remove any one of these elements, your roof is no longer stable.

Now lets look at what you're trying to do. You're probably popping this shed dormer out from the ridge or very near it, so you're likely going to have to remove at least one collar tie - probably several. You're raising the bearing elevation of the rafter, but not the rafter tie (which is the floor joist of this space). I'm guessing you'll have a sloped ceiling affixed to the bottom of the rafter. So you no longer have a tie to take the thrust. So you've probably destabilized the roof framing in this area. The most effective way of getting that stability back while also keeping the desired dormer and open space is to do as XR250 mentioned - ridge beam with posts and/or transfer framing to get the loads to the foundation.

 

[Simba13]

This is how I'm envisioning it in my mind, they want it to go across almost the entire house. Please excuse my drawing skills btw.

You're right phamENG, I wasn't thinking about the thrust. I guess I was thinking that the roof would be flat here and it would just be a bearing condition, but there certainly is a slope even if it's a smaller one. There is actually a bearing wall right below the edge of where they want the dormer, the house was previously extended so this used to be the edge of the house. I agree with the ridge beam idea XR250, but correct me if I'm wrong, even if you support it at the ridge with a beam, there is still the thrust to contend with right? How do you usually deal with thrust in this situation?

 

[phamENG]

You're right that there is still some thrust, but it's more of a second order condition and is dependent on the rigidity (or more accurately the flexibility) of the ridge beam. As long as your ridge meets code required deflection criteria, you can typically neglect it in a house. Unless they're going to put horse hair plaster on the wall and hire a renaissance painter to put up a mural. But that's probably not likely.