Existing Tied Roof

[L212]

I'm working on a reno/addition to an existing residence with a tied roof. There's a compression "ring" (square) at the top and then the attic floor laps with the roof framing at the bottom. My thought process was that we should either leave the entire existing roof as is and not disrupt it at all (or else the entire thing has to become structural instead of tied. Which would result in posts where they're not convenient and members that need to be reinforced) or take the whole roof off and redesign. If we leave the roof as is, I was thinking we could connect to the new portion (which will be going on the top of the page with how the image is oriented) by taking off part of the hip at the top dormer and using that as a bit of a hallway, without really breaking the current system. In the (very basic) plan attached, the gray is the existing roof framing, blue is attic (mostly hidden in roof other than the dormers) and green is 2nd floor walls. What they really want to do is cut back the one dashed valley to make a room more usable in that corner. I just want to make sure I'm looking at all my options. I was trying to see if I can place bent steel plates anywhere to open that corner and resist the loads without needing to design the whole existing roof as structural, but I can't see a good way to do it. Any ideas? Or is it more of a: don't touch it or make the whole thing structural or rip the whole thing off as the only solutions?

 

[lexpatrie]

I rather doubt this kind of framing will "calculate out" as-is. In reality these things are oddly strong compared to rational computation based on design standards, meaning they tend to fail on dead load alone. While you can throw in some connectors where needed, they tend to be shallow framing so there's not enough space for the connectors, and the framing is also usually overstressed at some level in the load combinations.

I haven't looked at it too closely, this is more based on my experience with similar "flat center" roofs with hip rafters from say, the early 1900s - 1950s.

 

[phamENG]

Step 1 is to analyze the roof and determine the loads in the members. You're not checking to see if the framing is adequate or not, just checking to see what all the forces, moments, and shears are everywhere.

Step 2 is to analyze the roof as you want it to be. Find those forces, moments, and shears.

Step 3 is to compare the results of step 1 and step 2. How much are the load effects in the existing framing changing? If the change in forces, moments, and shears (specifically at the location of the peaks!) are negligible, the framing is in good shape, and you don't see anything that worries you, then you can probably leave it as is. If the changes are significant, then make the necessary adjustments/reinforcements/etc.

Just because it doesn't fit nicely into one of two categories doesn't mean it's not possible. It's standing now, so it's possible. You just have to figure out if it's reliable enough to let somebody live underneath it.

 

[XR250]

I wouldn't touch the thing.