Framing rafters over existing trusses for cathedral ceiling

[Jc67roch]

I have an interesting project where the owner and architect are requesting that an existing truss framed roof be made open - like a cathedral ceiling - with the trusses (and associated bracings) and roof sheathing underside to remain and be painted on the interior. In order to get the code required insulation in the roof, we are looking to lay some form of insulation on top of the existing roof sheathing and create a SIP. However, when I checked the existing trusses for loading, they fail as now all load is on the top chord and not shared with the bottom chord as would be the case with a conventional flat ceiling at the bottom chord. Rather than reinforcing (sistering) the top chord of the truss, we are looking to frame rafters over the top of the existing roof. I have designed these to be structural, conservatively assuming no contribution of support from the trusses. I found that 2x6 rafters @ 16" o.c. are sufficient and this is likely a minimum thickness of insulation we would need anyway. However, it seems that this plan would require good connection of the rafters to the trusses in order for the trusses to act as ties to provide the necessary tension/kickout resistance at the rafter bottoms to prevent wall spread. Any thoughts on how this could be accomplished? I am thinking of either Simpson framing angles fastening the rafter ends down thru the wall top plate. OR simply toe nail the rafters into the top chord of the trusses? Or maybe fasten a sleeper over the top of the wall, on the existing roof sheathing and perpendicular to the rafters, and notching each rafter base to fit over it to act like a cleat? Any input on this design concept would be appreciated. A sketch is attached. Or premise is that framing in this manner, rather than down under the existing roof sheathing, also prevents notching gypboard or other finishes around all the truss diagonal members.

 

[XR250]

Assuming standard shingles, seems like your TC dead load is about 8 PSF. Does not seem like an issue for most truss designs.

OSB x 2 = 3 psf
1/2" sheetrock . - 1.5 psf
Shingles 2 psf
Insulation 1.5 psf (guess)

 

[kipfoot]

Or maybe fasten a sleeper over the top of the wall,

I like this idea, with a long screw through the sleeper and into the truss.

But, I also think that if a new 2x is aligned over the truss top chord these could act compositely, perhaps enough to get the reinforced top chord to work.

 

[XR250]

Another thing to consider is that you have effectively eliminated storage loads which will reduce the compressive force in the top chord.

 

[Jc67roch]

Thank you both. The existing trusses are open (its current a storage barn being converted to a residence). I think that is the reason the trusses failed also - they were not designed for any storage load, and maybe not any ceiling either.

 

[HouseBoy]

OP, you write "they fail as now all load is on the top chord and not shared with the bottom chord as would be the case with a conventional flat ceiling at the bottom chord"

I'm not sure I follow you on that one. I assume that when you write that they want the bottom to be "open", you mean no ceiling and the trusses are exposed on the inside. Presumably the truss itself will remain intact. In that instance, the bottom chord does not have normal load on it but it is loaded ..... by the ordinary truss behavior it will be in tension. The load that you seem to be referring to is not, not shared, it is eliminated. I'm not clear on why THAT would cause the top chord to fail.

In order for composite action to occur, I would expect that shearflow must be accommodated in the way that the new top piece is connected to the existing top chord. That doesn't seem like an easy thing to do. Maybe the fact that the top chord is continuous over the panel points will help. Although your sketch doesn't suggest they are continuous, I would expect them to be. Curious that they are 16" o.c. also. That might be a regional thing.

If a 2x6 "rafter" is installed over the top of the existing top chord (assuming 2x4), I would expect the rafter piece to be stiffer and therefore the load is more readily transferred into the panel points. Seems liek that would help the top chord issue somewhat. Again, maybe I am not understanding the terminology.