Toe Nailed Rafters 3

[IsaacStructural]

I've got an existing building, roof slope 9.5/12 in which the existing rafters are toe nailed into the attic floor decking, and possibly the joist below (I haven't been in the space to verify, though I will ASAP). Due to loading changes I'm planning to sister the joists, and I wonder, anyone have a recommendation for a better connection to resist the outward thrust? There is no ridge beam or ridge board, the rafters simple meet directly at each other and I can't see any connection other than bearing. Any suggestions on how to connect these as well? I'm thinking some kind of light gauge clips/straps in both cases, I don't see anything specific in Simpson that will work. Since the slop is fairly steep, the kickout thrust shouldn't be too high.

Thanks for any assistance,

Isaac



M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[IsaacStructural]

And here is the rafter detail.

M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[dhengr]

Isaac:
At the ridge, I would cut a triangular piece of plywd. 9.5/12 & 9.5/12 on the top edges (surfaces) and horiz. on the bottom edge. Nail this across the butted rafter joint and clinch the nails. Then block btwn. the adjacent rafter pairs to maintain spacing. Or, as your photo seems to show, put a continuous 2x4 or 2x6 on the underside of the rafters to maintain spacing. This would do everything that a ridge board does. Although, I have seen what your photo shows before and it seems to work fine as long as the rafters never become misaligned.

At the bottom bearing, it appears the rafter is bearing on the top plate of a wall, and on an unknown floor sheathing. The bearing on the wall top plate is the important one, but I would try to tie all three of them together with an angle connector. Something like a Simpson L90, or something similar might work to span the three and tie them together. Are the rafters right over ceiling joist which act as vert. support and lateral ties? Otherwise, supporting a rafter out at its tip and right under its upper edge is not a good detail. That rafter tends to act like it is notched 2/3s of its depth and it will develop a horiz. shear split up its length and starting from the inside upper corner of the wall top plate. Check to see that this has not happened on any of the rafters. The angle connector should help prevent this. How are you going to ventilate that rafter space? It also looks like it may have been wetted before.

 

[IsaacStructural]

Thanks Dhengr,

It is actually a finished attic space, and the owner is reluctant to let me tear open the walls. It is a chicken and egg problem, I want to open things up and investigate so I can design the proper details to support the new solar panels (yup, more load on this wonderful framing) they want to install, and they want to wait to do the demo until after we have the permit and right before installing the solar panels.

As for attaching with an L90, are you talking about attaching to the broad face of the rafter, essentially in the same spot as the current toe nails?



M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[focuseng]

like dhengr said at the ridge -some kind of gusset.

Then at the heel: If you can convince them to tear into the roof sheathing I have used a standard 2x6 joist hanger laid flat with the "bucket" part on the plumb/heel cut of the rafter. Your picture doesn't have a plumb cut so that is why I bring up cutting the roof sheathing and then create a vertical heel on the rafter end. Then use the face mount 2x6 hanger and fasten into the structure below. Might need some blocking between the floor joists to get something to bite into. Use longer structural wood screws instead of 16d nails to get through the floor deck into joist or blocking below. Maybe this is more invasive than you want but...

______________
MAP

 

[IsaacStructural]

focuseng,

I like that detail in terms of getting the thrust force into a bearing condition, seems very solid, however, I think the client will suffer an aneurysm if we have to expand the scope to include removing and replacing some of the roof structure.

M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[dhengr]

Isaac:
Yes, the L90 or something similar would be nailed to the side of the rafter and then down into the wall top plate and the floor sheathing and ceiling joists if possible. Just wack the heads of the toenails in so the angle connector lays flat. I’ve also used joist hangers the way Focuseng suggests, but that might be tough to accomplish at your detail. Call Simpson or USP and ask them if they don’t have any angles with wider legs, say 3 or 4" and 10" long.

As for the owner, he wants to add the loads, and he wants you to bless it without any structural changes so that he has someone to sue when things go south. Explain that adding loads to a questionable roof system sometimes involves adding more structure too, and some investigation work to uncover latent deficiencies, that except for the grace of God haven’t bitten him in the A$$ yet. Explain to him that you are a Structural Engineer, not a re-insurer of inferior past work. As for the blessings, we should have a new Pope in a week or so and he may want to go see him. A local Priest or a local Rabi are the best dispensers of house blessings, and their fees aren’t as outlandish as ours are.

 

[msquared48]

IssacStructural:


I don't know what the specific framing is, but if you could add Simpson LPT's to either side of the existing rafters, nailed off to the floor sheathing, then add Simpson CMST strapping over the floor sheathing, and nailed off to the closest floor joist below, that could work as a collar tie arrangement. If necessary, to cover the strapping, you could add ne sheathing over the existing.


Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[IsaacStructural]

Thanks Mike, that is kind of my plan at this point. The LPT and L90 are quite similar, so I will select one that works for my load and plan on using either another set of LPT/L90's or straps to attach the sheathing to the floor joists. My plan is to include explicit notes about verifying information in field. If the framing is different than what I'm expecting to be there, obviously we'll have to deal with it at that time and produce a functional detail. Not much else I can do.

M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[Backcheckrage]

Can you install collar ties to remove the thrust?
face nail/screw into the existg rafters to resolve the kick-out load?
Then install a plywood gusset at the "ridge"?