Scabbed Roof Rafters 1

[zero1238]

Completed an inspection on a residential hip roof that had five 2x6 roof rafters on each side scabbed with short 2x6 sister members. The original rafters are 16' long but several of them were either spliced with a scab plate or broken at midspan by a scab plate. In a perfect world, I would have them remove the scab members and just sister all 10 rafters with full length 2x6's but there is no way to get 16 foot pieces up there. Any thoughts on making this more secure/reinforced? Each rafter scab had minimal fasteners (two 1/2" diameter thru bolts and some nails, max).

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[XR250]

Realistically, that is likely the least of the problems this roof has.
Maybe try to post them down to ceiling joists.

 

[RPGs]

Would sliding full-length sisters up through the soffit at the eave be possible? I've seen it done that way before.

 

[zero1238]

Posting down is going to be difficult given the floor plan below. And you can't slide full length sisters through the soffit because there are no soffits on this house.

 

[XR250]

Posting down is going to be difficult given the floor plan below. And you can't slide full length sisters through the soffit because there are no soffits on this house.

Have you checked to see if the ceiling joists have the capacity to support the rafter load? You can try spreading the load out with an up-set beam if possible. You really only need belt and suspenders here as it appears the roof has been like this for a long time and hasn't failed.

 

[zero1238]

Not sure what you mean by up-set beam. Can you clarify?

 

[XR250]

Beam sitting on top of the ceiling joists to distribute the load. Sometimes called a "strongback"

 

[zero1238]

Oh yeah, I understand now. That could potentially work along the right side where this is occuring but not the left side. The left side has an area of the ceiling that is vaulted and framed up inside the attic so that might be a little more difficult.

 

[dvd]

It may be possible to convert rafters into scissor trusses inside of that space.

 

[SWComposites]

so ... what is driving the need to do anything? while those rafters look rather hinky, I should have kept photos of the 2x4 rafters in the 1914 house I used to own in Seattle; some of those would have really freaked you all out.

 

[zero1238]

To dvd's point, I can't create a truss scenario because it's a hip roof with a chimney in the middle.

To SWComposites, if my name is going to approve this, I'm going to make sure, without a shroud of doubt, it's not going to sag. I think there is a potential for that happening here because none of the side rafters run full length without scabbing.

 

[SWComposites]

ok, so the homeowner wants you to approve the condition? or a fix?

there is some sort of vent on the roof; can that be removed to allow full length rafters to be inserted?

 

[zero1238]

He wants to know that it is structurally sound.

 

[KootK]

A lot of these old, conventional framed hips wind up working via a combination tied arch behavior and sort of a folded plate shell action that relies on the in plane stiffness of the sheathing. That tends to put your rafters into compression significantly. And your offending splices would actually tend to be quite capable in compression (butt joints).

I'd be inclined to tell the owner that the roof is fine as is but will be quite sensitive to future modifications that might upset the system behavior that makes it work. So any significant alterations should be reviewed by and engineer (new dormers, sheathing removal, etc).

If those splices were getting overloaded in bending, I would expect to see some distress in them such as splitting at the fasteners.

 

[zero1238]

@KootK - You make a good point. I didn't think about it that way. Thanks!

And thank you to everyone else for responding to this post.

 

[XR250]

To SWComposites, if my name is going to approve this, I'm going to make sure, without a shroud of doubt, it's not going to sag.

You can't make sure of anything without a shred of doubt.
Sounds like an unrealistic project.

 

[StrEng007]

folded plate shell action that relies on the in plane stiffness of the sheathing

Is this older style plank (or board) sheathing just as much of a candidate for this type of analysis? I learned in the era of sheet sheathing, so forgive my ignorance.

Also, given the age of the roof, I would have expected to see signs of re-roofing at this point. From the inside surface here, moving towards the outside of structure, what would be the typical finish system? I see no signs for new nailing. I've been trying to familiarize myself with architectural systems...

 

[lexpatrie]

There are things out there that do not yield themselves to analysis via generally accepted principles of mechanics. Applying rational analysis will produce large overstresses (even with stronger historical wood) and/or connection forces that cannot be reasonably resisted in the space for a connection.

These kinds of roofs, broadly speaking, tend to occupy that category. There's some kind of load path, and it's held whatever loads it's experienced but that doesn't make it "safe" and it's unlikely to meet the current (or legacy) framing codes, let alone the difficulty of determining nailing after the fact with limited access.

 

[KootK]

s this older style plank (or board) sheathing just as much of a candidate for this type of analysis?

I would say that it is a candidate for that type of analysis but less so than panel sheathed systems would be. If the deck can resist shear, it is to some degree a candidate and it's just a matter of degree. Roof boards can resist some diaphragm shear in the same way that patio deck boards can. Just much less than panel sheathing.

From the inside surface here, moving towards the outside of structure, what would be the typical finish system?

I will abstain from speaking to that since the answer can vary based on region and build age. Most residential roofing systems are quite light. The roofing shown in the photo appears to be in pretty great condition so it would not surprise me in the least if it's been re-roofed at some point.

 

[KootK]

Is this older style plank (or board) sheathing just as much of a candidate for this type of analysis?

One thing that I like about OP's roof is that the hip is nearly square in plan. Those tend to do well be cause the best folded plate action occurs at the ridge lines as a result of the roof plane geometry. Roof diaphragms on square hips tend to behave almost like a circular shell structure with a tension ring. If the roof were more rectangular in plan, I would be more reluctant to employ such arguments.