jacking out deflection at ridge line and rafter midspan

I am in need of some guidance regarding repairs/renovations to an existing wood framed house. This house has a complex roof layout with several different spans and heights. One section of the roof is noticeably sagging along the ridge line and there is a sag in the rafters at midspan. The specs are - 2x8 rafters @ 16" o.c. w/30'-2" house width, roof slope is 8:12, there are 1x6 collar ties @ 5'-5" above the ceiling on every other rafter set. There is no ridge beam only a 2x10 ridge board.

The owner wants the deflection taken out as well as the structure reinforced to meet code. I would like to propose a way to concurrently jack up the ridge line and mid span of the rafters (maximum 1/4" per day w/screw jacks). Is this the best approach or should I consider jacking in phases (ridge line first then rafter midspan second).

My concerns with jacking are the rafter/ridge connection separating as we push upwards and the rafter/exterior wall connection separating causing possible cracks, leaks or crushing of wood members.

Any thoughts?

Cheers!

 

[msquared48]

So the collar ties are on every other rafter and are about halfway up to the ridge. The collar ties are probably pulling the rafters inward causing their deflection.

As for the deflection in the ridge, probably due to lateral spreading of the exterior walls.

You are not going to solve the problem by working with what is there, you need to adjust the method of framing.

You need deeper rafters (2X10's or 2"12s) so they do not deflect as much vertically, and you need to lower the collar ties. I would use one tie on each rafter.

I know the owner will not want to hear this, but this is the problem.

Mike McCann
MMC Engineering

 

[dhengr]

Dupy23:
You need a lot more info. and detail all around, before you start to unravel this mess. Show us actual connection details of the rafters, to scale, at the ridge, at the collar ties and at the exterior walls. Maybe photos too. Do they show any movement, consistent gaps in the framing which indicate the settlement/deflection you are seeing. Are the exterior walls plumb? Are there other parts of this complex roof framing into or onto the area which show the worst deflection? What kind of tension ties do you have at the attic fl., show those and their distress in your sketches and photos. Show a roof framing plan of the area, so we/you can start to understand how this roof works structurally. Material species and grades for the various members?

These are all things, bits of info. I would need to gather before I could determine where to start, and you will have to do this too. You are right to be concerned that you could pull thing apart in your jacking effort, so you have to watch that as you go. And, you also have to determine what discrete jacking reaction points you can use, and then apply that load to three or four rafters at a time.

 

Okay, here are all the details.

I have had a colleague working on this, and this is his first try it. It strikes me from the start that the vertical deflection at the ridge probably cannot be completely attributed to the deflection of the rafters (perhaps exacerbated by the collar ties). Thus, I would think that the ends of the rafters will need some kind of release.

Thanks in advance for your help and suggestions!

 

[BAretired]

Is the extent of remedial work contained within the "AREA OF FOCUS" as shown on plan? What about the rest of the roof? Is it considered to be okay as built? Otherwise what is meant by "AREA OF FOCUS"?

The 4-2x10 Girder shown on Section 1 should be shown on plan.

The intent of the remedial work is not clear. Are you intending to make the roof act as a truss spanning 30' or do you intend to carry a reaction on the existing 2x4 wall below? Or both?

BA

 

[emmgjld]

Regardless of the issues raised above, the length of time the roof members have been stressed is an enormous issue.
--- If they have been deflected for 6 months, adjustment within a couple of weeks is feasible.
--- If they have been deflected for 6 months to 2 years, Partial adjustment (70%+/-) within a couple of months is feasible.
--- If they have been deflected for for more than 2 years, partial adjustment (40% +/-) within a couple of months is feasible. Any more adjustment will take at least as many years as the framing has been deformed, but may not ever really 'come back'.

If the roof has this kind of problems, rebuild (partial/total) may be the best recommendation

 

[msquared48]

I agree with the above regarding the memory of the wood. I have seen the load from an old unsheathed residential studwall bend a steel beam, and keep it's original shape in doing so.

Mike McCann
MMC Engineering

 

[msquared48]

As a corollary, by doing the jacking, in order to straighten the members, you could overstress the existing connections causing the trusses to fail.

Usually, as for wicker chairs, this adjustment in member geometry is done with either green or wetted wood, not dry.



Mike McCann
MMC Engineering