Roof repair (sagging roof)

[94Sport5sp]

Hi:

I am looking for information or repairing a storage building roof. The building now has a saddle look. The center is sagging and I would like to know why and if my plan to fix is on the right track. I have a good general knowledge of building things but do not work in the construction field.

The building was built some 20 to 25 years ago. It has 2 x 4 rafters on 2 foot centers. The center board is .75 x 4 inches. There are 4 lengths of 2 x 4 which run from the top sill across the width of the building (stringers I believe they are called) which are on every third rafter. There is a .75 x 4 inch board (collar ties I believe they are called) located at about the halfway point on the rafters but only located on every second rafter and not on rafters with stringers. In general I believe there should have been more stringers and collar ties. Am I right is assuming this is why the roof sagged?

As for fixing the roof I am planning to raise the roof, add stringers and collar ties, and then run two wire cables fastened to the outside of the top sills. The wire cables would be about 1/3 from each end of the building. I plan to use building screw jacks to lift the building slowly until the roof is level (or close to level). I am using two 4 x 4 x 8 foot beams, one for each roof half and positioned near the roof peak. Am I on the right track?

Thanks for your comments.
PamND

[scuba_dave]

I have never built a roof on 24" centers, even for my shed
In my shed the old rafters are 2 x6 for a 10' x 12' shed
I added a 10' x 10' additiona & used 2x10's that I re-used from the old house rafters

A 2x4 rafter at the low end on 24" centers can span approx 8' 5"
depending upon snow load & wood grade:

Span Calc
http://www.cwc.ca/design/tools/calcs/SpanCalc_2002/

I think the problem lies with the rafters. They may have been acceptable 20-25 years ago, but I think they are your failure point

Are you in a snow area? I'm in MA, one reason I go heavier on the rafters, plus I had them to use. I still would have stayed with a min of 2x6 with my project

[joed]

Are the walls straight at the top. They could be bowing out. A cable with a turn buckle or a comealong could be used to pull the walls back in.

[94Sport5sp]

Hi:

Snow as in the white stuff falling from the sky with great abandon? Yes we get lots and I have seen several (older than mine) buildings with 2 x 4 rafters on 24 inch centers doing quite well. Although, if I were building this building I would have gone with 16 inch centers for extra strength.

Yes the top of the building is now bowing out in the center which is why I was going to add the two wires to pull the centers together and then to keep them together once the repairs are done.

Thanks
PamND

[Wgoodrich]

20 pound snow load will allow a rafter 2x4 at 24" on center to span about 8' per IBC rules.

Depends on your area and what your snow load rating is.

I would site down the outside walls on the low side of the roof. I suspect you will see those walls bowing out as others are concerned in prior replies to your post. The cables and turnbuckles are a good advice they gave.

Good Luck

Wg

[Bob Cougham]

It sounds like you have a pretty good plan already.

I might install the turnbuckles before raising the roof. Then setting a string line or something between the two endpoints of the top plate of the wall. Then I could use the turnbuckle to pull in the top plate once the weight is off.

My other two cents is to check the connections on the existing collar ties and rafters. The fasteners/holes are probably stretched. you might want to pop them off and reinstall them on the other side of the framing.

Bob

[94Sport5sp]

I might install the turnbuckles before raising the roof. Then setting a string line or something between the two endpoints of the top plate of the wall. Then I could use the turnbuckle to pull in the top plate once the weight is off.

My other two cents is to check the connections on the existing collar ties and rafters. The fasteners/holes are probably stretched. you might want to pop them off and reinstall them on the other side of the framing.

Bob

Yes that makes sense to put up the cable wires first. I take it the string line you are referring to is to allow me to see when the bow becomes straight. I never thought of that but that is what I will do.

As for the collar tie the nail holes are definitely stretched. I was contemplating making new ones of different length to avoid the existing holes, but putting them on the other side would certainly save me some extra work. Thanks. I will probably have the same problem with the stringers but I cannot see me nailing them to the top plate because there is very little room to swing a hammer. Would long wood screws work?

Thanks for the comments.
PamND