Roof uplift at gables

[gpaul]

In several homes (usually brick) built by the military in the 60'-70's, I have encountered a situation where the roof at the gable ends appears to have lifted, sometimes quite noticeably in comparison to the profile of the rest of the roof. Kind of a saddle look (that's a slight exageration). The actual roof line as viewed end-on at the gables sometimes has a slight bowed up effect as well. I feel it must be some kind of settling/heaving/truss uplift situation but can't really nail it down in my head. Have heard mumblings about trusses being placed on top of brick end (gable) walls,but nothing specific. Has anyone seen this? Can anyone shed a bit of light? Any thoughts appreciated. BTW, I am Northern Ontario where seasonal temperature differences are fairly extreme, and frost penetrates to 48"(+) at times. Thanks. Glenn.

[Richard A. Hetzel]

Sounds like inadequate rafter ties causing the "saddle" appearance...it's classic.Do you know for certain that the roof is typically framed with trusses? Maybe the interior trusses deflected some, and the end trusses did not. Often an entirely different configuration of truss is used for the gavble ends, with vertical web members instead of triangular, to act as studs for the sheathing and siding.

Would be easier to comment if we saw pictures.

[Jeffery L. Haynes]

Are they trusses?
Trusses often have camber to them....some framers mistakenly try to cut the camber
out while other use special clips that allow for movement.

On gable ends it is more noticeable depending on how they installed lath / drywall catchers.........as Richard stated.....pictures would be beneficial.

One other thing.....many framers do not fasten trusses correctly......pay attention
to any interior fastening of trusses as well as proper bracing.

[David P. Valley]

Does it look like this?



If so, I'll show you the results on the underside....

[gpaul]

Thanks David. It does look something lke that, and I have seen a few exactly like your pic, although the greatest deflection takes place near the roof edge (eaves)in this particular case. Would be interested in seeing the underside shots.

[David P. Valley]

Ok, now you are ready for the underside. When I went into the attic, I looked up at the center ridge board and the defect was obvious...




As you can see, the center support post was placed right on the center of the ridge board (with no bracing on the two outer sides of the ridge board to hold the load bearing weight firmly in place). This improper support caused the ridge board to twist out of place.

The builder should have custom cut this support so that it could be placed firmly and directly under the roof rafter area. That would have prevented the twisting action of the ridge board.

Keep in mind, that every roof sag tends to have different issues. There are times where I find missing collar ties and the sag is simply settling from snow loads.



Glenn,

Did you have access to the attic cavity in question?

[Brian E. Kelly]

Great pictures David, thanx for sharing.

[gpaul]

Thanks for sharing the pics David. Yeah, everything looked fine on the inside. With trusses, obviously there are no issues with collar ties or ridge boards, but I could see nothing to link back to the problem that was visible from outside. I'm curious about the pics though ... if the support in your situation was placed in the centre, or thereabouts in the roof, would it still not be the high point, with the rest of the roof settling around it

[Richard A. Hetzel]

As you can see, the center support post was placed right on the center of the ridge board (with no bracing on the two outer sides of the ridge board to hold the load bearing weight firmly in place). This improper support caused the ridge board to twist out of place.

A ridge board needs no center support! I'm sure that post was added after the fact by someone who didn't know what he was doing. Ridge boards are not structural; they are only a nailer. They probably added that post after their ridge began to settle, and that would happen because the rafters were thrusting outward, possibly because of inadequate rafter ties, or undersized rafters. I'm sure they didn't ask themselves what would hold up that useless post, either, because obviously, it didn't solve their problem.

[David P. Valley]

Originally Posted by Richard A. Hetzel

A ridge board needs no center support! I'm sure that post was added after the fact by someone who didn't know what he was doing. Ridge boards are not structural; they are only a nailer. They probably added that post after their ridge began to settle, and that would happen because the rafters were thrusting outward, possibly because of inadequate rafter ties, or undersized rafters. I'm sure they didn't ask themselves what would hold up that useless post, either, because obviously, it didn't solve their problem.

This particular ridge board obviously needed support, or it wouldn't have twisted out of place like it did. The problem is, the homeowner or builder reinforced (sub-standard repair) this roof structure improperly.

The snow loads were probably putting stress on the roof field and causing the center of the field to sag. The installer of this post thought that this additional center support would assist in preventing this sag to worsen. But due to it's improper placement and incorrectly diagnosing the sagging problem, the twisting occurred.

[David P. Valley]

Originally Posted by gpaul

Thanks for sharing the pics David. Yeah, everything looked fine on the inside. With trusses, obviously there are no issues with collar ties or ridge boards, but I could see nothing to link back to the problem that was visible from outside. I'm curious about the pics though ... if the support in your situation was placed in the centre, or thereabouts in the roof, would it still not be the high point, with the rest of the roof settling around it

It depends on the bearing point of the post. In this particular situation, the post was placed on a portion of the attic floor that had no load bearing advantage whatsoever on the underside. It's one of those (scratch your head-Harry the homeowner) installations.

If anything has to be supported in an attic, the brunt of the load must sit on top of load bearing wall studs.

[gpaul]

Thanks David, and everyone for the feedback. Interesting discussions as always. I may go snap a pic of the outside and re-post it here. I do see lots of sagged roofs but these ones don't seem to fit the profile. I suppose the sagging in the centre of the roof could cause some deflection at the eaves at the gable ends. Thanks again guys!

[David P. Valley]

Originally Posted by gpaul

Thanks David, and everyone for the feedback. Interesting discussions as always. I may go snap a pic of the outside and re-post it here. I do see lots of sagged roofs but these ones don't seem to fit the profile. I suppose the sagging in the centre of the roof could cause some deflection at the eaves at the gable ends. Thanks again guys!

Glenn,

Here's a nice link on Roof Thrust

[Richard A. Hetzel]

One problem in the roof framing pictured by Mr. Valley is that the ridge was not continuous, and neither was it spliced properly. It appears that the framer simply lapped the two sections of ridge, instead of butting them and applying scabs to their sides. But I repeat, a ridge never needs a center support, beause it is not supporting anything itself. The rafter ends at the ridge pictured are pulling away from the ridge, exactly as shown in the web site for which the link was given. Thus, obviously, the rafters have spread, twisting the ridges as they did so, and that is the cause of the saddleback roof. Some pictures of the condition at the point where the rafters bear on the walls would be helpful, as well as photos of the presence or absence of collar ties. That stupid post does nothing...never did, never could.

[Tim Spargo]

Some Ridges are known to be 4 x 12 (or worse), engineers can design DF and engineered Glu Lam Applications to support diff. applications such as Cathedral amongst many others. It does change prescription of Rafter, load and span. I would say what we are talking about is not covered under any of what I just said though. Probably the saddling is because the Gable Ends are the only part of the roof that hasn't sagged and as mentioned, poor design ie ties and others are of cause. On most cut and stacks, the ridge's only use is to help erect the roof and a starting point for you after you figured run, rise etc..

But to clarify, Ridges can be structural depending on design. Probably just not on the house that is being discussed. The structural engineer can come up with many designs.

Tim