I’ve always been curious about how this occurs…From the expressway here in Chicago, you can see right along the roof lines of a lot of houses with pitched roofs. The brick homes always seem pretty straight, but a lot of the frames are sagging in the middle of the roofline like a horses back. Because of the lot sizes here, these are always long narrow houses with a gable at the front and back, maybe a small dormer in the middle. One house near me looks like a Salvador Dali painting, extreme dip in the roofline and all the windows are severley racked towards the middle of the house.
My question is, how are these houses failing? My first thought was the foundation, but the brick homes don’t seem to have this problem, so I guess the framing is doing something.
Replies
Sag doesn't equal failure, although it does look a bit ugly. A lot of older houses have more roofing on them than they should, because people roof over rather than strip. My own house (1921) has undersized rafters that have some sag. Many houses have settling problems around the perimeter, in the middle, or both.
You remind me of a funny story. My first investment house had four layers of shingles , 1923 house with 2x4 rafters. We spent the day stripping layer after layer including the original cedar shingles. As the day came to an end we started to clean up. Seems the quick diet we put the house on shifted it somehow, as we had no luck in closing the front door. We actually had to move the striker plate to lock the place up.
Sounds like a lot of the older houses in my area, except the roofs over the living spaces are fine and it's just the garage roofs that sag, probably on at least half the houses I'm talking about, which were built in the 40's.
Definitely catches your eye, and annoying to look at.
the house next door here, has osb on trusses. I think 7/16. you can see each truss from outside.
Are the walls bulged out in the middle?
"Are the walls bulged out in the middle?"
Bob, that be a good question.
There is a beautiful 1920s gable-ended garage here that has beautiful catenary lines: the ridge, the tops of the long walls, and the drip lines. The foundation is still level. All organic forms now.
My house has a slight sag - maybe 3-5 inches over 40ft. But it was built in 1790 and carried a slate roof for a 175 years. I bet you could hear a sigh of relief from the rafters when the roofers took the slate off.
-Norm
People in time need more room and thus go up into the attic. Or they raise ceilings to the rafters, either way they cut collar ties and ceiling joists.
The walls may bulge a little as well as the structure settle down. Compound this with extra layers of shingles and an occassional heavy winter snow load to boot, and violayou got a sway back ridge.
There are basically two ways to do a roof ridge. One is to have the ridge be a big strong structural beam that carries the weight of the upper half of the roof, with the rest of the weight on the outside walls. The other is to have pairs of rafters on opposite sides lean against each other, with ceiling joists or rafter ties resisting the spreading forces at the bottom. That puts all the weight on the outside walls.
With a big strong beam, you can open up the space and have a "cathedral" ceiling. Eventually such a beam may sag a little under the load.
If somebody cuts away necessary parts of the ceiling structure in a building without a big structural ridge beam, the sag can get real bad real fast. ;-)
-- J.S.
Basically, the walls are being pushed out by the weight of the roof on the roof rafters. This causes the rafters to spread (flatter angle at top, sharper angle where they meet the walls), resulting in a sag at the peak. The geometry is such that the sag in the peak is much more obvious than the bulging of the sides.
The direct cause is obviously a lack of sufficient support for the rafters, either at the peak (vertical support) or where they cross the top of the wall (horizontal support), but these can in turn be caused by a dozen different things. (In the case of these barns it's usually due to a lack of structural members tying opposite walls together. The horizontal members are placed lower than the eaves to create the floor of an unobstructed hay loft, and spreading occurs as the top ends of the vertical members bend outward over time.)
I see a lot of barns like this out in the country, and some get so bad it's hard to believe they're still standing. However, usually the structure doesn't out-and-out fail until the roof is so badly compromised that major rot sets in.
It's quite likely that the homes you're seeing are constructed similarly to the barns, with the wall studs framed "balloon" style to run past the floor joists of the second story and partly up the second story wall. The result is a weak pentagon profile vs a strong triangle.