[Author's Note - By popular demand, I'm posting this Builder's Engineer article. It was previously published in NAHB's Nation's Building News, and can be found in my book, CRACKS, SAGS, AND DIMWITS - LESSONS TO BUILD ON, available at Amazon or this website. The concept of ridge beams and collar ties continues to perplex.]
Dear Builder’s Engineer,
My house was built in the early ‘70s, using 2×6 rafters spanning 13 feet between ridge board and wall. I live in New York State with a good-sized snow load. The roof has developed a sag between the ridge line and the rafter tails. I attribute that to the rafters being undersized and sagging in the middle. The sag is not horrible; I would guess it’s about an inch, maybe a little more. Still, I would like to do something now rather than wait to see if it gets worse.
A few details:
- roof pitch is 4 on 12
- a plain ranch style house…no hips or valleys…just front and back
- rafters are nailed directly to the ceiling joists
- rafters and joists are 2×6
I’m wondering if collar ties would help. I could take 12-foot 2×6’s and position them underneath the paired, opposing rafters, miter the tie ends to fit under the rafters and then use tie plates or gang nail plates to attach the ties to the rafters. I’ve read that is stronger than just nailing the collar tie to the side of the rafter. I thought that it would be best to get the collar ties supporting the midpoint of the rafter or as close as possible, that’s why I thought 12-foot length collar ties would be better.
Any thoughts or suggestions would be appreciated.
Daniel in NY
Dear Daniel,
Following is a sketch of your roof framing as I understand it. I analyzed this using a computer and the following assumptions:
- Snow load = 35 psf (pounds per square foot)
- Dead load from comp roofing and self-weight of framing = 15 psf
- Spacing of rafters = 2 feet
First, a little theory:
Truss vs. Rafter. There are big differences between rafters and trusses. A rafter bears at both ends; typically on a wall at the low end and on a ridge beam at the high end. There is no outward thrust at the low end of a rafter.
But what about a rafter that really isn’t one? I’m speaking of a sloping “rafter” with a pitch of 2:12 or greater, with no ridge beam—usually a puny 1x or 2x instead—and no connection to a ceiling joist, truss bottom chord, or other horizontally-restraining member at the low end. If you’ve got this, you’ve got trouble. Without the support of a sturdy ridge beam at the high end, there is nothing to keep that high end from going down. The low end can’t go down because it’s sitting on a wall providing vertical support, so when the ridge sags, the low end must move outward. Bad, bad situation. At the end of this chapter is a case study exploring a fix for such a “non-rafter” system.
The absence of a beefy ridge beam is fairly common, but in such a case, the low end of each (rafter) must be connected to some other member providing horizontal restraint, such as a ceiling joist. We call this connection the heel, and the overall system a truss. In this case, the rafter is no longer called a rafter, but a top chord.
A triangular truss has very large forces at each heel. It is the heel connection that keeps the top chord’s low end from moving outward. Also, the top chord of a truss has two kinds of stresses: bending and compression; whereas a rafter has only one: bending. Basically, a triangular truss has to work very hard because it is paying the price of a long span with no interior support.
So whenever you come upon a distressed truss, the best remedy is to add interior support(s), thus lessening the span and member stresses. You can bolster trusses, but without additional support, you’re simply spreading stress around—stress that must still be dealt with, and which becomes troublesome, particularly at connection points.
Let’s look at the truss in question. Note the 2×6 top chord is 217 percent overstressed and will sag 1.6 inches under a full snow load. The force at each heel is 1,700 pounds which requires twelve 16d nails to keep the top chord from moving outward. It is a miracle this roof hasn’t imploded.
Can anyone tell me how to cram twelve 16d’s into the small overlap space where the top chord and bottom chord (ceiling joist) come together? I think if you tried, you’d massacre the wood so badly, none would be left to hold the nails. It is for this very reason that gang-nail plates were invented.
Everyone wants to fix their sagging trusses with collar ties, presumably because they’re relatively easy to install. Here is the subject truss with a collar tie at the top chord mid-span.
Now the top chord is in better shape: only 70 percent overstressed with 0.82-inch sag. But look at the connections. Each collar tie connection point must resist 2,100 pounds of force, which would require fourteen 16d nails. Can’t be done. Also, the collar tie is in such compression that a 2x won’t cut it; a 4x is needed. But even more troubling is what happens at the heel. The top chord/bottom chord force has ballooned to a whopping 2,850 pounds. No way to make this connection. In short, the collar tie took load from the top chord and shuffled it around to other places, but those other places can’t take it.
When this column originally ran, I suggested the following upgrade: sister 2×6’s on to the existing rafters, like so:
This reduces the stress and sag of the top chord by half, but more importantly, does not increase the load on the heel connection. Note the sistered 2×6’s need only cover the middle 70 percent +/- of the rafter, not the entire span. This is because the sag is a bending problem (as opposed to shear, tension, or compression), which occurs in the middle two-thirds or so of the top chord. Bending stresses go to zero at the top chord ends; thus, no bolstering is needed there. I like this solution better than the collar tie. For more on beam theory, see my book, Basic Structural Concepts for the Non-Engineer, available at www.constructioncalc.com.
If we really want to solve the problem, we’ll find additional interior support. Like so:
Note that this completely solves the sag and top chord overstress problems, and it hugely reduces the heel connection force. The rub is making sure the interior support can truly take the load. In our example we’re adding about 600 pounds per foot (that comes from 1,200 pounds per truss, which are spaced every two feet) to the interior supporting wall—no small amount. If this wall has a continuous footing below it, it is probably okay. If this wall has large openings in it, those must be spanned with a beam, and the ends of said beam(s) need proper support all the way to a proper footing.
I once was involved in an old schoolhouse with a badly sagging roof. It was a hip system, about 8:12 pitch. There were no trusses, it was a rafter system; but amazingly, there were no beams at ridges, hips, or valleys. So the outward thrust at rafter heels had to be taken by the walls, but they couldn’t, so there were big outward bulges in the exterior walls. It is a miracle this building didn’t implode in one of our snowy northwest winters—a testament to the toughness of wood. Here is a sketch showing the problem and the fix (shaded).
We did our best to winch the exterior walls back to plumb and jack the roof back up. Then we installed a series of beams directly supporting the rafters, ridges, hips, and valleys. Of course, the new beams had to bear on something, so we positioned them over existing walls below and then retrofitted footings in the crawl space where the new loads came down. Interestingly, the contractor’s first suggestion was to install collar ties as a fix. No, no, no.
In summary, there is no easy fix for an improperly designed roof framing system. Collar ties are almost never recommended. Rather, find a way to add interior support, taking loads all the way to a good footing. And certainly, the best alternative is to design it right the first time.
