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Kerins Woodwork 11-28-2009 12:20 PM

Sagging 2 x 6 rafters on a 4:12 pitch
I have a split level ranch with sagging 2x6 rafters on a 4.5/12 pitch. I am having the 1/2 inch osb sheething replaced and putting on 30 year acht. shingles. The house was built in 1983 and the walls show no signs of spreading, there are 2x6 ceiling joist working as rafter ties. the sagging doesn't bother me so much but I do live in the north east and don't want to worry about snow load anymore. I would like to put in 2x6 collar ties about 1 1/2 to 2 ft down from ridge beam (2x6 ridge beam currently not sagging) and call it good. Will this be effective? I am also considering putting in a support knee wall which I think will be effective on one side because there is a load bearing wall 2 ft off center of ridge beam, on the other side there is a interior wall 2 ft off center of ridge beam but is not load bearing through to foundation like other side. However it framed over 2x10 floor joists 16 oc. this interior wall is 4 ft off center of load bearing wall on other side that I previously refered to and 4 ft from exterior load bearing wall. please help need to finish before winter really sets in.

Kyle Keever 11-28-2009 12:45 PM

Adding collar ties could help. You could also sister on 2x6 s and double them up which would strengthen the rafters. Your knee wall idea could also work. With the wall that is not bearing to the foundation you could double up the floor joist for added support under that wall. You could also use a strongback instead of the knee wall which you make by forming an L out of 2x? and use in post and beam manner. For that matter you could use a beam and post it where you have bearing for support but the strongback can be built in the attic so may be easier to handle. Without seeing your exact situation its hard to say which option is best but you are on the right track. Hope this helps. Good luck.

Gary in WA 11-28-2009 06:58 PM

"I am having the 1/2 inch osb sheething replaced" ------- was the old sheathing rotten? From a roof leak? From condensation in the attic rotting it due to inadequate ventilation? Was the problem corrected? I would replace with real plywood:

"walls show no signs of spreading, there are 2x6 ceiling joist working as rafter ties." ----- something is moving or they built it that way.... Check the walls for straight with a string-line stretched outside just below plate height, pulled tight, a 2x4 distance away from house. Measure it at questionable areas. OR.... The rafters may be sliding back away from the wall, check the fascia board for outward bow.
Collar ties are used mainly to reduce the span of rafters:

Pictures would help, of attic, of two walls, of rafters at ridge board, and of floor joists under two walls.

Be safe, Gary

Kerins Woodwork 11-29-2009 09:17 AM

sagging 2x6 rafters in 4:12 pitch
Yes the attic wasn't vented at all previously I have since vented on gable ends and soffet and will have a ridge vent cut in with new roof. I feel that the rafters haven't been compromised to much do to this but feel that the osb is probably delaminated somewhat. The osb is going to be replaced with 1/2" ply, would 5/8' ply be much better. I am considering having the roofing contractor sister the rafters where needed when they rip off sheathing. I believe it would be best to have them run from ridge beam all the way to wall plate, and have them use 2x8 or 2x10 is this correct or unnecessary. I thought I would wait until the roof was done to do any interior bracing if still needed, my thought is that if I did it prior it might be in the way of any sistering or replacing they might need to do or maybe the rafters will spring back a little, not likely? thanks

tpolk 11-29-2009 09:39 AM

I always use 5/8 if possible gives a better roof. You will need to add if you are getting rid of a sag. what is the horizontal span from ridge to inside of exterior wall and does bottom edge of 2x6 hit at this point. 2x4 exterior wall?

Daniel Holzman 11-29-2009 09:49 AM

The actual function of collar ties is to improve the load distribution between one side of the roof and the other during uplift (high wind) events. At the location you propose, they would have no significant effect on the sag of the roof rafters.

The 2x6 is not a ridge beam, it is a ridge board, since it has no structural function, but is simply there to supply nailing surface for the rafters. It is no doubt performing its function adequately.

Since the rafters are already sagging, the only way sistering will restore the correct shape would be if the contractor pries or wedges the existing rafter into the correct position when they sister the new one in place and nail it to the old one. This can be done, but is not as easy as it sounds, and it is relatively easy to crack the old rafter in the process. Make sure your contractor has done this before and is comfortable with the process before you ask him to do it.

The other techniques discussed, i.e. knee wall, brace, could be effective, but again you would need to straighten the rafters, which as I mentioned is not quite as easy as it sounds.

You should check the adequacy of the rafter size for the span, so far I haven't seen you mention the actual span of the rafters, if they are overspanned that would certainly account for the sag. If they are overspanned, you could consider installation of an actual ridge beam (a ridge beam is a structural member typically supported by a wall or column on either end, and designed to hold up the rafters at the top). Installation of a ridge beam requires proper sizing of the beam, typically by an engineer or architect, and would require some relatively significant framing changes, but could be a cost effective solution if it avoids the need to sister or replace rafters with larger boards.

Kerins Woodwork 11-29-2009 10:22 AM

The ridge board spans 26' and the rafters span is 16' from ridge to wall plate and an additional 1' to facial board. The end of the rafter at ridge fall just below ridge board do to the angle cut on the 2x6. I thought by having a new fafter at a larger dimension running the full span from wall plate to ridge board would basically be replacing the rafter and they wouln't have to worry about trying to get rid of the sag in the old rafter and go through the trouble of removing the old ones. My exterior walls are 2x6 framing.

tpolk 11-29-2009 06:10 PM

with 2x6 exterior walls you could go to a new 2x8 sistered to the side of your 2x6 with no need to straighten existing. the bottom of the rafter at the ridge should be in contact with the ridge board as this is a stress point. You can add a 2x4 or 6 as needed below existing ridge so there is support at this point

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