Can you sister sagging 2x6 rafters?

Reply

  #1  
Old 06-18-20, 04:51 AM
B
Member
Thread Starter
Join Date: Jun 2014
Location: Canada
Posts: 154
Received 0 Votes on 0 Posts
Can you sister sagging 2x6 rafters?

My home is old, my sagging rafters are 2x6, but surprisingly the rafter ties are only 2x4, and there is no collar tie. I wanna add support to the rafters but I don't wanna do anything with the rafter ties because they are only 2x4. Can I sister the sagging 2x6 rafters? And do I use bolt and nut to sister rafters?
 
Sponsored Links
  #2  
Old 06-18-20, 06:14 AM
P
Group Moderator
Join Date: Mar 2003
Location: NC, USA
Posts: 23,950
Received 673 Votes on 622 Posts
Yes, you can sister rafters. There is no need to bolt. You can nail them in place. The fun part will be straightening the old rafter.

Rafter and collar ties are often smaller lumber as they are in tension which tends to pull them straight.
 
  #3  
Old 06-18-20, 12:53 PM
B
Member
Thread Starter
Join Date: Jun 2014
Location: Canada
Posts: 154
Received 0 Votes on 0 Posts
How do u straighten old sagging rafters? I thought u can't unsag them, and u can only prevent them from sagging more?
 
  #4  
Old 06-18-20, 03:53 PM
P
Group Moderator
Join Date: Mar 2003
Location: NC, USA
Posts: 23,950
Received 673 Votes on 622 Posts
I've straightened 2x4" rafters by using bracing to push them straight and hold them there while I sister a 2x6" to hold it straight and provide additional strength. Straightening a 2x6" would be difficult. If it's long enough you might get some of the bow out.
 
  #5  
Old 06-18-20, 06:57 PM
H
Member
Join Date: Nov 2012
Location: USA
Posts: 1,800
Received 104 Votes on 92 Posts
Originally Posted by BurgerKing
How do u straighten old sagging rafters?
Time, patience, and a screw jack.

You can straighten wooden rafters, but it takes 1) time and 2) a good jacking point.

I had a 30' x 30' garage with a 6' front overhang, over many decades the corner post had rotted and the double 6" x 2" beam supporting the loft/overhang had sagged almost 2 feet.

Thanks to an accident of family history, I had inherited 2 train jacks from my grandfathers freight yard.

So over the course of around three months during one summer, I was able to slowly take the sag out of the barn. Basically, every other day, turn one jack one-quarter- turn to one-half-turn. Then leave it be, and let the wood react to the strain.
 
  #6  
Old 06-18-20, 08:42 PM
XSleeper's Avatar
Group Moderator
Join Date: Dec 2004
Location: USA
Posts: 26,189
Received 712 Votes on 659 Posts
One thing to add. Jacking up bowed rafters transfers downward stress to the sistered member, so when the jack is let go, that downward pressure is exerted on the new member. So it's often wise to select boards that are "slightly" crowned. Mark them with an arrow to denote which side is crowned up. Then when you sister them to the rafter that is being supported by the jack and you let the jack down, they are more likely to stay straight.

A straight board will often not be strong enough to take the downward pressure put on it when you let the jack go, but a crowned board will. You are just using the slight crown to your advantage.

Wood that has taken 100 years to bend doesnt just straighten back out (meaning perfectly straight) no matter how much time you give it. When you jack it you usually end up with 2 sections on either side of the jack that are still slightly bowed, just half as much as they used to be. Which is better than it was.
 
  #7  
Old 06-19-20, 05:05 AM
M
Member
Join Date: Sep 2019
Location: Canada
Posts: 1,514
Received 158 Votes on 145 Posts
I doubt that you will be able to straighten them.
I see a bunch of possible problems doing this.
Also you did not give any info on the amount of sag, rafter length, etc.

The only way I see this working is if you jack horizontally to push the rafter out.
So both sides of the rafter would have to be sagging the same amount and then cross your fingers that they will straighten evenly otherwise you may end up with a bulge on one side.
Also that the rafters do not detach at the peak.

Since you are jacking on a sloped surface, attaching to the rafter is essential otherwise it will just slip.

There are a lot of other things that will cause you problems.

Also you are probably working in a tight attic space with a finished ceiling. Trying to do this on an open ceiling would be a pain. Trying it in an enclosed attic would turns it into a nightmare.

Perhaps adding collar ties between the sag points would b easier and just as effective as sistering the rafters.








 
  #8  
Old 06-20-20, 03:38 AM
B
Member
Thread Starter
Join Date: Jun 2014
Location: Canada
Posts: 154
Received 0 Votes on 0 Posts
Rafters are about 14'. Rafter ties are about 12'. Rafters slope is about 7.5/12.
Rafters are just tiny bit sagging. I think it's hard to straighten them because they are very old, like 100yo. If anything I might crack them.
I don't wanna jack horizontally to another sagging rafter from the other side.
I prolly will sister them first, and then install collar tie.
The most annoying thing is that rafter ties are only 2x4, not 2x6. So I don't wanna rely on them at all.
 
  #9  
Old 06-20-20, 06:08 AM
P
Group Moderator
Join Date: Mar 2003
Location: NC, USA
Posts: 23,950
Received 673 Votes on 622 Posts
I wouldn't worry about the ties being 2x4's. The load on them is totally different than the roof rafters. They do not carry any roof load. Their only job is to prevent the rafters from spreading apart.
 
  #10  
Old 06-20-20, 07:07 AM
M
Member
Join Date: Sep 2019
Location: Canada
Posts: 1,514
Received 158 Votes on 145 Posts
How old is the house?
If as one poster said it is 100 years old and you said they only have s slight sag, why are you messing with it?

You are adding a lot of weight to the roof !!!!

 
  #11  
Old 06-20-20, 08:01 AM
H
Member
Join Date: Nov 2012
Location: USA
Posts: 1,800
Received 104 Votes on 92 Posts
Originally Posted by BurgerKing
they are very old, like 100yo
... rafter ties are only 2x4, .
Actually, a 100 year old 2x4 is LIKELY old-growth hard wood, that means it is probably about twice as strong as a modern big-box-store yellow pine 2x6.

I can break a standard yellow pine 2x4 with my bare hands, or simply by standing on it.
Totally different outcome if it's maple, hickory or pin-oak...
 
Reply
Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: