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Haney1 07-23-2007 10:25 AM

Sagging roof
My husband and some friends of his re-shingled our roof 6 years ago, and event put down new plywood. Now, the roof is sagging in the middle. We are planning on selling our home in a few months, so we need some good ideas on how to support the roof, without breaking the bank. Any suggestions on how to do this? ANY HELP IS APPRECIATED!!! THANKS!

jkrodger 07-23-2007 10:39 AM

We just bought a house that had a little bit of a sagging roof in the garage. The owners called out some contractors to come in and sister the rafters (I believe this is what was done). This appears to have improved the sagging a bit but we know at least it won't get worse and can easily be fixed when we eventually replace the roof. If you either get the sistering done or at least disclose that you will get the sistering done then you might be able to talk a buyer into ignoring the roof issures.

Haney1 07-23-2007 10:44 AM

Thanks jkrodger!

I will let my husband know. Do you know if this is something a "regular" man can fix, or do you need to be a professional? I don't want to spend money having someone else fix it if we can save money with him doing it.

jkrodger 07-23-2007 12:01 PM

Without knowing how handy he is, I'm not sure. If he's experienced and feels confident enough, he probably could, just remember that if it's done poorly, an inspector (a good one at least) will catch it. Here's some info I got by googling "sistering rafters"

Double the rafters by sistering a 2x6 or larger to the sagging ones. These need not extend over more than the middle three-fourths of the sagging member, unless the saggers exhibit signs of distress at their ends (cracking, splitting). In this case, the sistered rafter(s) should extend over the full length and bear solidly at both ends. Nail or screw new to old at six-inch intervals, using a staggered pattern. (from NBN)

Depending on the damage to the existing rafter, you can decide of sistering it is a good idea or not.

I don't like to see sistering of a rafter if the damage is on either end of the rafter. The ends of the roafter is where the weight is transfered to. If the damage is in the middle, the sister rafter can easily have the weight transfered to it and then the weight transfered back to the original rafter on the ends and to the ridge beam and the top plate of the walls.

If the original rafter is damaged on the ends, it is hard to get the birdsmouths cut or the top angle cut and the new rafter fit into place. It will never fit as good as the existing rafter and you will almost always have a sag or a hump in the decking.

But a sag or hump, depending on the price to install a new rafter, may not be too bad ?!?! (from forum)

concretemasonry 07-23-2007 01:16 PM

Sagging roof
Sistering will not get rid of a deflection or sag. All sistering does is make it more difficult to sag or deflect more.

To do it right, you have to get everything back in place and where you want it. After that, you can sister to make sure it does not go back to where it was.

Ron6519 07-27-2007 09:33 AM

You need to first find out why it is sagging. It wasn't sagging before and now it does sounds like a structural issue. If you can't diagnose the problem, call someone in who can. He should be able to tell you how to fix it and then you can decide if you are qualified to do the work correctly.

the roofing god 08-02-2007 10:18 PM

you should probably install a kneewall in the attic to bring the rafters to where you need them.sistering bad rafters w/out going for the whole length between support ares adds a lot of weight and can create further problems because of it:whistling2:

Ed the Roofer 08-03-2007 11:27 AM

Sistering is a good idea and so is the knee wall concept too.

But, if you do not repair the sag, you just have reinforced the sagging condition so it will remain a cosmetic and potential structural deficiency.

You must run a board or plate across the entire span and then jack it up making sure that the floor area where the jack is on is reinforced and transmits its load bearing weight across the entire floori of the attic rather than just on one joist.

Once the sag is jacked up sufficiently, then you can sister in the rafters and potentially also install a knee wall to prevent future sagging conditions.


the roofing god 08-04-2007 01:03 AM


Malcolm 08-04-2007 07:22 AM

I would like to build a knee wall for 3 slightly sagging rafters. Should I add a top plate or just brace the rafter directly with a 2X4? I will put a 2 X4 across the bottom to spread the load. I just wasn't sure about the top.

Ed the Roofer 08-04-2007 08:25 AM


If someone does not know much about construction, they may just build a knee wall and fit it in as where the rafters are already sagging, thinking that that would be good enough.

They need to jack them up first to alleviate the bow as much as possible from them prior to sistering the new rafters in to the existing sagged ones.

After they have the sagged rafters jacked up as much as possible, without creating any seperation from the ridge board or top wall plate, then they can install the sisters and then they can install and fabricate the knee wall.


Ron6519 08-04-2007 10:26 AM

What happened when they did the roof that caused this problem? Roof rafters don't all of a sudden sag unless there's a reason.
You can't just jack up these rafters, it won't work. As you push up on the roof rafters,they push back causing floor deflection. You need to build temporary walls under the jacking positions all the way down to the basement so when you push up, it has resistance below.

AaronB 08-04-2007 10:46 AM

I have never had to do this, but laying a couple of framing members across the floor joists has provided excellent even support without floor deflection. The roof framing members can be "retrained" by jacking a little at a time over a long period of time without anything dramatic happening to the floor or roof.

Ed the Roofer 08-04-2007 11:00 AM

That is exactly the point I was tying to make. By spanning the attic floor joists, I have been able to spread out the dispersement of the weight distribution.

Also, this usually is observed on homes which had too much weight from too many layers of old roofing on them, and when the tear-off is complete, the rafters move much easiear.


Malcolm 08-04-2007 12:45 PM

OK, it worked. All the rafters are level again. When the previous roofers did the roof about 16 years ago, it looks like the cracked a rafter. It is a 2X8. They just wedged a 2X4 under it. It only had a 1/2" sag because of the cracked rafter. I guess this caused the two rafters on either side to sag 1/4" each. That is why I had a wavy roof deck. Here is what I did. I placed a knee wall half way down the rafters. It goes under all three rafters. The load doesn't seem that great. I could lift it with my hands level with very little effort. There isn't any shingles on the roof of course. I do not, however, get any snow in my area either. I sistered a 8 foot section of the 2X8 with another 2X8. The top portion rests on a load bearing wall. The bottom portion of the sister rafter extends a foot beyond the cracked portion of the rafter. I then used 1/2" bolts on either side of the crack to connect the sister to the rafter. The sister and the original rafter both rest on the knee wall now. I notice sometimes on long trusses they are spliced together. This seems similar to that construction, but with better reinforcement. What are your thoughts?

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