Am going to replace several of my 100 yr old lathe & plaster ceilings upstairs (attic above) with 5/8 drywall. The 16″ oc joists currently sag 1 -2 inches in center. They are 2″x6″x16′. I’ve heard of sistering on plywood to the sides of old joists after jacked up to level. Is this a good way to address the problem. If so, how exactly would any of you recommend I go about it (plywood size, glue, nail pattern, brace from joist to rafters, etc)
I really respect your opinions and would be very thankful for any help you could offer.
Replies
Odd are pretty good that these joists are not structural in any way, meaning they were designed to hold up just the weight of the ceiling itself (and failed to do so).
This means that the jacking part may not be worthwhile.
It would be easier to sister new joists onto the old ones set around 2 inches lower than the old ones to establish a level ceiling plane.
I would probably screw & glue the sisters on, but this is certainly overkill.
Another approach to consider would be to jack up the plaster ceiling without removing the plaster, and sister the joists from above. I have had some success with this technique. Could save you from some messy demolition, and real plaster is a nice material in many ways. Patching labor may not be so bad, or you could do a thin blueboard or drywall veneer.
I would not consider plywood sisters. Too much trouble for no benefit I can think of. Also would not take screws into edge well.
Thanks for the response.
I was under the impressions that sistering the plywood to the old 2x6s (after jacking up the sag) would keep them straight. Was kind of hoping for this, in that it might save me a few bucks over buying all new joists (3 rooms at approximately 16' x 20'). Maybe I'm not thinking right here?!
Also... the old ceilings are already ripped out. There was a large fire up in the attic many years ago. Most of the lathe was brittle and was no longer supporting the plaster. So... I have easy access to the old joists at this point.
I do not think 8' plywood would do much on a 16' foot span. I suppose you could get the long sheets. Though it does not seem that you would save much money, and it would take much more labor to cut and splice the plywood than simply leaving the sagged joists in place, and sistering a new 2x6 or 2x8 onto each level. I would probably choose the 2x8s. The combined strength (of new and old) would likely be stiff enough to prevent additional sagging, particularly since the 5/8 board weighs less than the plaster did. If you intend to store things in the attic, you should take that load into consideration when choosing the depth of the sisters.
There are other options.
You could run a 20' built-up beam perpendicular to the sagging joists (down the middle), jack joists up to the beam, and secure with angle clips or hurricane ties. To lessen the burden of the 20' span of this beam, you could build up the centermost joist with a triple sister, or just replace it altogether.
This approach would require good support on either end of the crossbeams, would make a new 'tripping hazard' in the attic, and might be difficult to place.
Yet another option would be to tie the sagging joists up to the rafters. Several possible methods of accomplishing that, though it might become difficult to navigate the attic space with such bracing in place.
Edited 10/17/2003 2:31:58 PM ET by csnow
Appreciate the help.
If I were to add new 2x8s alongside the old, should I notch the ends where they would rest on the plate - so that the new joists fall below the old sagging joists? Or can I just blast them to the sides of the old and not rest the new on the wall plates?
I do not think the notching is important, and it would be time consuming. The distance between the plates and the start of the sisters would be short, so there is little chance the old 2x6s would have any significant flex through that section. You are really only looking for stiffness and leveling in this case, not maximum structural capacity.
As for the steel stud method, these would do little to stiffen the existing joists, which is why I would not use them. Seems this particular ceiling needs a bit more structure to it to prevent further sagging.
Well, it depends. Plywood is only practically available in 8' lengths, so your joists couldn't be longer than 10-12'. The sistering doesn't need to run the entire length as the bending moment is greatest in the center. You could try doubling the joists. You could also try sistering another material to them. Screwing light gauge metal studs to their side has been done in the past. I just did a job where we attached 3/16"x3x3 angle to the side of the joist. We used a self tapping screw with little wings so the screw could drill through the wood and metal and then thread into the metal. Worked out pretty well actually. Ultimately, you will need somebody who is able to determine what your loads are and how the ceiling needs to be reinforced. Just remember to jack the joists up a little too high because they will try to spring back to their existing shape. Also be sure to adequately distribute the jacks load over the floor joists. As item of interest, there was a article in a recent issue of FHB that showed how to straighten a sagged ceiling.
Please don't waste your time. Tried it with 3/4"x 6"x 8'-0" sheathing both sides. The only possible way is to cut joist in half and sister in 2x's. I wound up removing pine flooring ,old joists and installing engineered joists.You could if space permits sister in a larger joists. I would remove finish floor and subfloor if you have one and then add new joists.It is not possible to jack joists level, the sag is permanent you would just pop the ends of the joists up while jacking.
mike
How about the method in the most recent FHB. using steel studs attached to the existing joists, dropped slightly, to provive level surface to nail new ceiling ?
I read that article. Wondered about doing that except that I thought the old joists should probably be beefed up first. What do you think?
Steel stud method may work but does not level finish floor. The span is too much for 6" joists, possible drywall ceiling could crack when walking on floor above. The steel studs screwed to joists would flex with undersized joist I believe.Unless the stud was a structural stud. I did not read the article so I am not sure which type was referred to.
mike
No way should a 2X6 span 16'. Adding plywood to the side would be a waste of time.
I'd suggest setting new 2X10s alongside the old ceiling joists. Notch the ends a bit so the bottom edge of the 2X10s is lower than the lowest point of the old joists.
Behind every successful man is a surprised woman. [Maryon Pearson]
If your just using them for ceiling joists you can use 2x8's and nail them right along side the existing 2x6's but don't notch the ends. Nail them right on the same top plate as the existing 2x6's and then snap a line on the sagging 2x6's from one end to the other and cut the sag right out.
At mid span on the new 2x8 nail down a 2x4 on a flat and a 2x6 on edge Strongback and you'll be fine. That's how we frame every house or addition and what we've done in your situation.
Or you can nail the new 2x8's on first and just cut the 2x6's right out and get rid of them.
Joe Carola
2 other possibilities for you to consider:
1) If the joists are exposed at the moment, and easily removed, and reasonably square, you might be able to flip them over. I was able to do that recently with equally old and sagging joists, I put some weight on them from above, and they came back to pretty close to straight - and so it has worked fine. Then use idea 2 below to really beef it up.
I have also glued and screwed some 1" plywood to the sides of some 16' 2x8 joists that had sagged more than the 14's beside them, and they are now so stiff - and straight that they feel like 2x12s. If found I had to plane or sand the sides of the old 2x material to get a clean gluing surface: they were fir and the rough sawn surface had deteriorated a bit. I also sprayed water on the wood to damp it a little before applying the glue because the moisture content was less than ideal.
In another instance, I was trying to countersink my subfloor to allow 3.5" of concrete on top (dead weight of 50 lbs/sf) to allow for in floor radiant, without raising the overall floor level. I strengthened each of the 12' 2x10s by sistering 2 1" sheets of ply - 1 per side - along the bottom 7" of each joist. I used an 8' and a 4' piece butted together - screw and glue (and even clamp) on one side and then reversed the 8 and 4' on the other side. That gave me a 1" platform on either side - and then I put the same 1" ply down on top of that platform between the joists, effectively countersinking the sub floor to be two inches below the the top of the joists. Then I poured 3.5" of concrete on top. The floor did not flex or move any measurable amount. Before the concrete pour I suspended a bob in the centre of the floor exactly 1" above the floor below. It didn't move. 2 months later, bottom of the joists are still exactly the same distance from the floor as they were before the concrete was added. So to the people who question the effectiveness of plywood, I can only say it worked well for me. I will point out that several others here questioned whether the stiffening using plywood would work and there were many suggestions that I first increase the joist size as 2x10s were undersized for the span - before the concrete.
2) In an almost identical situation to the one you describe, I went to an architect friend and asked for a suggestion. The 2x6s in the attic dated to 1890 - 112 years old - and so they are true 2" by 6". However, they were only spanning 12' - but the attic is to become a bedroom. I didn't want to use bigger material because I would lose ceiling height on one of the two floors - and in the attic would rapidly contribute to losing useable floor space. His suggestion - and I followed it - was , in a 12' ceiling, to take 2 overlapping sheets of 8'x4' 1/4" ply and screw and glue the sheets to the underside of the joists, and then use 1" plywood on top. The resulting floor is extremely stiff - like a drum.
Details: the long edge of the plywood follows the joist line. The 8' piece is on one side (i.e. west side) and the 4' piece on the other on the first layer and then reversed for the second layer. The second course started with a 16" x8' strip so that none of the seams would loverlap. I screwed the ply to the joists on approximately 12" intervals. The first layer was obviously glued only on the joists. The second layer of sheets had glue in a bead along the joist line, and zig zags in between.
Then I put drywall on the underside of the 1/2 inches of ply. The ply is a lot lighter than the lathe that was there beforehand and the drywall a lot lighter than the plywood.
In a 16' application, I would probably do the first layer as 6', 8' and then 2' on the first layer, and then 2,8 and 6' on the second layer. That should provide greated overlap and strength / stiffness at the center point where you need it most.Gavin Pitchford
"Sail fast - live slow"
I can't believe you guys all missed this - ther has been a FIRE in the attic!
Heat from a fire can charcoal the wood without seeming to have charred it.
I would not build a ceiling to span 16' feet today with 2x6@ 16"oc but I have seen several really old ones with full sized 2x6s that do very well. Either the fire has destoyed the integrity of these framing members or the attic haas been used for heavy storage, or both.
As they are now described, these 2x6s are worthless. If severly charred(blackened) or charcoaled(dried showing alligatoring) they may not even hold a srew, nail or glue for sistering.
I would replace them one at a time with new 2x8s or if the attic is used for storage, with 2x10s.
Excellence is its own reward!
Good catch, Piffin. Gavin Pitchford
"Sail fast - live slow"
Fire or none, the sistering is probably still easier, and should yield a good result.
The sisters would be doing most of the work. If you can rely on the last foot (or less) of the old joists, you can save a lot of trouble over fitting in new ones.