Old Dry wall and wood lath ceiling - how to restrain

[ajk1]

A portion of the plaster of an 80 year old house ceiling suddenly fell down. There has never been any leakage or other apparent signs of distress, although I won't know for sure until I go into the attic on Wednesday to look around.

I think the most probable cause is the age, shrinkage of the wood lath and the fact the there is AC equipment installed in the attic maybe 15 years ago that is annually serviced by someone who goes into that attic. Perhaps the small deflection of the joists under his weight was eventually enough to break the palster to wood lath interlock.

I think the ceiling in all the rooms could give way and I can think on no way to test them for this. I tried hammer tapping the ceiling but does not tell me anyhting.

Assuming that the wood ceiling (attic floor) joists are sound, I was thinking that instead of removing all the existing plaster (a messy job while people are living in the house), we could install new 5/8" thick drywall to form a new ceiling and thereby confine the old plaster. However this will mean the new drywall will be supporting more load than would usually be the case.

Now the Question:
Does anyone know the size and spacing of screws normally used to fasten drywall, and how I can calculate the "pull-over" strength of the drywall on the screw head, so that I can determine the required screw spacing for a safe instalaltion?

 

[msquared48]

AC equipment in the attic? Check and make sure it is adequately vented and that the condenser vents to the exterior of the structure. Any excess moisture could have compromised the ceiling structure connections over time.

Drywall screws same spacing as cooler nails.

Mike McCann
MMC Engineering

 

[ajk1]

Hi Msquared48;
The a/c is what is called a "split system", as I am told by a mechanical engineer, and is used on old houses that do not have ductwork. I believe that all that is in the attic is a fan, not the condenser. I believe the condenser is outside. I will be going into the attic to look around this morning. My question is where to find the "pullover strength" of a standard coarse drywall screw with bugle head. I need this information because the new drywall will be put on tight to the underside of the old existing plaster ceiling on old wood lath, to restrain any more plaster from breaking free from the wood lath and falling to the floor. This will be a plaster remediation project. Don't want to spend a lot of money and then have the new ceiling fall down. By the way, inspection to date gives no indication that water is any way involved, but will see better when get into the attic.

So any idea of where I could find the "pullover strength" of drywall screw on the drywall?
I will try calling Hilti this morning, but so far I do not see this particular data in their literature on line, or anywhere else on line, but I am not that great with the computer. I suppose I could rig up a test to measure this, but is a fair bit of work to do so. Someone somewhere must know the answer.

 

[SLTA]

I'd bet it was more vibration of the mech unit than anything to do with deflection.

 

[Jc67roch]

In my experience, old plaster and lathe systems just simply fail with time.... the mortar keys into the lathe break with time/vibration/house movement, the nails holding the lathe loosen in the wood and pull out - especially on ceilings (gravity). I have repaired in two ways - 1) new, THIN drywall overlay like 3/8" drywall, and use screws with ceiling button washers (see link below), OR 2) just use the screws with button washers on the plaster/lathe alone. In either case, try to layout the screws into the ceiling joists - otherwise you aren't accomplishing anything. Some electronic stud finders with "deep scan" can find the joists or get you close. Verify that the locations make sense with a typical spacing like 16 or 24" on center. I would recommend a spacing for the screws in the joists of say 16" to 24" on center long each joists. Hope that helps.

 

[Ron]

ajk1...for ceiling application of gypsum panels, the fasteners are Type W, 1-1/4" long, spaced at 12" o.c. For your application, I would drop the spacing to 8" and use a 1-3/4" or 2" fastener to accommodate the extra thickness of the plaster you are holding.

Another option is to put metal lath up below the existing plaster and re-plaster the ceiling. Follow ASTM C1063 for the application of the lath, but reduce the fastener spacing, again to accommodate the weight of the existing plaster.

 

[ajk1]

Jc67ROCH - I think you are bang on! I cannot identify any particular condition that caused a portion of the second floor ceiling to fall. No identifiable plaster staining or signs of leakage. There is only a fan in the attic for the "split system" air conditioning, and that is supported on a platform hung from the roof rafters. It is not near the area where the plaster fell off. We started up the air conditioing and there was no perceptible vibration of the ceiling (attic floor) joists during start up or while running. I also learned that the basement ceiling under the first floor joists is coming down in some areas. So your diagnosis of old age and its toll seems about as good a diagnosis as can be made. I did notice that the heads of the nails holding the wood lath genrally all had some surface rust on them...perhaps that was from the original wetness of the palster when it was first applied.

QUESTION: The idea of washers is very interesting but can you tell me whether the washers are going to be visible in the finished installation or are they totally hidden?

Thank you for you help. Much appreciated.

Ron - the length of the fastener has to accommodate new drywall (5/8") plus existing plaster (0.38" measured with electronic calipers) and wood lath (0.46" measured with calipers) for a total of 1.5" plus at least 5/8" penetration into joists = 2.09". My better judgement tells me we should aim for more that 5/8" penetration into the wood, so my feeling is that we need 2.5" long dry wall coarse screws with bugle head.

I was coming to the conclusion that we should put the screws at 8" centres (based on sme pull over data I found for plastic fasteners, so it looks like we are thinking the same way, although I still have no definitive strengths for pullover with the steel drywall screws.

Your other option of metal lath and plaster sounds very interesting. Perhaps we should ask the contractor to price it that way too, as an option. How long would it take the plaster to dry...the residence are planning in staying with elsewhere whil the work is in progress.

Thank you for your help. You always have good and interesting comments. Much appreciated.

 

[ajk1]

As a follow up, I think we are moving towards a decision to take down the exisaiting ceiling.

 

[Ron]

ajk1...if you use gypsum plaster or portland cement plaster, it should be dry in a couple of days. I would use portland cement plaster, but count on a few cracks.

 

[AELLC]

Check for moisture damage too - in a split system A/C unit, the evaporator and fan is in the attic. Moisture in the interior air condenses on the evaporator, and a pan collects the water. There are usually 2 drain pipes to the exterior from the pan. If both get clogged, then the pan overflows.

 

[ajk1]

Ron - thank you for the advice. Much appreciated.

AELLC- I found no evidence of mositure staining or damage and the house occupants say there has never been any known leakage. I did notice that the heads of the nails holding the lath were slightly rusted, but I suppose this may be from the wet plaster when it was applied when the house was built in 1934. About the condenser, i will ask a mechanical engineer with whom I have worked about that. Thank you for you comment.

 

[BAretired]

Removing the existing ceiling is the route I would take.

BA

 

[ajk1]

BARetired - thanks for you thought.

 

[Jc67roch]

ajk1 - with the washer heads I proposed, you would need to plaster over them to cover them for a finished look.