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how to patch this ceiling


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06-24-11, 09:39 AM   #1  
how to patch this ceiling

Our house was built about 1948-52. To repair a long-term leak that created some wood rot, I had to rip out large sections of ceiling and wall below the master-bath and sister the joists with LVL. Now I'd like to repair the wall and ceiling.

What's there now (see pic) is about 1/8" thicker than (xxx3/4"xxx) 5/8" drywall. Would I install xxx3/4"xxx 5/8" drywall and then trowel on a 1/8" skim-coat directly to the drywall? If so, what is the name of a good skim coat product that is designed to adhere to a ceiling?

If the edge is irregular where the original was torn out, so that there would be gaps in some places as wide as an inch between the edge of the drywall and the original, could I affix a fiberglass mesh over the gap and skimcoat directly onto the mesh? Or would a void behind the mesh lead to cracking and fallout? Do these voids have to be filled with some other product beforehand?

I don't have any experience with skim coating over drywall but I have put up drywall that looked professional at the joints, and I've also used a trowel-on clay wall treatment that turned out ok. I think my troweling skills would be up to the task if I knew what product(s) to use and if the instructions were clear about water/mix ratio for ceiling use.

Thanks for tips/suggestions.





Last edited by tr888; 06-24-11 at 11:21 AM.
 
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06-24-11, 10:15 AM   #2  
Are your original walls/ceilings plaster?

I've never heard of 3/4" drywall - do you mean 5/8"? Drywall comes in 1/4",3/8" and 5/8" thicknesses.

The most common way to get the wall/ceiling level is to shim out the drywall so it will be flush or just shy of flush with the plaster. How uneven is the cut? Ideally you would trim it straight so there isn't much of a gap. Mud and tape will work if the gap isn't too large. Gaps wider than 1/2" can get a little iffy.

Most any joint compound will adhere well to drywall. The regular [green lid] is best for the tape coat because it has the best adhesion. I often use a setting compound like durabond when repairing plaster. It dries harder than regular j/c making it similar to the plaster - it's harder to sand too, so try to apply it neatly.


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06-24-11, 11:01 AM   #3  
5/8", sorry.

I don't know what to call what we have now. As you can see in the picture (clicking on it makes it bigger) there is no wooden lath, and it looks like there are three layers: some sort of paper-covered gypsum board nailed onto the bottom of the ceiling joists; a heavy gauge wire mesh tacked onto it with a thick grey coarse cement layer trowled onto the wire mesh; and then finally a thin fine skimcoat.

It's hard to get the edges straight and clean when tearing out this old multi-layer stuff --the coarse cement layer tends to break jagged even if I score the skimcoat layer deeply with a knife.

 
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06-24-11, 12:48 PM   #4  
The wire definitely complicates things. Drywall lath was fairly popular back when your house was built, not sure why they used the wire. I forgot to mention earlier that we have a few plaster pros that frequent the forums - hopefully one of them will chime in later. While I've made repairs to plaster using drywall techniques - I'm not a plaster guy.

After you hang your drywall, if you take durabond and try to prefill the gap between the drywall and plaster that will make the taped joint stronger. It might take a couple of times to get the compound to fill most of the void.


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06-24-11, 04:18 PM   #5  
This is gypsum plaster over gypsum lath. Angles, corners and the rentrant corners of openings were reinforced with expanded metal lath. I would fix this with metal lath and plaster. Probably beyond your skill level. Plaster always fits.

So shim the drywall as close as possible to flush BUT NOT PROUD, then use a setting joint compound to fill any gaps. Mesh tape and extra wide mesh WITH SETTING COMPOUND will make a good transition between the plaster and drywall.

There are many posts about this kind of repair here.

 
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06-29-11, 03:19 AM   #6  
Posted By: marksr The wire definitely complicates things. Drywall lath was fairly popular back when your house was built, not sure why they used the wire. I forgot to mention earlier that we have a few plaster pros that frequent the forums - hopefully one of them will chime in later. While I've made repairs to plaster using drywall techniques - I'm not a plaster guy.

After you hang your drywall, if you take durabond and try to prefill the gap between the drywall and plaster that will make the taped joint stronger. It might take a couple of times to get the compound to fill most of the void.
Thanks for the info and the Durabond tip.

 
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06-29-11, 03:24 AM   #7  
Posted By: tightcoat This is gypsum plaster over gypsum lath. Angles, corners and the rentrant corners of openings were reinforced with expanded metal lath. I would fix this with metal lath and plaster. Probably beyond your skill level. Plaster always fits.

So shim the drywall as close as possible to flush BUT NOT PROUD, then use a setting joint compound to fill any gaps. Mesh tape and extra wide mesh WITH SETTING COMPOUND will make a good transition between the plaster and drywall.

There are many posts about this kind of repair here.
Now that I know what it's called, I'll have an easier time finding those posts. Thanks for that info. The coarse gray middle layer is also gypsum? Did it have cinders or something added to it?

 
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06-29-11, 04:39 AM   #8  
Tightcoat will know for sure but I believe you have the gypsum lath, then a brown coat of plaster followed by the plaster finish coat.


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06-29-11, 07:53 AM   #9  
Gypsum is one of the most versatile and common of all building materials. The plaster at the time your house was built came in 100# bags and to this was added 2 - 3 parts of sand and enough water to make it workable. Some of that water still remains in the plaster as part of the molecular structure of the gypsum crystals. The rest evaporated. It is the remaining water in the molecular structure that makes gypsum plaster such a good fire resistive material. This is more than you want to know. Gypsum plaster on gypsum lath is a wall and ceiling system superior to drywall. You are fortunate to have such a well built house.

You asked about cinders. The most common aggregate was sand. Sometimes vermiculite and more often perlite were used as lightweight aggregates. In some places I suppose it is possible that cinders were used. I've never run across cinders in plaster but plasterers are known for improvisation and you could have cinders or some cinders and sand.

If you could find a plasterer to do this repair you would get a better repair than with drywall. This being a do it yourself forum you have plenty of information here to do it.

 
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