# Ceiling rock lath repair



## Mike Milam (Mar 3, 2017)

Lots of help in this thread.

http://www.diychatroom.com/f101/big-ceiling-plaster-patch-560570/


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## ClarenceBauer (Mar 4, 2005)

The cracking is normal for rock lath due to a high moisture at some time like when you have a hurricane and power is off for weeks the rock lath pulls in the moisture than when power is back on the Gypsum shrinks causing the cracks along the board joints. Get a USG hand book and look up Hygrometric Expansion. Another thing that may aid in the cracking is there were no USG R-1 , R-2 or R-5 clips used at the board joints. See if you can find this booklet at a book sales. USG Red Book ( 1964 ) on page 26 & 27 it has all the info need for Rock Lath installation. Also that ceiling had a over lay finish applied that was most likely a dry wall product applied over the original plaster finish again high moisture causing the cracking due to suction rates of the finish products. Plaster washers may help but in most cases there is no wood support behind the short cracks this is were the R clips should be. For repairs i would recommend removing all the loose existing finish than apply a good quality plaster finish DO NOT USE a dry wall type finish over plaster or you will get failure at some time. If you are going to retexture use USG Diamond finish or Gold Bond Cal- Coat.
If you want a smooth finish for a DIYer I would recommend the use of Master of Plaster due to the fact it is VERY user friendly cost is a little high but so is the cost of a high quality car.


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## miamicuse (Nov 13, 2011)

ClarenceBauer, thanks. I also looked up the link Mike Milam provided where you also had some useful suggestions there.

The difference in my case is I do not think there is another ceiling sagging or delamination from the framing. The crack must have been there a while and was previously repaired and it is the repair that is now coming apart.

I have already scraped off all the loose finished with a putty knife.

So to make sure I understand I should be applying a high qualify plaster finish directly over it? Do I need to mist the area before hand? Will it help to apply any bonding agent?

What is the best way to apply the finish to the edge of the existing sound finish without spreading over to it as the existing finish has paint and ugly texture over it. Should I use masking tape to mask along the irregular edge best I could?


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## ClarenceBauer (Mar 4, 2005)

If you use the USG or Gold Bond than yes a bonding agent would be best. If Master of Plaster is used no bonding agent is required. As for finishing apply the finish than use a wet sponge to remove any plaster on the existing texture up to the new applied plaster edge.


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## miamicuse (Nov 13, 2011)

OK I am going to try the Master Of Plaster product and see how it comes out.

So no need to prep the cracked plaster, no mesh tape just remove all loose materials, dust it off best I can and apply the finish coat. I think they call it "Restoration Plaster" on the MOP web site.


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## ClarenceBauer (Mar 4, 2005)

On the mesh tape YES use it on cracks & joints the M.O.P. supplier should have the tape it should be a white color with a finer mesh than the dry wall mesh tape. I use to order it in wider rolls than the normal 2 inches. If you can't find it locally Contact Lauren @ 803-513-5078 she will lead you in the right direction. Yes the restoration Plaster is a little less expansive & will work very well, use thin layers rather than one or two thick layers it will come out better in the long run.


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## diyfriend (18 d ago)

ClarenceBauer said:


> The cracking is normal for rock lath due to a high moisture at some time like when you have a hurricane and power is off for weeks the rock lath pulls in the moisture than when power is back on the Gypsum shrinks causing the cracks along the board joints. Get a USG hand book and look up Hygrometric Expansion. Another thing that may aid in the cracking is there were no USG R-1 , R-2 or R-5 clips used at the board joints. See if you can find this booklet at a book sales. USG Red Book ( 1964 ) on page 26 & 27 it has all the info need for Rock Lath installation. Also that ceiling had a over lay finish applied that was most likely a dry wall product applied over the original plaster finish again high moisture causing the cracking due to suction rates of the finish products. Plaster washers may help but in most cases there is no wood support behind the short cracks this is were the R clips should be. For repairs i would recommend removing all the loose existing finish than apply a good quality plaster finish DO NOT USE a dry wall type finish over plaster or you will get failure at some time. If you are going to retexture use USG Diamond finish or Gold Bond Cal- Coat.
> If you want a smooth finish for a DIYer I would recommend the use of Master of Plaster due to the fact it is VERY user friendly cost is a little high but so is the cost of a high quality car.


Hi Clarence, if you have a copy of the USG Red Book, then pl can you share the pages relating to rock lath installation.


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## ClarenceBauer (Mar 4, 2005)

diyfriend said:


> Hi Clarence, if you have a copy of the USG Red Book, then pl can you share the pages relating to rock lath installation.


I have the U S D Red Book Dated 5/64 there are many types of installations what are you looking for Rock Lath on wood studs / ceilings or application for Radiant heat, or ceiling system on metal grid?


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## diyfriend (18 d ago)

ClarenceBauer said:


> I have the U S D Red Book Dated 5/64 there are many types of installations what are you looking for Rock Lath on wood studs / ceilings or application for Radiant heat, or ceiling system on metal grid?
> View attachment 725351
> View attachment 725352
> View attachment 725353
> View attachment 725354


Thanks for sharing. I am interested in rock lath on wood joists. My house is a 1955 built and is all plaster walls and ceilings. I just love the plaster-work; but unfortunately several rooms have pillowing in the ceiling. I am attending to the worst room (ref my post Pillowing (sagging) plaster ceiling) as a priority. In other rooms the ceiling plaster has solid integrity and I plan to only veneer coat those. (I read your advise on few other posts regarding Master Of Plaster as a good product).
The reason I requested info is for my own learning. I am not a plastering professional. I have keen interest in the art of plastering and especially wanted to know the science behind how the plaster application is done on rock lath and the how and why of repairing it so I dont do anything that will harm the plaster. There are not many master plasterers where I live (Michigan). The one I know does not undertake projects anymore due to an injury.


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## diyfriend (18 d ago)

I actually wanted to ask another question relating to your reply in post Big Ceiling Plaster Patch

In that post you mention that a bonding agent is required with most products except Master of Plaster and then you go on to say we can also make own recipe of 60 % lime putty 35 % Guaging Plaster & 5 % Moulding Plaster. Will this recipe not need a bonding agent and why?


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## ClarenceBauer (Mar 4, 2005)

diyfriend said:


> Thanks for sharing. I am interested in rock lath on wood joists. My house is a 1955 built and is all plaster walls and ceilings. I just love the plaster-work; but unfortunately several rooms have pillowing in the ceiling. I am attending to the worst room (ref my post Pillowing (sagging) plaster ceiling) as a priority. In other rooms the ceiling plaster has solid integrity and I plan to only veneer coat those. (I read your advise on few other posts regarding Master Of Plaster as a good product).
> The reason I requested info is for my own learning. I am not a plastering professional. I have keen interest in the art of plastering and especially wanted to know the science behind how the plaster application is done on rock lath and the how and why of repairing it so I dont do anything that will harm the plaster. There are not many master plasterers where I live (Michigan). The one I know does not undertake projects anymore due to an injury.


Plastering over Gypsum Lath (rock lath) you can use any of the following plaster mixes.
Structo-Base Gypsum Plaster rating very good.
Structo- Lite Gypsum Plaster rating excellent.
Red Top Gypsum Plaster rating very good.
Red Top Wood Fiber Plaster rating good.
Structo - Lite is the easiest to work with.
As for sagging in Rock Lath plaster there is no way to correct the sagging you will have to eighter remove it or apply the furring strips & drywall.
Rock Lath has a special paper that has fibers to grab the wet plaster.
Master of Plaster is the best product for skimming Old Plaster, do repairs on old drywall & can be used on new work. I just completed a bathroom 18" X 7' walls & ceiling had no problems & I am 85.
If you need pictures, I will comply.
A good Handbook for products is the USG Gypsum Construction handbook I have a couple dated 1987 if you would like one let Me know.


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## ClarenceBauer (Mar 4, 2005)

diyfriend said:


> I actually wanted to ask another question relating to your reply in post Big Ceiling Plaster Patch
> 
> In that post you mention that a bonding agent is required with most products except Master of Plaster and then you go on to say we can also make own recipe of 60 % lime putty 35 % Guaging Plaster & 5 % Moulding Plaster. Will this recipe not need a bonding agent and why?


Master of Plaster does not require a bonding agent.
The Lime Putty- Guaging Plaster & moulding would need one if applied over old work.


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## diyfriend (18 d ago)

Another reason I was wanting to learn about the rock-lath installation basics is because I am unable to understand the reason behind sagging pattern on the ceiling.
The ceiling joists are 24" O.C. and the 4' rock lath board is attached with the longer 4' side perpendicular to the joist. 
This means one rock-lath board spans 3 joists. I assume the rock-lath is nailed to the joists at both ends 4' apart AND the middle.

The sagging in every room presents the same pattern with the max sag at the middle of the rock-lath board. I am wondering why is it that only the middle nailing is getting separated and not the ends? If moisture was the cause then the separation could well be at the ends as well?


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## ClarenceBauer (Mar 4, 2005)

diyfriend said:


> Another reason I was wanting to learn about the rock-lath installation basics is because I am unable to understand the reason behind sagging pattern on the ceiling.
> The ceiling joists are 24" O.C. and the 4' rock lath board is attached with the longer 4' side perpendicular to the joist.
> This means one rock-lath board spans 3 joists. I assume the rock-lath is nailed to the joists at both ends 4' apart AND the middle.
> 
> The sagging in every room presents the same pattern with the max sag at the middle of the rock-lath board. I am wondering why is it that only the middle nailing is getting separated and not the ends? If moisture was the cause then the separation could well be at the ends as well?


One explanation could be 3/8" rock was used for 24" span it requires 1/2" rock lath.
Also, you may have some clips that were not installed see attach page # 28. ( Bridjoint Clip )
As for only the center sagging it may be the center would carry most of the weight & moisture causes the center nails to pull thru the lath.


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