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Strange wall composition in my 1950's house in NJ - plaster/concrete/gypsum

Strange wall composition in my 1950's house in NJ - plaster/concrete/gypsum


Old 03-21-16, 06:23 AM
Thread Starter
Join Date: Oct 2013
Posts: 12
Strange wall composition in my 1950's house in NJ - plaster/concrete/gypsum

The walls in my 1950's era house have the strangest composition I've ever seen. Cutting holes and repairing cracks is a challenge.

From inside out, the wall consists of the following;
1. ~1/2" Gypsum board sheets - seem to all be 2' x 8', running perpendicular to studs/joists.
2. ~3/16" cement on top of the gypsum. The gypsum was mounted spaced apart so cement has keys between the gypsum sheets. Wire mesh at all corners.
3. ~1/8" plaster finish on cement, and this is what is painted and visible on the interior of the house.

As you can imagine - 1 square foot of this wall material weighs about 10lbs! On the plus side it is an excellent sound deadening material. On the downside the ceiling walls are cracking along the keys - and I don't know the right way to fix this.

Any help in identifying what this type of wall construction is called would be appreciated!
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Old 03-21-16, 06:40 AM
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Join Date: Mar 2005
Location: USA
Posts: 43,823
That is plaster with a gypsum lath.
I generally repair plaster with a setting compound like Durabond.
pics would give us a better idea of what repairs you need to make http://www.doityourself.com/forum/el...your-post.html
Old 03-23-16, 04:20 AM
Join Date: Jan 2014
Location: usa
Posts: 466
Gypsum lath is usually 3/8" in residential work but it could be 1/2". The base coat of plaster should be 3/8" at a minimum for a first class job. The finish should be 1/32" to 1/8".

Can you determine if your base coat is a mix that contains sand as the aggregate material? Many times, if the crew was blending their own material on the job, too much aggregate could be added for the amount of pure gypsum used. This can create a weak mix that would be more prone to cracking, especially if applied too thinly.

If you have a great number of cracks you may want to skim coat the entire surface and use either "fibatape mesh" or " fibafuse tape", both available in 36" rolls.
This approach would be most appropriate if you are anticipating continued issues associated with cracking.
Old 03-29-16, 03:30 PM
Thread Starter
Join Date: Oct 2013
Posts: 12
Thanks to all for your input!

I was finally able to grab some photos - see below.

Below photos show the largest crack I have along my ceiling. When I moved in 2 years ago, PO seems to have repaired it the wrong way, as it wasn't there when I bought the house and showed up 1 year later. There appears to be another crack perpendicular to this one which is barely visible in this photo, but that repair hasn't failed yet.
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Other minor cracks and uneven-ness on corners. Looks like maybe previous water damage?
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I had to remove a piece of this stuff last year and saved the chunk for future reference. Here is it in its full splendor;

Back side. The nails were on studs;
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Cross-section - this time I measured with a caliper;
Gypsum = 3/8"
Concrete = ~3/8"
Plaster = <1/16"
Total thickness = 13/16"
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Two questions here;
1. What is the best way to address the cracks permanently? I'm a DIYer - and am extremely comfortable doing HVAC, gas, electrical, and plumbing work, but I feel lost with this. I do have access to the ceiling here (attic) in case this helps.

2. I want to install can lights and will need to run a new switch and wiring, which means cutting holes in the walls to get the wiring through. How do I do this and repair the surface the right way?

To clavert's question, I'm not sure if sand was used in the base coat mix. Maybe the pictures will help you determine if this is the case or not?

Thanks in advance!
Old 03-30-16, 02:39 AM
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Join Date: Mar 2005
Location: USA
Posts: 43,823
Often when patching cracks in plaster you can get by with just etching out the crack and filling with a setting compound. If there is any doubt whether or not the crack might reappear it's best to tape and finish the crack like you would a drywall joint.
Old 03-30-16, 04:27 AM
Join Date: Jan 2014
Location: usa
Posts: 466
Is the attic used for storage? I'm wondering if you have stresses on the framing that are causing the cracks or if there is perhaps some point load like a support at a hip/valley roof framing member that is putting a significant stress in the area of the cracks that run diagonal and longitudinal.

Although I can't tell for sure if this plaster base coat contains a sand aggregate, if you break up a piece from the section you cut out and pulverize it with a hammer you should be able to see sand grains. If the material crumbles easily, it could be a weak mix.

Your pictures seem to indicate some light texture on the surface. I'm not sure if that is just paint roller texture or a sand finish that has been painted over numerous times. When you patch, achieving a similar texture is usually the part of the job that is most difficult.

I would first make sure that the cracking is not due to some framing issue and then if you want to try to dig out a crack and fill with the hard setting compound and see if it holds up. Usually, it is best to tape the joint to add extra reinforcement as marksr alluded to. I've repaired many where I dug out the crack, filled with hard setting compound and tape and then skim coated the entire ceiling to give it all a uniform appearance. If you skim, make sure you wash the surface first to remove any accumulated dirt and films that may have been deposited from cooking, smoking,etc..
Old 03-30-16, 09:26 AM
Join Date: Nov 2007
Location: MI
Posts: 2,527
My first house--built in '58--had the exact same walls.
Loved the solidity & quiet.
Loved the coved ceiling/wall joints.
Loved ZERO nail pops, ever.
Loved how easy it was to strip wallpaper.

Hated anything requiring work on it because of its hardness & non-standard thickness.
Old 03-30-16, 09:39 AM
Thread Starter
Join Date: Oct 2013
Posts: 12
Thanks guys - I feel more comfortable tackling this now. Appreciate your input!

Calvert - attic is not used for storage, and is empty. I'll verify framing does not have any undue stress before proceeding here.

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