Repairing lime plaster walls

We are finally plastering the interior of our 1927 craftsman home restoration. Project pasge

Many of the walls are severely cracked from a massive earthquake in Richmond that brought down a few buildings including the historic stone church a block away. There has also been delaminations from 40 years of leaky roof(no exaggeration).

We air sealed(Pro Clima DB+) and insulated the exterior walls(Roxul R-23 batts), so they have been covered in drywall. The DB+ paper sticks out on the edges and is hot-mudded(gypsum) to the plaster walls with some fiber tape to prevent cracking.

Here are the first steps

First the cracks are packed with plaster, then fiberglass tape is added to the wet mud to prevent the cracks from telegraphing through the new plaster.

Then plaster on top and into the fiber tape to complete the repair and it's ready for the first coat of lime plaster.

We use our own custom blend of lime, gypsum, sand and manure to repair the cracks in the plaster and even to plaster right onto the wooden lath where the plaster is missing.

The lime putty was mixed 2 weeks ago from bags of hydrated type s lime.

Basic recipe; 7.5 gal lime putty, 7.5 gal all purpose joint compound (I like National Gypsum for all my gypsum needs) 2 big shovels full sharp quartz sand and 1 big shovel full horse manure or if you would like to do the work of the horse, you can shred the straw yourself.

Sometimes we must remove a large portion on plaster from a severely damaged wall and just replaster.

The first thing is to repair any damaged or loose lath. Splice any cut boards.

Then gather the ingredients;

Lime putty 10 gal
Sharp sand 20 gal
Horse manure(well aged dry) 4 gal
Wood shavings 1 gal
Wood ash/charcoal 1 gal
put additives in a bucket and break them down with an electric paddle mixer

Mix all this up with a straight hoe or paddle

Total cost for this large bucket of plaster - $18

Once the plaster has been mixed well, apply to a well wetted wall ensuring proper adhesion and key by splatting each bit of plaster onto the wall. Then after a dozen or so plats, trowel smooth. Let dry just a bit and trowel off, holding the trowel at a 45 angle to the wall, which will raise the sand so the next coat can bond well.

If we cut open a wall like in the first photo where we cut open the wall for a walk-in shower, we save the lath and re-install the wooden lath on the new section. I don't like to use drywall for this because of differences in expansion/contraction causes cracking.

This first coat is thin, just enough to cover the lath well. When this has dried for a few days, then we will apply another coat of the same mix, but sometimes even more sand like 1 lime to 3 sand. This coat levels everything up.

Some photos of the original plaster.

We've got the first coat of 1:1 lime to pumice/ash on

hi Bill,

could you please elaborate on how you insulated the exterior walls? is it correct to understand that you added a layer each of (1) pro-clima (2) rpxul, then (3) drywall over the existing plaster wall? if so, how much thickness did this add to the wall? also, could you also explain what you mean by hot-mudded?

thanks mate.

The exterior wall assemblies are 1- 1 1/2" lime plaster, 3 1/2" dense packed cellulose, 1" lime plaster, 5 1/2" Roxul R-23 batts, DB+, 1/2" drywall, topped with 1/2" lime plaster. This adds 6 1/2" to the wall thickness, so the doors need to be remounted in new frames and the windows just get more trims.
I mean setting gypsum, but we have started to just use our taping mix for this as well.

Did you test the old plaster for asbestos? In buildings that I demolish, about a third of those your age, contain it.

 Even when the product contains no asbestos, I wear a full face asbestos grade mask,  to avoid filling my lungs with silica and mouse droppings.
........
Great pictures and documentation.

Hi Dale,
Yes, old gypsum does often times contain asbestos, but not ours thankfully. There was a fair bit of asbestos in the boiler room, but nowhere else.

I buy a full PPE set up for each new apprentice, after that it is up to them to wear what they wish. I keep these N95 masks on hand as well and they choose them more often than not.

So, sometimes you've got a complete delamination and you must rebuild the lime plaster from the lath up. The well haired base mix is pressed firmly, but gently into the lath in one clean motion from the hawk to the wall and up. This ensures proper key while not spreading the mud too thin.
Once the render is set up but still wet, scour the surface lightly with a wooden float. Then scratch diagonally across the surface with a pointed lath board to provide key.

The second coat or float coat has more sand and no hair.

This is applied when the render is dry to the touch.

I like to apply the fill coat and then wait an hour or so and give it a 1/2 coat to get the wall really straight for polishing.

I use the same technique for floating large areas of cracked plaster after it has been taped.

For the polish/color coat, we want a very smooth surface with no trowel marks.

I'm going to share my secret!

I add 1 cup of Murco to 15 gallons of plaster, this gives the very wet plaster some body as it seizes up the mix and helps to ensure smooth troweling.

Another little secret is to save or scrounge leftover roofing membrane as it makes the very best plastering tarps. I don't like to use my good canvas tarps because they get crusty and ruined.

What did you use for a bonding agent?