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How To: Repair Old Plaster

One of the easiest mistakes to make when renovating a historic home is to tear down the old plaster walls and replace them with modern drywall and joint compound. This not only destroys the historic architecture and features that make a historic home great, but it also adds to the overall costs of the project exponentially. Despite the rumors you can repair old plaster yourself.

Lime plaster has been in use for thousands of years from Japan to Egypt and has been employed in many historic structures around the globe. Lime plaster is a far superior product than today’s modern wall coverings. With its crystalline structure, it repels moisture well while allowing for the contraction and expansion that often occurs in older homes during changing weather conditions. In fact, as the plaster’s structure calcifies (ages), it increases in durability and strength!

A Little History First

Traditional lime plaster was used for wall coverings until WWII. It was applied in a 3 coat process over thin wood furring strips called lath that were made of cedar, cypress or some other rot resistant non-staining wood and attached to the studs. The lath was soaked in water prior to installing the plaster to prevent it from sucking too much water out of the the plaster too quickly and spaced similar to the above picture with room between each piece for plaster to be pushed thru when applied. This spacing allowed the plaster to “key” in the lath and gave it extraordinary holding power when done properly.

The plaster was then applied in successive coats, typically 2, and a smooth finish coat was applied on top. Plaster took weeks to dry properly and fully cure before the walls could be painted. The whole process was slow and required a skilled plasterer which cost more money. After WWII, the building industry needed a faster way to cover walls and the relatively new product, drywall (getting its name from the fact that it didn’t go up wet like plaster), slowly crept into everyday use.

The Repair

  1. Assessing – There are many reasons for lime plaster to fall into disrepair. Knowing the cause of the damage is crucial to applying the correct solution to the problem. From water damage to vibrations from nearby traffic to peeling paint, historical plaster damage can be caused by many problems. Is the plaster peeling from the lathe? Is the plaster soggy or crumbly? Are new coats of paint peeling from the walls? If you answered yes to any of these questions; don’t panic. While each condition is unique to each situation, lime plaster can be repaired easily, economically and effectively. A common occurrence in older plaster, cracks are commonly caused by expansion and contraction of an exterior wall. In the case of heavy cracking, it is possible to that the house it settling improperly. This should be inspected by a Building contractor or structural engineer immediately! Repairing small cracks is the focus of this article though.
  2. Dealing With Cracks – This repair can be done by drilling several small pilot holes in the materials at various intervals. By measuring the depth of the penetration, you can determine if the lathe is detached. Many times a few well-placed screws can draw the lathe and plaster back together. In extreme cases of detached lathe, more holes are drilled into the affected area and an elastomeric adhesive is injected between the separations. Clamping washers are then applied to the surface of the plaster to press the loose plaster back tightly against the lathe and allowed to dry. Once the plaster is secured, the holes and cracks can be filled flush with a mixture of lime and gypsum, allowed to dry and then painted. Check out my video on fixing cracked plaster to learn how to do it yourself!
  3. Peeling Paint – Another common dilemma when dealing with older homes is peeling paint, and it can be repaired with just a few simple techniques. More often than not, peeling paint occurs when many layers of paint have been applied to the plaster over the years. Calcimine is the common culprit of many peeling paint plaster problems. Calcimine is a water soluble paint material that was typically used in older paint products. The calcium in the paint reacts with the moisture content of the calcium in the lime plaster and creates a bond between the water molecules. This bond easily allows water to slip in and out of the paint’s surface, so even if you apply new paint, it peels over time. To remedy this problem, the old paint must be removed. A wallpaper steamer is the perfect tool for removing old paint from lime plaster. Gently use the steamer and a plaster knife to remove the paint without gouging the plaster. Once you’ve removed all of the old paint, wash the plaster with a rag and room temperature water. Don’t get the walls too wet; a light wipe down will suffice. Allow the plaster to dry 24 hours before repainting.
  4. Patching – Patching plaster in anything more than small amounts is something best left for the pros, but if you are a brave DIYer then you can try to tackle the task on your own. Old plaster is made from much different materials than current drywall joint compound. For decent sized holes the best thing to use for repairs is called Big Wally’s Patching Plaster. You simply mix with water and apply. Once your plaster is mixed apply it with a putty knife and press it firmly into the supporting lathe. Make sure the plaster isn’t sagging in the hole. If so, your mix is too watery and you should add more plaster until it is firm enough to hold under its own weight. Once the plaster has dried, give it a light sanding to smooth out the surface and wipe it down with a damp rag before painting.

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While some damages can be easily repaired, it’s easy to quickly get in over your head when it comes to plaster repairs in a historic structure. Plaster work is an art form and many delicate cornices, crown molding and ceiling medallions were sculpted by hand by skilled craftsmen. So, before you rip that old plaster down to insulate and cover with drywall think twice. You might have a unknown work of art on the walls or ceiling of your home that deserves restoring.

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78 thoughts on “How To: Repair Old Plaster

  1. It’s helpful to know that removing old paint will help newer layers of paint to stick on plaster better. I will be hiring a plaster repair services soon because I’d like to make some of my walls look new. That way, my refurnishing efforts will make my home look very clean and orderly.

  2. Our house was built in 1910. The walls are mostly still lath and plaster. Our plaster has a sand finish. We have a guy that does plaster work on historic homes, but he can’t seem to achieve the same finish. Does anyone know of a source that can show us how to create different period finishes on plaster?

  3. My stone built tenement flat was built in 1869. I think it is old plaster and lath but not sure about that or when it was installed. There is strip of damaged plaster – not deep – where a boiler was installed this year (2in by 14in roughly – there is still sound plaster in it but I had to scrape out loose plaster so it is uneven in depth). At one end of this strip there is an actual hole about 1.5 inches square (no wood behind it properly – just a wee but wood sticking up – the hole is deep and dark and there seems to be quite a space from the hole to the wall. I am taking it this will need a professional and that I can’t do it myself ? Grateful for any advice, many thanks.

  4. I have a 1940’s home with plaster and lath walls. With the paint stripped we’ve found this “paper liner”. Does this have to be stripped? Can we apply a coat of some sort of adhesive for new primer and paint?

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