How to Repair Plaster Walls
Your average homeowner looks at the walls of their home as a decorative surface--for painting, texturing, and hanging family photos. Beneath the surface, however, is a system--a wall system. By today’s standards, this wall system is likely constructed of drywall, now the Kleenex of wallboard products.
In a drywall wall system, panels of paper-wrapped, packed gypsum are affixed (nailed, screwed, or glued) to the framing system below. The edges, or joints, between hung panels are then taped and are ultimately coated with additional layers of gypsum paste. This applied compound itself then hardens to create a homogeneous surface, rigid and sound.
While the first iterations of wallboard (ironically called plasterboard) appeared in the early parts of the 20th century, it is generally accepted that it didn’t gain wide popularity until around the time of World War II.
It was into the 50s before drywall wall systems unseated its primary predecessor, the plaster and lath wall system. In a plaster and lath system, a mix of lime-based mortar was troughed over narrow wood strips, themselves called lath. This lath runs between the studs found in the framing system below.
The application of plaster, akin to other building practices like stuccoing, was typically performed in a three-stage application process. While certainly not a dead form, many antique plaster installations still survive today. This article will focus on accessible techniques for repairing common issues with antique plaster wall systems.
|Tools||Materials||Skill Level||Estimated Time|
|• Spray-water bottle||• Plaster (basecoat and veneer)||Intermediate||1 day or more|
|• Tiling sponge||• EZ Sand powdered joint compound|
|• Disposable gloves||• Multi-purpose joint compound|
|• Respirator||• Plaster washers|
|• Mud pan||• 1 1/4"-1 5/8” Fine thread drywall screws|
|• Taping and finishing knives||• Self-adhering 6" crack stop or wide mesh tape for drywall joints|
|• 1" - 2" flexible putty knife||• 3ft mesh plaster repair fabric|
|• Drill driver (#2 screw bit)||• Latex-based adhesive caulking|
|• Caulk gun||• Water conditioner|
|• Shop vac (with hepa filter)||• Surface protection|
|• Protective clothing|
Some would surely argue that plaster systems are far superior to today’s wallboard-based systems. Plaster over drywall yields a denser surface (possibly because it is fully site-built). It is generally accepted that plaster altogether is both more resistant to common wear and tear (such as nicks and dings), while also providing superior sound-deadening characteristics.
Historically, plaster was applied over wooden lath, but various forms of metal lath were also used. On removing wooden lath today (say in the context of a remodel), it can be quickly noted that there is often a good bit of irregularity between the individual wooden (rough-hewn) strips used within a given installation. Because of its nature, and perhaps because of the state of building practices, in general, at the time of its installation, precision lath to lath (as well as in frame quality and spacing underneath) was not an absolute necessity for a quality finish.
That said, it is commonly accepted that the universal dimension of a wood lath is around 5/16" thick by 1 ½” wide by about 4 feet long. Each lath is nailed to studs using 3-penny (3d) "blued" lathing nails (still available today). While spacing is often varied based on the craftsman responsible for the installation, 3/8" is likely the most common lath spacing increment. Exact spacing could be traced in part to the installation's finished thickness. Where very basic installations have a thickness of around 3/8", it is not uncommon to see plaster thicknesses of 5/8" or more.
In its most traditional form, plaster was applied in three coats. Not dissimilar to the installation of stucco (sometimes itself called exterior plastering), these three coats included two base coats--individually, the scratch coat and the brown coat, as well as a third finish or veneer coat. On removing plaster today, these three distinct layers can often be identified. The base coat(s) commonly have a distinct brown color (as dictated by a high use of sand in mixes), while the finish coat is instead often notably a bright white.
These installations get their form and strength from this build-up. The scratch coat, troughed directly onto the lath subsystem (itself pre-conditioned with a bonding agent), was forcibly pushed through the spacing between lathes. Here, it ultimately slumps to form what are known as "keys". These keys, put plainly, lock the plaster to the lath subsystem, and are, coincidentally, the most common point of failing in a plaster wall system.
The brown coat, applied second, gives the installation its integrity (if you will) and its form. Horse hair--yes, hair from a horse's mane or tail--was often mixed in the slurry to reinforce and add strength to these base coats. The veneer coat, more refined than the others, gives the installation a smooth(ish), paintable and wall-like finish.
Understanding the makeup of these systems can go a long way in attacking a plaster wall repair. Repairs often become necessary, when, as noted above, the system's keying fails, and hairline cracks appear. While of course not the only type of repair required, cracks are simply the most common. This article will focus not only fixing these hairline cracks, but will also briefly cover the process of addressing dings, and even damage that may have led to holes or crumbling.
Living in a home containing plaster walls, you know, some cracking on these walls (as well as on ceilings) is inevitable. Think about it, these walls have been in use, in some cases, for a century or more.
Some might argue that cracking is simply a sign that the plaster is approaching the end of its functional life. Fair, but one might counter quickly saying instead that the sole and primary reason for its survival to this point is due largely to plaster's reparability. Being able to quickly and "permanently" repair minor issues (like cracking) will easily extend plaster’s life into the foreseeable future.
Is Cracking in Plaster Structural?
If you have been around homes, you have heard some form of the phrase,"Well, that's settlement damage." It's true, gravity is tough on houses. Before we digress, let's say that home's must stand against this basic force daily, and all homes, for the entirety of their lives, will experience some amount of settlement, though at lesser degrees over time. Still, it happens, and the breaking off of your plaster's keys can be a consequence.
To that, there are a few things that should be noted about homes that still contain plaster surfaces. As most of these installations are approaching near 100 years of life one can assume that the wood (again the subsystems of lath and framing below plaster surfaces) are doing very little in terms of expansion and contraction.
First, the framing lumber used at the time of antique installations was simply of higher quality, i.e. denser cuts of mature wood. Second, it is fair to say also that this now antique wood material has likely thoroughly dried.
All that said, it is possible that plaster cracking can point to some sign of a structural deficiency in the home. Typically not, but if there is, however, any concern (due perhaps to repeated cracking), it may be wise to consult either with a contractor that possesses extensive experience in older homes or with a certified structural engineer.
Other Need-to-Knows about Plaster
In the United States, lead-based paints were widely used before 1978. While these products unquestionably performed well, they ultimately proved to be hazardous.
While some plaster installations themselves are said to contain lead, know that any antique plaster wall has, without question, been coated with lead-based paint. While rendered inert if encapsulated, special care must, however, be taken if plaster is to be removed.
For removal of larger areas, consult with a local EPA-approved authority for testing, and employ only plaster wall repair professionals who have been certified in safe lead removal, containment, and handling practices.
Prepping for Repairs
The largest percentages of plaster cracks can be traced to a failure in the keys over an isolated area. Further, how you approach your specific repair will come down to one simple test.
Standing in front of the damage, place a gloved hand about three inches off the edge of a crack and apply light pressure. Do the same on the opposite side of the crack. Can you see movement in the wall surface? If not, you may proceed to patching the area, as detailed below.
If on performing this “touch test,” movement in the plaster is distinctly noted, you must then take steps to immobilize the surface prior to addressing the crack itself.
Take appropriate measures to protect floors and other surfaces prior to beginning your repair.
Securing Failing Plaster
There are a handful of “pre-patch” systems on the market. In these systems, adhesive is injected through a series of site-made holes. Once cured, this adhesive re-bonds the plaster directly to the lath. While somewhat time consuming, this method is very effective, provided manufacturer's instructions are followed.
Opting for mechanical means may instead be a more universally-available and more time-sensitive option. Plaster repair fasteners appear in several unique forms, including both as staples or as capped nails. Plaster washers, however, are possibly the simplest, most straightforward option. While designs may vary based on maker, each measures around an inch in diameter and incorporates a subtle recess at its center, provided in this case to accept the head of a drywall screw.
Using a drill-driver, a drywall screw is driven through the washer, through the plaster surface, and into the lath behind. Based on the dimensions common in plaster installations, 1 1/4" fine-thread drywall screws are ideal for this operation.
When installing screws and washers, be mindful of misses--those efforts that fail in finding lath behind. When a miss is perceived, correcting up or down (along the course of the crack) by about 3/4" should guarantee success in re-securing the plaster to lath.
When successfully driven into place, the screw and washer combination should both draw the plaster back into plane, and also dimple downward to create an easily patchable surface. If your drill-driver has a clutch, it may be wise to back it down as not to risk damage from over-driven screws. If your drill-driver has a variable speed option, it is best to install washers with the drill set at a lower speed.
Install washers appropriately 1 ½” from the crack’s center. Once lath is successfully discovered, space washers on both sides alternating at maximum every six inches. For areas where you find the plaster to be particularly loose, you may choose to "pair" washers on either side of the crack in that location. For added insurance, you could pre-fill the crack with an adhesive water-soluble caulking. Plan to wipe down the surface with a wet sponge after washers are installed.
If you are consistently missing when setting your screws and washers, move to a longer screw, such as ones that are 1 ⅝” long. If you are having difficulty bringing both sides of a crack into plane, try setting a washer or two at the center of the crack. This will help keep your washer installation true by grabbing the surface on either side of the crack and setting them flush. While fine-thread screws will offer more holding power in antique lumber, a coarse-thread drywall screw may offer more grabbing ability.
Important! It is always a good idea to keep a bucket of clean water nearby. Add to this, a spray bottle, which can be used to spritz down and pre-condition the existing surface prior to repair.
Patching and Finishing
If your touch test above showed no sign of movement along the crack, you may simply fill the hole. (Note: Typically, movement is only lacking in the smallest of cracks.) While Plaster of Paris can still be found at local home centers, and will work in a pinch, pros typically reach for high performance plaster products, developed usually by the makers of drywall-related products.
Plaster, while not always in the same vein as traditional applications, is still being installed today--something that benefits us in our repair endeavors. Plaster products are found in powder form and are mixed with water in a mud pan or in a construction pail.
Tip: To ensure a perfect lump-free and peanut-butter-like consistency, plan to fit your drill-driver (or low-speed mixer) with a drywall compound mixing paddle.
Like other cementitious products, existing plaster will have the tendency to draw moisture from any water-based mix used in repair. As mentioned above, it’s important to spritz exposed edges in the existing surface prior to repair. As we patch, work initially to fill voids and not much more. If needed, feather patches out onto the surrounding wall surface using a tiling-type or drywall sponge. For a sense of security, a (larger) repair can be misted every eight hours for one day, prior to priming and painting.
If your project had you employing some means of securing the existing plaster prior to patching, your patch will then take a course akin to a standard drywall repair. A drywall repair, you may know, employs many of the same skills needed for finishing newly installed drywall.
Important! Prior to beginning any patch, it may be necessary to remove loose plaster material, especially at the edges of the area to be repaired. Do this with the outside edge of a putty or drywall knife. At minimum, you may find it necessary to run a drywall sanding sponge down the length of the crack to help minimize any irregularity in plane.
Prepping for Repair of Dings and Holes
Dings, dents, and even slightly larger issues, such as holes or crumbling, are approached with and employ not only skills described in this article, but draw further on knowledge of simple drywall repairs.
Minor dings, dents, and nicks are treated much like damage in any other wall surface. In most cases, spackling or a multi-purpose drywall compound will do for filling holes. Touch-up texture and paint can often complete the camouflage of these repairs.
Some issues, however, require the squaring up of an area surrounding damage. If cut-outs from a plaster surface are required, use these tips:
- Be mindful of the dangers of working with plaster.
- Prior to cutting into a plaster wall surface, effort to identify stud locations.
- Always score the outline of your intended cut with a utility knife.
- While a patch into a plaster wall can effectively be achieved using standard wallboard, do not assume that the most common thickness of wallboard (1/2") is the correct choice. Instead, measure the thickness of your plaster, and select a drywall thickness that will allow you to build up (with plaster or joint compound) to the finished plane.
- Securely attach your patching piece to "nailer strips" using coarse-thread drywall screws. (Nailer strips themselves are attached at minimum to two edges of the area to be repaired.)
- Finish as outlined below.
Note: Most consumer-oriented stud finders are often of little help here, so other techniques may need to be employed.
Note: 3/8" is often a preferable thickness when patching into a plaster surface.
Patching after Plaster is Secured
- Mix an appropriate quantity of veneer plaster or fast-drying, powdered drywall compound in a drywall mud pan. (Follow manufacturer recommendations for mixing and working time.)
- Spritz the existing wall surface.
- Using a one- to two-inch flexible putty knife and fill any space in the crack with your mixture. Scrape off any excess mix from the surrounding wall.
- Once this fill has cured completely, roll out self-stick plaster repair fabric and mesh tape over the length of the crack (or over the edges of a patched repair).
- Starting at the middle of the mesh tape and working out towards an end, use a four- to eight-inch taping knife to apply either a mix of veneer plaster or pre-mixed multi-purpose joint compound. (Repeat working towards the other end of the tape. Try for a reasonable job of feathering the edges of the repair during this step.)
- Allow this tape coat three to six hours to dry. Once dried, apply a second thin coat of either veneer plaster or slightly thinned drywall compound. This time use an eight- to twelve-inch drywall finishing knife.
- Once this second coat has dried, sand lightly with a drywall sponge. For some, this may mark the end of the repair.
- If you are thoroughly satisfied with the blending of your repair into the surrounding surface, skim and fog coat the entire area using a 12- or 14-inch finishing knife.
- Sand again with a drywall sponge, prime, and paint (attempting to match both the paint, and the paint tools, used on the rest of the wall, exactly.)
Note: If plaster washers were employed, select a mesh that will span the entire space between washers. Employing standard drywall joint tape will conversely run down the center of your washer installation and is equally acceptable. (For larger areas, or in areas where cracking is cluttered, it may be worthwhile to incorporate a large-format repair fabric. As a measure, this only adds overall integrity to the repair.)
Note: For smooth finishes, it's important to do an even more reasonable job of floating the edges of your repair at this point.
Important! Smaller knives may be useful if you are having difficulty feathering out the edges of your repair.
Important! The above describes the basic steps required in making repairs on smooth plaster surfaces. It is not uncommon, however, and especially in the plaster medium, to find variations in the finished texture on surfaces. If your repair is executed on a wall where a unique texture appears, additional steps (and some creativity) will be required for successfully blending in the repair.
Plaster walls often show signs of their age by way of hairline cracking. For many, plaster walls represent not only a tie to the past, but additionally, a still superior interior wall system. In fact, many would argue that lacking additional motivation there is simply no reason to replace them in favor of a newer, and consequently, less mature wall system. Instead, learning to repair them not only allows you to participate in the preservation of a piece of history, but also allows them to remain high performing and functional for years to come.
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