Repairing Plaster Cracks

Clues from Cracks

Cracks are wonderful communicators, and literally "point" to what is going on within a building. They indicate a high spot or low spot - in other words, where the building is settling from or to. Horizontal cracks usually happen "at birth" of the plaster. The wood lath shrinks and moves consistently through the setting up phase of plaster application, so as the plaster dries it cracks along lath lines, if a wall board product is used, it may crack along intersections and joints. Unless the plaster is clearly loose and floppy (delaminated from its substrate), horizontal cracks are usually stable and can be left alone unless you have an aesthetic dislike for them.

Vertical and diagonal cracks tell us that the building is moving, follow the crack and it will point out a sill or structural shift that my be a problem. Something has happened structurally to compromise the plaster, and it has moved from where it needs to be (broken keys, torn from substrate). These type of cracks need to be address before the weight of the loose plaster continues to pull more off.

Sometimes surface cracking will appear in more haphazard patterns, indicating plaster problems that are not structural. This kind of cracking is called map cracking or alligatoring. Old age will cause plaster to crack along intersections and seams in substrate materials (lath lines, beams and studs), and sometimes just the finish coat of plaster will let go from old age or poor-quality workmanship.

It is very rare to see older plaster without any cracks, and there are those of us that find cracks part of plaster's aesthetic character and charm - love plaster, love cracks. Cracking, however, can also be serious and lead to further plaster damage if not taken care of. Cracks occur for a variety of reasons, many of which are simply the natural reactions of plaster compounds and building materials. Climate and temperature changes, building settling and moving over time, weight loads, chimney movement and environmental stresses (heavy traffic, nearby trains, construction blasting) all contribute to plaster cracking. Cracking is further exacerbated by any structural disturbances or repairs to a building (foundation work, sill repair), leaving a building unheated during the winter, deteriorating framing and timber (rotting sills, weak floors and joists, insufficient framing), or water leaks. Some or any of these conditions are usually present in older homes and buildings.

As a caretaker of an older building, it is a good idea to learn some basics of plaster repair. Cracks are a good place to start with and will provide a foundation for other repairs such as patching and skimming. Cracks can be repaired, and if done correctly you will have long-lasting to permanent results. Using spackling, taping, and/or repainting are quick fixes that usually result in a crack coming back. We don't usually use fiberglass tape over a crack because it requires more layers to cover it and blend it in, so unless we will be skimming the entire surface we prefer a dig-out and infill method.

For a sound repair to any crack, the plaster on each side of it must be stopped from moving or shifting, and the substrate material (wood or rock lath) must be solid and secure with no bouncing. Push gently on the plaster on each side of a crack to test for movement. If it gives, or has delaminated from its substrate, more than 1/8" than further repairs such as reattachment is necessary. If the substrate is week, some sort of stabilization is necessary. Assuming we have a crack in stabile plaster in need of repair, we find this method very successful. Click here to view Crack Repair Illustration.

Loose plaster on each side of this crack has been marked for reattachment, the crack has been V-notched and is ready to vacuum.
tThis crack has been V-notched, vacuumed and brushed out. It is ready for infill.

First remove the plaster involved in the cracking by digging it out down to the wood or other substrate material with a sharp utility knife (have lots of extra blades on hand for this). Following the crack you will create a V-notch by removing the plaster along both sides of the crack, cutting at an angle until you see the substrate. You want to open up the V-notch that is approximately " - " wide at the opening, and angles in down to the substrate material. You are creating an angle so that your infill material will have maximum bonding surface without losing too much plaster around the crack. By opening up the crack in this way, you are minimizing the amount of debris that might be pushed into the lath and behind the plaster, causing more damage. If the plaster around this area is delaminated, has come away from the lath, then you can complete any reattachment at this point. Click here for our article on Plaster Reattachment.

Next, vacuum out the crack well, carefully removing additional debris and dust, brush out gently with a small hand broom or paintbrush, then use a spray bottle and damp sponge to clean the dust off the plaster and prepare the surfaces for infilling. With the crack cleaned out, infill can be approached two ways: using 1) a setting-type compound such as Durabond, or 2) plaster such as gauging or plaster of Paris.

Beginners will find a setting-compound easiest to use. Durabond is mixed with water and ready to use.

Durabond is a setting joint compound that has a bonding agent built in, plaster-like qualities, and different setting rates available, most common is 45 or 90 minutes. Durabond also comes in a sandable or non-sandable version. Non-sandable dries harder, and slightly stronger and we usually use this for the first coat. If you are less experienced you may want to use sandable Durabond throughout so you can sand away any excess as necessary. When you are ready to infill the crack, wet the inside of notch down well with a spray bottle, and within 3" of the sides so that you can remove build up from your infill products.

For the 1st coat, approach the crack from both sides with fill to push the fill both into and under the sides of the crack.

Mix the Durabond to peanut butter consistency. Push it into the crack, coming at it perpendicularly from each side of the crack so that you are gently pushing it into the notch and "smooshing" it under the plaster achieving a little reattachment. We use a 6" joint compound knife for this. After each infill application, stand your knife up on end to scrape off any excess filler, and to leave the fill flush with existing surface. It will shrink as it sets up, requiring 2-3 coats. We usually use Durabond sandable for the final coat so any final touch ups can occur by sanding the repair flush with 150-grit sandpaper block.

During the 2nd coat, the fill is applied again with a 6" joint knife, then the knife is used on end to scrape the excess fill off (shown).

If you are more of a purist and prefer to use "real" plaster to fill your cracks, plaster (gauging or plaster of Paris) can also be used to fill yourcracks. Be aware that plaster sets up much more quickly than Durabond, is very difficult to sand, and requires a little more skill in application. However, it usually requires only one coat to fill the crack. follow the same digging, vacuuming and washing process described above. When the crack is ready for infill, you will need to brush on a bonding agent because plaster on its own does not stick. We prefer acrylic bonding agents which are available from a plaster supply house, or many concrete products suppliers (USGS, Weld-O-Bond from Silpro, Plaster- and Concrete-Weld from Larson's). Many have the added feature of color so you can clearly see where you have been. Bonding agent works on the principle that it provides a consistent bonding surface for a new compound to adhere to. It does not need to be fully dry before applying the plaster, however, if it has dried, it remains active for up to one week. Bonding agent can only get wet once, softening enough to bond with the plaster and hold it onto the adjoining surface.

Peter applies the 3rd and final coat, using sandable Durabond. Again, the knife is used to scrape off excess fill once applied.
Completed infill that is wet and still setting up. The slight buildup on each side of the crack will be sanded off when fill is dry.

Mix the plaster to thick-yogurt consistency (tip: put water is a small container, add plaster in until the water disappears, this will give you the proper water-to-plaster ratio). Once mixed you have about 15 minutes to work with plaster of Paris, up to 30 minutes for gauging plaster. Fill the crack, coming at it perpendicularly from each side again to push the plaster into and under the crack well. Scrape the excess off the surrounding area, but leave it proud, or slightly bulged, on the surface. Use a damp sponge to wipe off any excess plaster on the surface surrounding the crack. As the plaster turns rubbery, spray it down slightly and begin to work it again, scraping excess fill off (it will have a creamy consistency) and pushing it further into the crack, or into other areas where the infill is dipping slightly. Keeping it damp enough, for 15 - 30 minutes depending on what kind you are using, so that the plaster does not dry out before it sets up is very important. If plaster does not have enough moisture to fully set up, it will not have the necessary strength and integrity intended for the product. Plaster mix starts out whatever color the dry compound is that you are working with, and changes gray as it sets up, gradually to white as it dries.

For Peter washes the repairs with a damp sponge to remove all residue and sanding dust. A 1:3 vinegar solution is used to neutralize pH levels.

If you finish your crack repair and it dents in slightly, you can go back with a coat of sandable Durabond to perfect your repair. To properly prepare repaired surfaces for repainting, we use a 1:3 vinegar and water rinse to restore the surface pH. At this point your plaster is ready for priming and painting.

Remaining residue and build up is sanded off following all repairs. Proper safety gear - Tyvek suit and charcoal mask are a must in older building!

Click here to view our Crack Repair Illustration. This article is also available in print in the October 2001 issue of Old House Journal.

©Copyright 2002 Peter Lord Plaster & Paint, Inc. 24 Moody Road, Limington, Maine 04049 (207) 793-2957