Submitted by Tom Foster

If you live in an old home, odds are you have plaster walls and ceiling and a “popcorn” ceiling at that. And if you have a roof or chimney leak, long fixed, there may be staining or efflorescence on the ceiling or molding. One of the benefits of pandemic downtime has been the opportunity to get caught up all the household projects that have been put off over the years. (Alright, some of the projects!) Just as the shoemaker’s children go barefoot, so the builder’s house is never finished first.

In my particular case, the challenge was our dining room ceiling. Minor water damage due to a leak from the floor above had left staining and efflorescence in the patterned plaster of our 1890’s Gothic cottage. Simply put, efflorescence is a result of excess water in a lime-based mixture. Lime will continue to reincorporate small amounts of water, in effect healing itself. Too much water can lead to efflorescence, and eventually to failure of the surface. It’s fascinating to look closely at the crystalline growths where they have blossomed out from beneath the surface, erupting or forming veins. It almost seems a shame to smooth them out… almost.

Luckily, the plaster hadn’t separated from the lath, so the repairs were mostly cosmetic. For those unfamiliar with the building practices of the time, walls and ceilings were generally constructed in framing lumber (2x4s, etc.), surfaced with strips of wood (lath), then a rough coat over the lath, and a finish coat of fine white plaster. The key ingredient in the mixtures for both the rough and fine coats is lime, or powdered limestone (calcium carbonate), an essential feature of home building prior to the wide adoption of Portland cements in the 1930s and gypsum plasters after the mid-century.

The first step was to remove the damaged area of finish plaster to expose the rough coat beneath, scraping or ‘raking’ away the softer material, and to leave a rough edge around the sides of the opening for the finish coat to grab onto for a more smoothly blended joint between new and old, rather than a sharp (and more noticeable) line.

Filling back out to the surface is simple enough. Indeed, most of this type of work is less about rocket science and more a matter of a steady yet gentle hand, first building back the depth of the rough coat to achieve a suitable depth to apply the finish coat, then applying the lime and plaster mixture to match the textured appearance of the original surface. As with repairs to historic stone and brick masonry, the vital ingredient to restoring interior plaster is lime. Its importance cannot be overstated. While modern manufacturers have attempted to provide substitutes, nothing can match lime for the combination of strength, chemical stability, workability, and finish that it provides.

Having repainted the ceiling, we come to the most important part of any household project, namely getting your damned tools out of the dining room before your dinner guests arrive.


Tom Foster is an accredited member of the Edifice Guild, and operates Deer Park Restoration in Hamilton, specializing in the repair and maintenance of historic homes.