"Do you recommend, as my regular hardware store owner does, preserving gutters with a 50/50 mix of raw (not boiled) linseed oil and paint thinner - or do you recommend something better?"
Our house is a small one bedroom federal brick house built around 1774. All of the walls are brick and plaster. In many places the plaster has separated from the brick, creating bubbles away from the wall. The plaster is between 1/2″ and 3/4″ thick, with a horsehair rough plaster and then a skim coat. In some places the brick behind the wall has deteriorated into dust. What is the best approach to these walls?
Before you do any repairs to the plaster you need to determine what is causing the deterioration. Causes will be related to movement and moisture. There may be structural movement within the wall. Look for patterns of cracks on the exterior of the wall that are near the interior plaster deterioration. Water may be penetrating the wall through these cracks or at other locations such as the roof and gutters above or down near the foundation and cellar. As moisture migrates toward the interior surface of the wall it can deteriorate the mortar and the bricks, reducing them to powder. Even if the bricks and mortar are in good condition the moisture can loosen the bond of the plaster to the wall. If liquid water is penetrating it can dissolve the lime or gypsum that holds the plaster together resulting in loose and crumbling plaster.
Correct any moisture and movement problems and repair the brick masonry before proceeding with plaster repairs. This might involve minor gutter and downspout repairs to major foundation work to stabilize movement in the wall and brick and mortar repairs to the wall itself.
Reattachment of the plaster is possible but usually justified only when the plaster has some particular importance such as an unusual texture that cannot be easily reproduced or decorative paintings that must be preserved.
If you have more ordinary plaster the deteriorated areas can be removed and filled back in again with hand-troweled plaster with the same materials and methods originally used. The same two-coat plaster system should be used. If you cannot find a plasterer who knows how to do this, or just want to try it yourself start with the book “Plastering Skills”, by F. Van Den Branden and Thomas L. Hartsell.
John Leeke is a preservation consultant who helps homeowners, contractors and architects understand and maintain their historic buildings. You can contact him at 26 Higgins St., Portland, Maine, 04103; or by E-mail: johnleeke@HistoricHomeWorks.com; or log onto his website at: www.HistoricHomeWorks.com
© John Leeke