Keeping Foundations Dry
Thanks to Reid Thomas, Restoration Specialist with the NC State Historic Preservation’s Eastern Office and instructor for C2C workshops, for this guest post.
Moisture is the primary cause of deterioration in historic structures. Foundation drainage problems are the most common and overlooked source of moisture affecting many older buildings. Without good drainage or a system designed to carry water away from a building, roof rainwater runoff will cause eventual damage. One inch of rain on 1,000 square feet of roofing produces 600 gallons of water. This makes any building susceptible to dangerously high levels of moisture collecting in the foundation. Older masonry foundations with softer brick and mortar are especially likely to wick moisture upward into the inhabited areas of a structure, resulting in a host of problems. An excessive amount of foundation moisture can cause cosmetic and structural damage, raise interior humidity levels, increase the likelihood of termite activity, and create an indoor mold problem, along with other issues.
Deteriorating mortar is often noticeable between the bricks in the foundations of historic structures and on lower sections of the 1st floor walls. An excessive amount of foundation moisture is the primary cause of this deterioration. Large mortar losses in pre-20th-century masonry should be repaired using a lime-based mortar, rather than Portland cement (which will do more damage in the long run). Any materials used as filler should match the original mortar material. But unless water can drain away from the house, foundation moisture and deterioration will persist.
In one North Carolina historic house museum, humidity levels were so high that staff began to run a dehumidifier in the first floor of the building. Although the idea was a good one, there must be a sound procedure to dispose of the water that the machine collects. In this case, a dehumidifier drain emptied the water collected by this unit to the south wall foundation, contributing to the indoor humidity problem. Some of this moisture likely returns to the parlor by wicking into the masonry and penetrating into the crawl space. Also note the corrosion on the exterior outlet and the nearby wood decay that the moisture problem of this foundation has caused.
Traditional style gutters, along with drainpipes that carry the rainwater runoff away from the building, are the most effective option for historic structures. In cases where staff believe that gutters will detract aesthetically from the façade, they can be installed along the sides and rear of the building. This is the compromise that NC Historic Sites decided upon with the Iredell House in Edenton. Flexible corrugated plastic extensions can then direct water from downspouts downhill, away from the foundation. As a disaster preparedness measure, extra long extensions can be used to drain moisture as far away from a building as possible. These can then be threaded through cinder blocks to weight them in place despite strong winds.
The North Carolina Historic Preservation Office staff is available for technical assistance with problems and repairs to historic structures. Remember, your building is your largest artifact. Please contact Reid or other knowledgeable specialists within the division for help in managing your institution’s older buildings.
Posted on September 18, 2012, in Connecting to Collections, disaster preparedness, guest bloggers, historic houses, historic sites, hurricanes, workshops and tagged dehumidifier, gutters, Historic Preservation Office, Iredell House, mortar, Reid Thomas. Bookmark the permalink. Leave a comment.