Forensic Mysteries

Article written by Jack Carr, P.E., R.S., LEED AP, CRITERIUM ENGINEERS

What I enjoy most about forensic engineering is solving the mystery. This often takes the form of being asked to investigate a particular problem at a condominium complex only to discover that the issue is something totally different. I thought it would be interesting to review some case studies and let others play Sherlock and try their hand at problem solving.

Let us call the first, The Case of the Levitating Wall. A property manager added a new condo complex to her portfolio. Before calling me she had just completed a tour with the buildings and grounds committee. Several unit owners were complaining that their buildings were settling, resulting in cracked ceilings and interior walls separating at their baseboards. Her visit confirmed the problem but she could not determine the cause of the structural problem and asked if I could stop by the next day.

Over the phone I asked about the nature of the buildings. She said they were simple one-story duplex units with full basements, typical wood stud walls, and manufactured roof trusses. Her inspection of the concrete foundation and floor girders revealed no cracks or movement. The site was a well drained strip of land between a river and marsh and none of the basements ever experienced water infiltration.

Her description was giving me a clue but one should not perform diagnosis over the phone. When I walked into the first unit the next day and saw that the conditions were localized in the center of the house, my suspicions were confirmed. What did I tell her?

I said “truss lift”. By her look I realized I needed to explain further. Truss lift is a phenomenon that occurs in new homes when roof truss bottom chord members arch in the center and pull up, causing ceilings to crack up to eighteen inches from the interior walls, and walls to be ripped from their floor bases. During the heating season, warm air rises to the ceiling where the attic insulation blankets the bottom chord of the trusses, keeping them dry. The top chords of the trusses, on the other hand, are exposed to cooler attic air that has relatively higher moisture content. As wood absorbs moisture, the top chord expands while the bottom chord stays the same, resulting in the truss bending upward.

This problem could have been avoided during construction by not nailing trusses to interior walls and the use of L-shaped metal clips to allow the truss to move up and down independently of the walls. Similar steps are taken to correct an existing condition. Often crown molding is used to mask the movement by attaching the molding to only the ceiling.

Let us call the next case, The Mystery of the Weeping Window. Once again, a new building was in trouble. This time it was a mid-rise luxury condo with a masonry façade and quality metal windows. The problem was that the windows were leaking, causing staining and damage to the surrounding drywall. The odd thing was that the windows only leaked when the winter wind blew from the north. The facility manager was stymied. His maintenance staff had tried spray tests and studied the window flashing but could find no solution

Fortunately, we were in the middle of a reserve fund study at this condominium and I was able to review the questionnaires that were returned by the unit owners as part of the study. Many of the owners complained that the units were too cold, too hot, or the air was poorly circulated. After studying the building drawings and visiting several units, it was clear that this was not a leaking window problem but rather an HVAC matter.

The windows were not leaking; they were sweating. The water source was not rain infiltration but rather condensation. The windows were facing a perfect storm of circumstances due to design and installation issues. The windows walls were built with deep (8-inch) sills and framed with drywall to create a window-well design effect. These in turn were covered with drapes.

A review of the HVAC system for the reported poor distribution of heat and ventilation air flow revealed that the building’s system was never properly balanced. To compound the problem, the ceiling and wall vents directed almost no air toward the window walls and whatever air reached the walls was diverted by the deep set windows and draperies. With inadequate ventilation to carry away the normal moisture from cooking and shower usage, the relative humidity in the units allowed the interior air to approach 100% saturation. When this air came in contact with the cold window frames, the moisture in the air reached the dew point and the water was released, giving the appearance of a leaking window. The good news is that once a mystery is unraveled the solution becomes elementary, my dear Watson.

© 2010 CRITERIUM ENGINEERS