At first glance, one might assume that the floors have settled. Actually the ceiling has gone up - sometimes creating a gap of as much as two inches where interior walls meet the ceilings. Trusses are prefabricated structural assemblies which hold up the roof and the top floor ceilings. Trusses tend to be a stronger lighter and less expensive approach to roof framing. Trusses are strong because they make use of the most efficient geometric shape we know of - the triangle.
Trusses are a series of triangles fastened together with gusset plates.
The outside members of a truss are called chords while the inner pieces are known as webs. Houses have changed over the years. Attics of newer houses have lots of insulation and ventilation. They also have roof trusses instead of rafters and ceiling joists. The bottom chord of a truss is buried below a deep blanket of insulation.