Attic With Trusses Not Suitable for Storage
QUESTION: I want to use the attic of my home for storage. The roof has trusses instead of rafters, and I’ve been told by some (carpenters included) that you cannot put sheets of plywood in the attic because truss roofs must be allowed to sway. Can you clear this up for me?
ANSWER: Your attic is not suitable for storage because the bottom member of a typical truss (referred to as a cord) is not designed to support a storage load, not because of truss movement. The cord is only designed for a dead load of 10 pounds per square foot. The truss will support its own weight, the gypsum ceiling below, insulation and light fixtures. It is not designed to support the weight of stored objects.
Trusses move, but they don’t sway, exactly. Truss movement occurs in a condition known as “truss uplift” or “truss rise.”
During the winter, trusses rise because of the different contraction between the bottom members of the truss, which are normally covered with insulation, and the upper truss members that are exposed to the cooler attic. The bottom members are exposed to warm temperatures and a low relative humidity.
Consequently, the lower members shrink and shorten slightly compared to the upper members. This causes the upper members to pull the bottom members upward, lifting the drywall ceiling below. As the ceiling moves up, cracks form where it meets the walls.
Lack of Ventilation Can Cause Wood Stain
Q: The 2-foot-wide eaves of my house have to be repainted every two or three years because stains that resemble mildew begin to appear. Would this be caused by the kind of wood used in the construction, or do I have a ventilation problem?
A: Your attic probably does not have adequate ventilation. If there are soffit vents, check and see that they have not been covered with insulation. The minimum amount of free air ventilation should be 1/150 of the ceiling area. For example, a 20-by-30-foot house has a 600-square-foot ceiling area. The minimum ventilation (1/150 by 600) is 4 square feet.
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Questions? Write to Popular Mechanics, Reader Service Bureau, 224 W. 57th St., New York, NY 10019. The most interesting questions will be answered in a future column.