Don’t store stuff in today’s attics
Almost all of us have seen, either in person or on television or in movies, old houses with large open attics filled with family lore and treasure. Many of us have also noticed that “they don’t build then like they used to” because today’s attics seem to have no useable space at all. All that open space in a traditional attic is now “filled with sticks!”
There are a couple of really good reasons for this. One is to control the costs of framing a traditional roof. Rafter and ceiling joist designs use very large pieces of lumber—2” x 10” or 2” x 12”s fifteen or twenty feet long. As more and more old growth forests have been harvested to fill the insatiable demands for lumber, paper and other similar uses, the cost of large timbers has risen exponentially. Roofers skilled in doing traditional framing are also in short supply.
Eventually, structural engineers and architects began to search for alternatives. The concept of the open web trestle had been around for years, extensively used in bridge building. In that use there were no choices—obviously no trees were large enough or strong enough to handle trains! But the idea that you could substitute interior reinforcement for less mass meant that roofing “triangles” could be made with smaller, less expensive lumber, with the proper reinforcement.
The analysis of strength of materials, methods of connection, how structures behave under various conditions and how strong is strong enough are all part of structural engineering. Engineers gather information about the anticipated loads the structure might bear, and then design open web trusses capable of meeting that load. (Those for a roof are triangular, but you may have seen some with the same flat appearance as the aforementioned train trestle used for floor systems.)
Why does any of this matter?
There are two different loads that any building components have to bear. The first is their own weight, called the “dead load”, which consists of the weight of the materials used, along with any other permanently attached components like large antennas, roof mounted air conditioners, elevated rooftop decks and anything else that is permanent and constant. Simple conventional roof systems are in the vicinity of 15 lbs per square foot. (If that seems high, pick up a sheet of 4’ x 8’ roof decking and a bundle of shingles and you will be surprised at the weight—and then imagine clay roof tiles or slate!) The “live load” is the anticipated load of temporary conditions—snow, workmen and equipment, etc.—which is generally 20 lbs per square foot. There is also another variable load from strong winds. Depending on where you live, all of these factors can vary considerably. These are the conditions and assumptions used in the design of the truss.
What is NOT included in the design specs is any allowance for storage INSIDE the attic. Where once were 2” x 6” or 2” x 8” ceiling joists making up the attic floor, now there are much lighter 2” x 4”s—clearly not as strong. In addition, trusses are designed to flex seasonally, which requires the bottom of the truss to rise and fall. This becomes impossible under an unanticipated weight. Storage also compresses your insulation, starving it of entrained air which in turn reduced its thermal performance….
I’m sorry to say it, but this means the correct answer is “Don’t store stuff in the attic.”