“Plank and beam” roofing (or flooring) is a structural system that is often combined with post and beam framing. The confusing part here is the use of the
word “beam” in each case. For our generalized discussion, up to this point, it has been convenient to use the word beam, but now we must leave it behind in favor of more accurate —and therefore less confusing — terms. The “beam” component of the “plank and beam” system will be either a floor joist or a roof rafter, not girts or girders.
Just as posts are the naturally strong part of a post and beam frame, it is the planking that is the strong component of the plank and beam system. To give you an idea of just how strong planking can be, two-by-six tongue-in-groove decking can easily support a heavy (eight-inch or 20.3-centimeter thick) earth roof and a 70-pound snow load, with supporting rafters at four feet (1.2 meters) on center (48 inches o. c.). The earth roof described, with dead (structural) load comes to about 170 pounds per square foot. Conventional roofs in our area are required to support 70 pounds per square foot.
So engineering problems will not be found in the strong planking. The situation described in the preceding paragraph calls for some extra heavy-duty girders and rafters to support such an earth roof if the rafters are 48 inches on center. By the way, with a parallel rafter system the term “on center” refers to the distance from the center of one rafter to the center of the next. With normal framing, on-center spacing of floor joists is typically 12 inches (30.5 centimeters), 16 inches (40.6 centimeters), or 24 inches (61 centimeters). With heavy timber construction, other on-center spacing may be appropriate. At our Log End Cave earth-sheltered house, it was convenient to use a spacing of 32 inches (81.3 centimeters) on center. This worked out well with the predominantly eight-foot (96-inch) planks we used.
The number of rafters used, a function of the on-center spacing, is also known as the frequency of the rafters. The strength of the roof system (all else being equal) is a direct linear function of the frequency. If “direct linear function” throws you, think of it by way of a simple example. If you double the number of rafters (with spans, loads and rafter quality staying the same), you will, in fact, double the strength. You will be able to support twice the load with twice as many rafters by placing rafters on 12-inch centers instead of on 24" centers. This is an easy, if expensive, way to increase the shear and bending strength of a roof system.
