A.Paul Fisette responds: When it comes to
using unusual materials, most building inspectors will require
a qualified engineer to review the plans, calculate the loads,
size the rafters, and stamp the plans. However, as long as
you’re aware of these requirements, the process of
calculating dead loads is not that difficult.
Dead loads are simply the sum of the weights of all the
building materials used to build a particular part of a
structure. For example, the dead load of the roof might include
the weight of the rafters, sheathing, roof covering, and
anything else that is permanently attached to the roof
structure, such as drywall on a cathedral ceiling. You need a
good reference, such as Architectural Graphic Standards,
to find a list of the weights of various building
Once you establish how much each of the materials weighs,
you simply add them to calculate how much the assembly weighs
per square foot. This will be the dead load you use when you
turn to the tables to select your rafters.
Asphalt shingles typically weigh about 3 pounds per square
foot. The clay tiles you are using weigh about 8 pounds per
square foot — adding 5 pounds to the design dead load.
So, all things being equal, if you were able to use a rafter
table for dead loads of 10 pounds per square foot for the
asphalt roof, you should be able to use a rafter table for dead
loads of 15 pounds per square foot for the tile roof.
When selecting rafters, remember that the tables define
rafter span as the horizontal projection, or
footprint length, of the rafter (see illustration,
above). If you were to use the actual length of the rafter
along its edge, you would be oversizing the rafter.
So if calculating dead loads is so easy, why do you need an
engineer? Well, all things may not be equal when you change
roofing materials. Deflection limits are based on live loads,
but excessive dead loads will affect deflection. An engineer
can tell you when this becomes a concern. Also, if you change
the assembly, this may change the way the structure performs.
For example, if skip sheathing is used in place of plywood for
the roof tiles, you may change the way the structure responds
to wind loading. Again, an engineer would be able to tell you
if additional bracing is required.
Paul Fisette is director of the Building Materials
Technology and Management program at the University of
Massachusetts in Amherst.