Q.We are designing a house with heavy clay roofing tiles (which weigh about 800 pounds per square). How should we calculate the dead loads for choosing our roof rafters?

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 materials.

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.