Expert: Richard Taylor, AIA - 7/27/2008

Question
I am designing a flat roof garage. It will ultimately be a green roof and I need to know of a source for the load bearing requirements due to snow in my state NJ. Also, where can I find data on load bearing capacities of both wood and steel beams for this application? I am looking for accredited sources for this information.

Answer
Hi Robin - thanks for your question!

By "green roof" do you mean it will be planted?  If so, the weight of the soil and water will have a far greater impact on the structure than the snow load.

Snow load minimums are specified in building codes.  New Jersey probably uses the International Residential Code (IRC) which is the most common Model Code in the Eastern U.S.  Your local government, however, may have increased local requirements for snow loads, so check with your building department first.

As far as the structural strength of wood and steel members, you'll need to work with several sources.  First is the building code, which sets minimums for your building type, snow loading conditions, and area of the country.

For wood, structural values are set by several agencies; for plywood it's the APA (American Plywood Association) for dimensional lumber it's the AWC (American Wood Council).  For engineered lumber, you can use the proprietary data supplied by the individual manufacturers for structural design.

Don't forget in your structural design to take into account the lateral bending of the wall structure as well as the "racking" forces on the walls.  Lots of garage walls fail under heavy loading while the roofs stay intact.

It's easy to get in over your head in something like this if you don't have training and experience in structural design.  If you get stuck, contact a local structural engineer or Architect for help.

Hope this helps!

Richard Taylor, AIA
www.rtastudio.com
www.rtaplans.com