Henry Staggs, RRO

May 21, 2020

Roof Loads

Buildings are designed for a specific amount of weight on that roof. If a client wants to install a new system, heavier than the current one. An engineer needs to assess the building and make recommendations for any modification necessary to handle the new load. But don't think of a roof load as only the roof system and the air condition units on the roof. There are different types of roof loads that need to taken into account when considering a roof system. 

  • Dead loads - Anything that is permanently installed on the roof, including the roof system itself.
  • Live loads - People accessing the roof various reasons. 
  • Seismic loads - The Earth shaking things up.
  • Environmental - The rain and wind we get in a monsoon season, and all the dirt and debris that accumulates on the roof (which is now a dead load, if not cleaned off)

What can happen?

Here is one scenario that fits Arizona well. The roof is an SPF (Spray Foam) roof. The monsoon comes along and drops dirt and debris from the nearby trees on the roof. The debris is not removed; it happens again and again. The weight of the debris begins to press down enough to deflect the spray foam. The debris clogs up the drains. Water stands on the roof, damaging the coating that protects the foam. Eventually, the coating blisters off, and the foam is exposed. The foam deteriorates, and the roof starts to leak.  

Or another example might be someone deciding to do an overlay of their asphalt shingle roof. The roof is designed for one asphalt roof system, not for two. The additional weight is too much, and the roof joist starts to bow and even brake. Now the roof has large visual dips in it, and the owners are wondering what happened?

The weight of your roof system and mechanical equipment installed and even the people who go up there are all important factors when deciding on which roof system to invest in.