loads for roof design - double roof

huangyhg

loads for roof design - double roof?
since the ibc 1510.3 allows one reroof before old roofs must be removed, do you design a new building for the weight of two roofs?  i have never seen an engineer do this, but it seems prudent.  
yes
reroofing is possible only in certain types, most commonly built-up roofing.  for built-up type roofing, we typically include the weight of reroofing in design (three if directed by the owner).
2001 california building code (modified 1997 ubc) section 2316a.2 table 2.3.2 footnote 3 is somewhat related to your question (this is for wood construction only).  the load duration factor (cd) is listed as 1.25 on the table, but the footnote is saying that this value can be used only if the dead load includes the weight of at least one reroofing.  otherwise cd shall be 1.15.
you design the roof for whatever loads it is expected to see.  that may or may not include a re-roof.  often engineers add in some extra load for contingency or just round up, such as using 20psf for roof dead load when the actual load may only be 15 or 17 psf.  that extra 3 psf may compensate for the weight of some extra tar, paper, shingles, or whatever.  
the other side of the coin is that if you tell your client that they can re-roof without retrofit, then they get upset about having paid the extra cost for overdesigning the thing in the first place.  client:  "well for how much of my buidling did i over pay because you over designed it?"  sometimes that can lead to more issues than just a bad phone call.