White smoke that is slow or lazy is most likely indicative 
  of early-stage heating. One more important note about smoke 
  color: brown smoke. Unfinished wood gives off a distinctive 
  brown smoke as it approaches late-stage heating (just prior 
  to flaming). In many cases, the only unfinished wood in a 
  structure are the wall studs, floor joists, and roof 
  rafters/trusses (photo 3). This can tell you that the fire 
  is transitioning from a contents fire to a structural fire. 
  Using our knowledge of building construction-especially 
  lightweight structural components and gusset plates-brown 
  smoke issuing from gable-end vents, eaves, and floor seams 
  becomes a warning sign of impending collapse.