The mass produced stuff yes, however I went to a sofa place in Nottingham where I was able to see them being made on the premises and I was able to see and inspect the wood, look at finished frames and talk to the carpenter about the different properties of various timbers, something I know a fair bit about [job related]. I am going to have to disagree with you about using a roof as an analogy, the load per square foot is very light indeed even allowing for snow build up, in fact balsa wood could do the job, not quite the case with a sofa, especially with the average weight of an adult today.