Posted By Carolc
I believe it is up to the Principal contractor concerened to set the rules, but in my experience it is banned on most responsible building sites because of the dangers of flammable materials, solvents etc that may be present.

Often there is a welfare area set aside for smoking at permitted break times, though most of the major contractors require smokers to leave the site completely to indulge their habit

Posted By Carolc
Examples: I believe Bovis banned it on the recently completed Drake Circus project in Plymouth, HBG on the university building, etc.
perhaps my use of the word "completely" was misleading ...on large sites the welfare portacabin is technically on site, but is a long way from work areas and operatives have to leave the work areas completely to smoke

Posted By mrs.seed
it is not just the flammability issues. There are health issues to with substances being ingested due to transfer from the hand to cigarette to mouth. Consider the environment being worked in, for instance I am involved in a lot of work on sewage treatment sites, where due to the possible contact with partially treated sewage, and the hand/mouth transfer described, smoking is banned until hands have been washed.