Same way as virtually all the rest. I simply worked for companies that took on commercial, industrial, civil engineering, council and the like contracts. In my day you where expected to be able to do the lot, gas, water, oil, air just about anything that went through a pipe. If you couldn't do it the door was open for you to leave. Must admit it was a lot freer then, I think they where more concerned about you being able to do the job than the number of qualifications you had to say you might be able to do it.