Marijuana has become a legal substance in Canada. So naturally many employees want to use it recreationally. This can become an issue at work as the THC builds up in a persons system and if a drug test is required the result is positive test readings even though a person is not actually impaired at the time of the test.
There is a saliva test that can be administered that is supposed to be more accurate in determining impairment, but the test is not 100% accurate, and there fore depending on the contractor you are working for, or your companies drug and alcohol policy this test could be not allowed and the blood test required.
Metal building erection is a high risk working environment and therefore it is a safety sensitive position allowing no impairment at work, and many companies have not changed their zero tolerance stance in regards to marijuana. Until the government is able to come out with a point system similar to the blood alcohol content index of impairment my suggestion to workers is that they refrain from using drugs recreationally. Many employers will ban workers from their sites if a pre-access or post incident drug test comes back positive. So it is up to the worker to decide what is more important, consuming marijuana recreationally or knowing that they have a job.