Unfortunately, your situation is really not the same as the majority of us here on Bluelight. When you are an individual, doing relatively normal, daily activities, you really don't have a lot of serious liability should you get too intoxicated. Sure, driving a car is the glaringly obvious thing here, otherwise, the worst is often a night in the drunk tank or embarrassing moments among your peers.
It's not about "will they notice", as that is sort of irrelevant. It's about the fact that you are clearly unsure of this. Sure, we can all be idiots in our daily lives and say for instance "my boss won't realize I'm on Diazepam". We might think we've got it all figured out only to get fired and find ourselves in a hole. As someone working on a roof, you are responsible for your own life, the life of your crew and the lives of those on the ground. It's not about "thinking" you're good, it's about knowing you're good because you're not on anything. As others have mentioned so poignantly, Benzodiazepines are notorious for making users feel sober even when they are often extremely, visibly fucked up.
Pilots, Firefighters, Heavy Machinery Operators and the like have different liability levels and principles that make them very different. I would have to say that the idea of taking any amount of Diazepam (Valium) that you're not totally understanding of, then going up to work on a roof is as unethical a move as getting behing the wheel of a car after having drinks. You could be fine, you could kill someone and the point is, you're not the judge of that. After the substances are in you, you're no longer fit to make that decision and simply must take a cab.
I think you could consider taking this dosage once you're totally familiar with it and how it effects you. You have to be able to say 100%, you have no doubts, otherwise you're responsible for anything that could or would go wrong as a result. I really would not do this or if you absolutely need Diazepam, take a dosage that does not in any way intimidate you or make you worry.