Tipping does not appear to be customary among the local population. The people who perfom the tasks you have listed typically make between 600 and 1000 riyals per month; when that's the going rate for labor, one might surmise that Qatar is not home to a culture of conspicuous generosity.
I suggest that you go with your conscience, bearing in mind that anything you give will probably be put to good use. With the exception of the car washers, none of these people have any control whatsoever over the prices of the services that they perform. What they can control is the service itself, and I generally use that as the basis for my own calculations.
Tipping does not appear to be customary among the local population. The people who perfom the tasks you have listed typically make between 600 and 1000 riyals per month; when that's the going rate for labor, one might surmise that Qatar is not home to a culture of conspicuous generosity.
I suggest that you go with your conscience, bearing in mind that anything you give will probably be put to good use. With the exception of the car washers, none of these people have any control whatsoever over the prices of the services that they perform. What they can control is the service itself, and I generally use that as the basis for my own calculations.