Hi Baros,
All you need to do is insert the roof, select it and you will get 4 arrows (assuming it is rectangular), three will be red and the side you selected will be green. What you have to do is change three of them to Gable. So click the arrows, so that the LOWEST edge of the roof is red and the other three are green. Right click and change the roof Type to Gable. You now have a roof sloping one way - down to your red arrow. You can now click that side so it is green and click the three that you changed to gable so they are red. Now right click / Properties and change the angle of the roof to the slope you want for your shed. For a flat roof set it to a slope of 1% which you will not see. See the image.
If it has to be TOTALLY flat you could use a floor instead (Select Floor by Picking Points) and right click Elevate it to the height you want.
Allan