Hi, what a great project !
The roof is just two rectangles which you can make pretty much as you have the sides but after (I suggest) you have made the gable ends This is how we would advise you construct it.
The bottom of the gable end should be 6 inches longer than the width of the shed. This will allow for a 3-inch overhang at each side. So thats one measurment thats cast in stone!
Cut a timber to this length and lay it on the ground. This is your base line. Measure half-way along the base line. Make a mark.Then place another timber at right-angles to the first, at the halfway mark. This is the ridge rod. The ridge rod is at 90 degrees to the base line.
Write down the half-distance measurement of your base line in inches. For an 8ft wide shed, half the base line would be 51 inches. If it's easier for you in metric it makes no difference, there are 25.4mm to each inch.
Divide this number by 12 and multiply by 7. The answer is how high your gable end is and gives you a roof slope of 30.5 degrees which is about standard. For metric, multiply the half base line length by 0.583 to get the height of the ridge rod.
You can use a sliding bevel to keep the angles consistant
https://www.diydoctor.org.uk/DIY-superst ... bevel.html Cut the ridge rod at this height and measure from the top of it to each end of the base line. These mesurements should be the same each side and form the hypotenuse of each triangle, or in shed terms, your roof line.
We know that the angle from the base line to the ridge rod is 90 degrees and that we have pitched the angle from each end of the base line at 30.5 degrees. As there are 180 degrees in any triangle we now know that the angle at the top (ridge) of the roof line must be 59.5 degrees. This allows us to cut all mitres at the correct angles and remove the ridge rod.
Remember you are measuring to the centre of the timbers at all times so make a drawing of how you are going to proceed first to be sure you have the measurements and method correct.
To strenghthen the gable (and ultimately the roof) I would suggest you use some of these mending plates (see image|)
https://www.diydoctor.org.uk/DIY-superst ... of-10.html a version of these is what is used (in larger, slightly different form) to hold roof trusses together in a house. They are only a few pence each and make a whole world of difference to the strength of a joint.
When you have made both gables you can measure for your roof sections which should sit on top of the gable ends.
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