our wood-shed we had a slightly different problem in that we had two (front and back) wall plates, one higher than the other, and the roof is on a 20% slope. So instead of cutting the bird's-mouth in the end of the rafter, two notches have to be cut out of the long edge of the rafter [see schematic L - our roof is considerably shallower so the notch is flatter and less symmetrical] with a good deal of precision. The horizontal cuts have to be exactly as far apart as the wall-plates and the angle of the notch has to such that it sits exactly upon and parallel to the top of that timber. Furthermore these two angled cuts in the rafter have to go through the wood so that the notch looks the same from each side. This is way beyond what my hand-saw skills are able for. One of the most useful things I learned in university was to point my index finger along the blade of the saw and hold the handle with only three fingers and the thumb. I was told this by a genuine chippy when I was helping to make stage flats behind the scenes at the college dramatic society. I can cut through a small timber at right angles so that the sawn face is all-square but angles are a different, more taxing, matter.
The rafters in our wood-shed were meant to be 4x2s = 100mm x 50mm on 3ft = 900mm centres and because our poles were 24 ft apart, I added 9 4x2s to the inventory. I had duh! neglected to allow for the roof's weather-protective over-hang which meant that the roof was at least 3ft longer than the floor below. So the spacing was nearer to 1100mm than 900mm. But we stuck with 9 rafters because the odd number meant that we had one rafter in the centre sitting on top of the middle poles and that meant that the roof was symmetrical; which turned out to be A Good Thing. To Be Continued . . .