Making a shelf

[jeff47]

11. I don't have to. I do it with wood.

Again, you can not assume that the corner is actually a corner where two flat planes come together at a single point. Houses are explicitly built so that this is not the case.

The corner will be slightly rounded, because you don't put plaster/drywall compound tightly in the corner. The compound/plaster is thicker in the corner. And unless your measuring device is about 3 feet long on each side, you are not going to accurately measure that corner. You are going to be measuring the angle of the rounded compound, yielding an angle that is too wide.

Further, your walls are not flat planes, because making walls perfectly straight is hell of a lot of extra work and they won't stay flat as the house settles. They will be built "close enough". If a stud sits too far back near the corner, your measuring tools will yield a wider angle. Yet the shelf which goes past that stud will need a narrower angle to fit.

Because of all of these variables, you need to use the walls themselves as a template if you want a perfect fit. Otherwise, cut the wood at 45 degrees and you'll be close enough.