We have made only two tents since 1994. Both are still in use.
The canvas we use is Sunforger, treated for mildew and fire resistance. This canvas is pre-shrunk, very strong and basically *the* material to be used. We are very happy with it.
The way good tent canvas is constructed you are looking at a regular weave. This means an equal count of vertical and horizontal threads (warp and weft) in the cloth, like a grid pattern. This will leave small, square holes between the threads.
However, canvas fibers will swell up when they get wet. As they swell they fill these holes, making the canvas watertight.
If you treat your canvas, the fibers will not swell and you will be treated to fine mist of water in the tent, during a heavy rain.